Astronomy Picture of the Day Search Results for "great"

APOD: 2026 February 13 - NGC 147 and NGC 185
Explanation: Dwarf galaxies NGC 147 (left) and NGC 185 stand side by side in this deep telescopic portrait. The two are not-often-imaged satellite galaxies of M31, the great spiral Andromeda Galaxy, some 2.5 million light-years away. Their separation on the sky, less than one degree across a pretty field of view toward the constellation Cassiopeia, translates to only about 35 thousand light-years at Andromeda's distance, but Andromeda itself is found well outside this frame. Brighter and more famous satellite galaxies of Andromeda, M32 and M110, are seen much closer to the great spiral. NGC 147 and NGC 185 have been identified as binary galaxies, forming a gravitationally stable binary system. But recently discovered faint dwarf galaxy Cassiopeia II also seems to be part of their system, forming a gravitationally bound group within Andromeda's intriguing population of small satellite galaxies.

APOD: 2026 January 10 - Jupiter with the Great Red Spot
Explanation: Jupiter reaches its 2026 opposition today, January 10. That puts our Solar System's most massive planet opposite the Sun and near its closest and brightest for viewing from planet Earth. In fact, captured only 3 days ago this sharp telescopic snapshot reveals excellent details of the ruling gas giant's swirling cloudtops, in light zones and dark belts girdling the rapidly rotating outer planet. Jupiter's famous, persistent anticyclonic vortex, known as the Great Red Spot, is south of the equator at the lower right. But two smaller red spots are also visible, one near the top in the northernmost zone, and one close to Jupiter's south pole. And while Jupiter's Great Red Spot is known to be shrinking, it's still about the size of the Earth itself.

APOD: 2025 September 10 – The Great Lacerta Nebula
Explanation: It is one of the largest nebulas on the sky -- why isn't it better known? Roughly the same angular size as the Andromeda Galaxy, the Great Lacerta Nebula can be found toward the constellation of the Lizard (Lacerta). The emission nebula is difficult to see with wide-field binoculars because it is so faint, but also usually difficult to see with a large telescope because it is so great in angle -- spanning about three degrees. The depth, breadth, waves, and beauty of the nebula -- cataloged as Sharpless 126 (Sh2-126) -- can best be seen and appreciated with a long duration camera exposure. The featured image is one such combined exposure -- in this case taken over three nights in August through dark skies in Moses Lake, Washington, USA. The hydrogen gas in the Great Lacerta Nebula glows red because it is excited by light from the bright star 10 Lacertae, one of the bright blue stars just to the left of the red-glowing nebula's center. Most of the stars and nebula are about 1,200 light years distant.

APOD: 2025 August 14 – M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core, upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison with our neighborhood of the Milky Way, the closest star to the Sun is over 4 light-years away. Early telescopic observers of the great globular cluster also noted a curious convergence of three dark lanes with a spacing of about 120 degrees, seen here just below the cluster center. Known as the propeller in M13, the shape is likely a chance optical effect of the distribution of stars viewed from our perspective against the dense cluster core.

APOD: 2025 July 14 – NGC 2685: The Helix Galaxy
Explanation: What is going on with this galaxy? NGC 2685 is a confirmed polar ring galaxy - a rare type of galaxy with stars, gas and dust orbiting in rings perpendicular to the plane of a flat galactic disk. The bizarre configuration could be caused by the chance capture of material from another galaxy by a disk galaxy, with the captured debris strung out in a rotating ring. Still, observed properties of NGC 2685 suggest that the rotating helix structure is remarkably old and stable. In this sharp view of the peculiar system also known as Arp 336 or the Helix galaxy, the strange, perpendicular rings are easy to trace as they pass in front of the galactic disk, along with other disturbed outer structures. NGC 2685 is about 50,000 light-years across and 40 million light-years away in the constellation of the Great Bear (Ursa Major).

APOD: 2024 December 10 – The Great Meteor Storm of 1833
Explanation: It was a night of 100,000 meteors. The Great Meteor Storm of 1833 was perhaps the most impressive meteor event in recent history. Best visible over eastern North America during the pre-dawn hours of November 13, many people -- including a young Abraham Lincoln -- were woken up to see the sky erupt in streaks and flashes. Hundreds of thousands of meteors blazed across the sky, seemingly pouring out of the constellation of the Lion (Leo). The featured image is a digitization of a wood engraving which itself was based on a painting from a first-person account. We know today that the Great Meteor Storm of 1833 was caused by the Earth moving through a dense part of the dust trail expelled from Comet Tempel-Tuttle. The Earth moves through this dust stream every November during the Leonid meteor shower. Later this week you might get a slight taste of the intensity of that 1833 meteor storm by witnessing the annual Geminid meteor shower.

APOD: 2024 November 4 – M42: The Great Nebula in Orion
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the featured deep image in assigned colors highlighted by emission in oxygen and hydrogen, wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2024 October 6 – The Magnificent Tail of Comet McNaught
Explanation: Comet McNaught, the Great Comet of 2007, grew a spectacularly long and filamentary tail. The magnificent tail spread across the sky and was visible for several days to Southern Hemisphere observers just after sunset. The amazing ion tail showed its greatest extent on long-duration, wide-angle camera exposures. During some times, just the tail itself was visible just above the horizon for many northern observers as well. Comet C/2006 P1 (McNaught), estimated to have attained a peak brightness of magnitude -5 (minus five), was caught by the comet's discoverer in the featured image just after sunset in January 2007 from Siding Spring Observatory in Australia. Comet McNaught, the brightest comet in decades, then faded as it moved further into southern skies and away from the Sun and Earth. Over the next month, Comet Tsuchinshan–ATLAS, a candidate for the Great Comet of 2024, should display its most spectacular tails visible from the Earth.

APOD: 2024 September 26 - The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shows itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core, upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The deep, wide-field image also reveals distant background galaxies including NGC 6207 at the upper left, and faint, foreground Milky Way dust clouds known to some as integrated flux nebulae.

APOD: 2024 September 25 – Comet A3 Through an Australian Sunrise
Explanation: Comet Tsuchinshan-ATLAS is now visible in the early morning sky. Diving into the inner Solar System at an odd angle, this large dirty iceberg will pass its closest to the Sun -- between the orbits of Mercury and Venus -- in just two days. Long camera exposures are now capturing C/2023 A3 (Tsuchinshan–ATLAS), sometimes abbreviated as just A3, and its dust tail before and during sunrise. The featured image composite was taken four days ago and captured the comet as it rose above Lake George, NSW, Australia. Vertical bands further left are images of the comet as the rising Sun made the predawn sky increasingly bright and colorful. Just how bright the comet will become over the next month is currently unknown as it involves how much gas and dust the comet's nucleus will expel. Optimistic skywatchers are hoping for a great show where Tsuchinshan–ATLAS creates dust and ion tails visible across Earth's sky and becomes known as the Great Comet of 2024.

APOD: 2024 August 7 – Milky Way Behind Three Merlons
Explanation: To some, they look like battlements, here protecting us against the center of the Milky Way. The Three Merlons, also called the Three Peaks of Lavaredo, stand tall today because they are made of dense dolomite rock which has better resisted erosion than surrounding softer rock. They formed about 250 million years ago and so are comparable in age with one of the great extinctions of life on Earth. A leading hypothesis is that this great extinction was triggered by an asteroid about 10-km across, larger in size than Mount Everest, impacting the Earth. Humans have gazed up at the stars in the Milky Way and beyond for centuries, making these battlefield-like formations, based in the Sexten Dolomites, a popular place for current and ancient astronomers.

APOD: 2024 May 19 – Jupiter Diving
Explanation: Take this simulated plunge and dive into the upper atmosphere of Jupiter, the Solar System's ruling gas giant. The awesome animation is based on image data from JunoCam, and the microwave radiometer on board the Jupiter-orbiting Juno spacecraft. Your view will start about 3,000 kilometers above the southern Jovian cloud tops, and you can track your progress on the display at the left. As altitude decreases, temperature increases while you dive deeper at the location of Jupiter's famous Great Red Spot. In fact, Juno data indicates the Great Red Spot, the Solar System's largest storm system, penetrates some 300 kilometers into the giant planet's atmosphere. For comparison, the deepest point for planet Earth's oceans is just under 11 kilometers down. Don't worry though, you'll fly back out again.

APOD: 2024 April 19 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula is more modestly known as NGC 3372. One of our Galaxy's largest star forming regions, it spans over 300 light-years. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye. But at a distance of 7,500 light-years it lies some 5 times farther away. This stunning telescopic view reveals remarkable details of the region's glowing filaments of interstellar gas and obscuring cosmic dust clouds. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the bright star above the central dark notch in this field and left of the dusty Keyhole Nebula (NGC 3324).

APOD: 2023 October 6 - Edwin Hubble Discovers the Universe
Explanation: How big is our universe? This question, among others, was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and on October 6, 1923 wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. Annotated 100 years ago, the featured image may not be pretty, but the variable spot on it opened a window through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.

APOD: 2023 September 1 - The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shows itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core, upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core.

APOD: 2023 May 1 – Carina Nebula North
Explanation: The Great Carina Nebula is home to strange stars and iconic nebulas. Named for its home constellation, the huge star-forming region is larger and brighter than the Great Orion Nebula but less well known because it is so far south -- and because so much of humanity lives so far north. The featured image shows in great detail the northernmost part of the Carina Nebula. On the bottom left is the Gabriela Mistral Nebula consisting of an emission nebula of glowing gas (IC 2599) surrounding the small open cluster of stars (NGC 3324). Above the image center is the larger star cluster NGC 3293, while to its right is the emission nebula Loden 153. The most famous occupant of the Carina Nebula, however, is not shown. Off the image to the lower right is the bright, erratic, and doomed star known as Eta Carinae -- a star once one of the brightest stars in the sky and now predicted to explode in a supernova sometime in the next few million years.

APOD: 2023 March 22 – M31: The Andromeda Galaxy
Explanation: How far can you see? The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy, over two million light-years away. Without a telescope, even this immense spiral galaxy appears as an unremarkable, faint, nebulous cloud in the constellation Andromeda. But a bright white nucleus, dark winding dust lanes, luminous blue spiral arms, and bright red emission nebulas are recorded in this stunning fifteen-hour telescopic digital mosaic of our closest major galactic neighbor. But how do we know this spiral nebula is really so far away? This question was central to the famous Shapley-Curtis debate of 1920. M31's great distance was determined in the 1920s by observations that resolved individual stars that changed their brightness in a way that gave up their true distance. The result proved that Andromeda is just like our Milky Way Galaxy -- a conclusion making the rest of the universe much more vast than had ever been previously imagined.

APOD: 2022 December 6 - M16: A Star Forming Pillar from Webb
Explanation: What’s happening inside this interstellar mountain? Stars are forming. The mountain is actually a column of gas and dust in the picturesque Eagle Nebula (M16). A pillar like this is so low in density that you could easily fly though it -- it only appears solid because of its high dust content and great depth. The glowing areas are lit internally by newly formed stars. These areas shine in red and infrared light because blue light is scattered away by intervening interstellar dust. The featured image was captured recently in near-infrared light in unprecedented detail by the James Webb Space Telescope (JWST), launched late last year. Energetic light, abrasive winds, and final supernovas from these young stars will slowly destroy this stellar birth column over the next 100,000 years.

APOD: 2022 September 14 - Waves of the Great Lacerta Nebula
Explanation: It is one of the largest nebulas on the sky -- why isn't it better known? Roughly the same angular size as the Andromeda Galaxy, the Great Lacerta Nebula can be found toward the constellation of the Lizard (Lacerta). The emission nebula is difficult to see with wide-field binoculars because it is so faint, but also usually difficult to see with a large telescope because it is so great in angle -- spanning about three degrees. The depth, breadth, waves, and beauty of the nebula -- cataloged as Sharpless 126 (Sh2-126) -- can best be seen and appreciated with a long duration camera exposure. The featured image is one such combined exposure -- in this case 10 hours over five different colors and over six nights during this past June and July at the IC Astronomy Observatory in Spain. The hydrogen gas in the Great Lacerta Nebula glows red because it is excited by light from the bright star 10 Lacertae, one of the bright blue stars just above the red-glowing nebula's center. The stars and nebula are about 1,200 light years distant.

APOD: 2022 August 4 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shows itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core. Distant background galaxies in the medium-wide field of view include NGC 6207 at the upper left.

APOD: 2022 July 17 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 357 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. Forty-five years ago this September, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

APOD: 2022 April 25 - The Great Nebula in Carina
Explanation: In one of the brightest parts of Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebulas. The Keyhole Nebula (NGC 3324), the bright structure just below the image center, houses several of these massive stars. The entire Carina Nebula, captured here, spans over 300 light years and lies about 7,500 light-years away in the constellation of Carina. Eta Carinae, the most energetic star in the nebula, was one of the brightest stars in the sky in the 1830s, but then faded dramatically. While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that much of the Great Nebula in Carina has been a veritable supernova factory.

APOD: 2022 April 8 - Hale-Bopp: The Great Comet of 1997
Explanation: Only twenty-five years ago, Comet Hale-Bopp rounded the Sun and offered a dazzling spectacle in planet Earth's night skies. Digitized from the original astrophoto on 35mm color slide film, this classic image of the Great Comet of 1997 was recorded a few days after its perihelion passage on April 1, 1997. Made with a camera and telephoto lens piggy-backed on a small telescope, the 10 minute long, hand-guided exposure features the memorable tails of Hale-Bopp, a whitish dust tail and blue ion tail. Here, the ion tail extends well over ten degrees across the northern sky. In all, Hale-Bopp was reported as visible to the naked eye from late May 1996 through September 1997. Also known as C/1995 O1, Hale-Bopp is recognized as one of the most compositionally pristine comets to pass through the inner Solar System. A visitor from the distant Oort cloud, the comet's next perihelion passage should be around the year 4380 AD. Do you remember Hale-Bopp?

APOD: 2022 January 31 - Carina Nebula North
Explanation: The Great Carina Nebula is home to strange stars and iconic nebulas. Named for its home constellation, the huge star-forming region is larger and brighter than the Great Orion Nebula but less well known because it is so far south -- and because so much of humanity lives so far north. The featured image shows in great detail the northern-most part of the Carina Nebula. Visible nebulas include the semi-circular filaments surrounding the active star Wolf-Rayet 23 (WR23) on the far left. Just left of center is the Gabriela Mistral Nebula consisting of an emission nebula of glowing gas (IC 2599) surrounding the small open cluster of stars (NGC 3324). Above the image center is the larger star cluster NGC 3293, while to its right is the relatively faint emission nebula designated Loden 153. The most famous occupant of the Carina Nebula, however, is not shown. Off the image to the lower right is the bright, erratic, and doomed star star known as Eta Carinae -- a star once one of the brightest stars in the sky and now predicted to explode in a supernova sometime in the next few million years.

APOD: 2022 January 9 - Hubbles Jupiter and the Shrinking Great Red Spot
Explanation: What will become of Jupiter's Great Red Spot? Gas giant Jupiter is the solar system's largest world with about 320 times the mass of planet Earth. Jupiter is home to one of the largest and longest lasting storm systems known, the Great Red Spot (GRS), visible to the left. The GRS is so large it could swallow Earth, although it has been shrinking. Comparison with historical notes indicate that the storm spans only about one third of the exposed surface area it had 150 years ago. NASA's Outer Planets Atmospheres Legacy (OPAL) program has been monitoring the storm more recently using the Hubble Space Telescope. The featured Hubble OPAL image shows Jupiter as it appeared in 2016, processed in a way that makes red hues appear quite vibrant. Modern GRS data indicate that the storm continues to constrict its surface area, but is also becoming slightly taller, vertically. No one knows the future of the GRS, including the possibility that if the shrinking trend continues, the GRS might one day even do what smaller spots on Jupiter have done -- disappear completely.

APOD: 2021 November 26 - Great Refractor and Lunar Eclipse
Explanation: Rain clouds passed and the dome of the Lick Observatory's 36 inch Great Refractor opened on November 19. The historic telescope was pointed toward a partially eclipsed Moon. Illuminated by dim red lighting to preserve an astronomer's night vision, telescope controls, coordinate dials, and the refractor's 57 foot long barrel were captured in this high dynamic range image. Visible beyond the foreshortened barrel and dome slit, growing brighter after its almost total eclipse phase, the lunar disk created a colorful corona through lingering clouds. From the open dome, the view of the clearing sky above includes the Pleiades star cluster about 5 degrees from Moon and Earth's shadow.

APOD: 2021 November 4 - NGC 147 and NGC 185
Explanation: Dwarf galaxies NGC 147 (left) and NGC 185 stand side by side in this sharp telescopic portrait. The two are not-often-imaged satellites of M31, the great spiral Andromeda Galaxy, some 2.5 million light-years away. Their separation on the sky, less than one degree across a pretty field of view, translates to only about 35 thousand light-years at Andromeda's distance, but Andromeda itself is found well outside this frame. Brighter and more famous satellite galaxies of Andromeda, M32 and M110, are seen closer to the great spiral. NGC 147 and NGC 185 have been identified as binary galaxies, forming a gravitationally stable binary system. But recently discovered faint dwarf galaxy Cassiopeia II also seems to be part of their system, forming a gravitationally bound group within Andromeda's intriguing population of small satellite galaxies.

APOD: 2021 October 26 - Jupiter Rotates
Explanation: Observe the graceful twirl of our Solar System's largest planet. Many interesting features of Jupiter's enigmatic atmosphere, including dark belts and light zones, can be followed in detail. A careful inspection will reveal that different cloud layers rotate at slightly different speeds. The famous Great Red Spot is not visible at first -- but soon rotates into view. Other smaller storm systems occasionally appear. As large as Jupiter is, it rotates in only 10 hours. Our small Earth, by comparison, takes 24 hours to complete a spin cycle. The featured high-resolution time-lapse video was captured over five nights earlier this month by a mid-sized telescope on an apartment balcony in Paris, France. Since hydrogen and helium gas are colorless, and those elements compose most of Jupiter's expansive atmosphere, what trace elements create the observed colors of Jupiter's clouds remains a topic of research.

APOD: 2021 May 20 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core. Distant background galaxies in the medium-wide field of view include NGC 6207 at the lower right.

APOD: 2021 January 19 - A Lunar Corona with Jupiter and Saturn
Explanation: Why does a cloudy moon sometimes appear colorful? The effect, called a lunar corona, is created by the quantum mechanical diffraction of light around individual, similarly-sized water droplets in an intervening but mostly-transparent cloud. Since light of different colors has different wavelengths, each color diffracts differently. Lunar Coronae are one of the few quantum mechanical color effects that can be easily seen with the unaided eye. Solar coronae are also sometimes evident. The featured composite image was captured a few days before the close Great Conjunction between Saturn and Jupiter last month. In the foreground, the Italian village of Pieve di Cadore is visible in front of the Sfornioi Mountains.

APOD: 2020 December 30 - Jupiter and Saturn Great Conjunction: The Movie
Explanation: Yes, but have you seen a movie of Jupiter and Saturn's Great Conjunction? The featured time-lapse video was composed from a series of images taken from Thailand and shows the two giant planets as they angularly passed about a tenth of a degree from each other. The first Great Conjunction sequence shows a relative close up over five days with moons and cloud bands easily visible, followed by a second video sequence, zoomed out, over 9 days. Even though Jupiter and Saturn appeared to pass unusually close together on the sky on December 21, 2020, in actuality they were still nearly a billion kilometers apart. The two gas giants are destined for similar meet ups every 19.86 years. However, they had not come this close, angularly, for the past 397 years, and will not again for another 60 years. If you're willing to wait until the year 7541, though, you can see Jupiter pass directly in front of Saturn.

APOD: 2020 December 23 - Jupiter Meets Saturn: A Red Spotted Great Conjunction
Explanation: It was time for their close-up. Two days ago Jupiter and Saturn passed a tenth of a degree from each other in what is known a Great Conjunction. Although the two planets pass each other on the sky every 20 years, this was the closest pass in nearly four centuries. Taken early in day of the Great Conjunction, the featured multiple-exposure combination captures not only both giant planets in a single frame, but also Jupiter's four largest moons (left to right) Callisto, Ganymede, Io, and Europa -- and Saturn's largest moon Titan. If you look very closely, the clear Chilescope image even captures Jupiter's Great Red Spot. The now-separating planets can still be seen remarkably close -- within about a degree -- as they set just after the Sun, toward the west, each night for the remainder of the year.

APOD: 2020 December 20 - A Volcanic Great Conjunction
Explanation: Where can I see the Great Conjunction? Near where the Sun just set. Directionally, this close passing of Jupiter and Saturn will be toward the southwest. Since the planetary pair, the Sun, and the Earth are nearly in a geometric straight line, the planets will be seen to set just where the Sun had set -- from every location on Earth. When can I see the Great Conjunction? Just after sunset. Since the two planets are so near the Sun directionally, they always appear in the sky near the Sun, but can best be seen when the Earth blocks the Sun but not the planets: sunset. Soon thereafter, Jupiter and Saturn will also set, so don't be late! Is tomorrow night the only night that I can see the Great Conjunction? Tomorrow night the jovian giants will appear the closest, but on any night over the next few days they will appear unusually close. Technically, the closest pass happens on 21 December at 18:20 UTC. Will there be an erupting volcano on the horizon near the Great Conjunction? Yes, for example if you live in Guatemala where the featured image was taken. Otherwise, generally, no. In the featured image captured last week, Jupiter and Saturn are visible toward the right, just above a tree, and bathed in the diffuse glow of zodiacal light.

APOD: 2020 December 19 - Conjunction after Sunset
Explanation: How close will Jupiter and Saturn be at their Great Conjunction? Consider this beautiful triple conjunction of Moon, Jupiter and Saturn captured through clouds in the wintry twilight. The telephoto view looks toward the western horizon and the Alborz Mountains in Iran after sunset on December 17. The celestial gathering makes it easy to see Jupiter and fainter Saturn are separated on that date by roughly the diameter of the waxing crescent Moon. On the day of their Great Conjunction, solstice day December 21, Jupiter and Saturn may seem to nearly merge though. In their closest conjunction in 400 years they will be separated on the sky by only about 1/5 the apparent diameter of the Moon. By then the two largest worlds in the Solar System and their moons will be sharing the same field of view in telescopes around planet Earth.

APOD: 2020 December 15 - Great Conjunction: Saturn and Jupiter Converge
Explanation: It's happening. Saturn and Jupiter are moving closer and will soon appear in almost exactly the same direction. Coincidentally, on the night of the December solstice -- the longest night of the year in the north and the longest day in the south -- the long-awaited Great Conjunction will occur. Then, about six days from now, Saturn and Jupiter will be right next to each other -- as they are every 20 years. But this juxtaposition is not just any Great Conjunction -- it will be the closest since 1623 because the two planetary giants will pass only 1/10th of a degree from each other -- well less than the apparent diameter of a full moon. In the next few days a crescent moon will also pass a few degrees away from the converging planets and give a preliminary opportunity for iconic photos. The featured illustration shows the approach of Saturn and Jupiter during November and December over the French Alps.

APOD: 2020 December 8 - Great Conjunction over Sicilian Lighthouse
Explanation: Don’t miss the coming great conjunction. In just under two weeks, the two largest planets in our Solar System will angularly pass so close together in Earth's sky that the Moon would easily be able to cover them both simultaneously. This pending planetary passage -- on December 21 -- will be the closest since 1623. Jupiter and Saturn will remain noticeably bright and can already be seen together toward the southwest just after sunset. Soon after dusk is the best time to see them -- because they set below the horizon soon after. In mid-November, the Jovian giants were imaged together here about three degrees apart -- and slowly closing. The featured image, including a crescent moon, captured the dynamic duo beyond the Cape Murro di Porco Lighthouse in Syracuse, Sicily, Italy.

APOD: 2020 November 26 - The Great Turkey Nebula
Explanation: Surprisingly reminiscent of The Great Nebula in Orion, The Great Turkey Nebula spans this creative field of view. Of course if it were the Orion Nebula it would be our closest large stellar nursery, found at the edge of a large molecular cloud a mere 1,500 light-years away. Also known as M42, the Orion Nebula is visible to the eye as the middle "star" in the sword of Orion the Hunter, a constellation now rising in planet Earth's evening skies. Stellar winds from clusters of newborn stars scattered throughout the Orion Nebula sculpt its ridges and cavities seen in familiar in telescopic images. Much larger than any bird you might be cooking, this Great Turkey Nebula was imagined to be similar in size to the Orion Nebula, about 13 light-years across. Stay safe and well.

APOD: 2020 October 20 - Saturn and Jupiter over Italian Peaks
Explanation: Saturn and Jupiter are getting closer. Every night that you go out and check for the next two months, these two bright planets will be even closer together on the sky. Finally, in mid-December, a Great Conjunction will occur -- when the two planets will appear only 0.1 degrees apart -- just one fifth the angular diameter of the full Moon. And this isn't just any Great Conjunction -- Saturn (left) and Jupiter (right) haven't been this close since 1623, and won't be nearly this close again until 2080. This celestial event is quite easy to see -- already the two planets are easily visible toward the southwest just after sunset -- and already they are remarkably close. Pictured, the astrophotographer and partner eyed the planetary duo above the Tre Cime di Lavaredo (Three Peaks of Lavaredo) in the Italian Alps about two weeks ago.

APOD: 2020 October 19 - A Flight over Jupiter Near the Great Red Spot
Explanation: Are you willing to wait to see the largest and oldest known storm system in the Solar System? In the featured video, Jupiter's Great Red Spot finally makes its appearance 2 minutes and 12 seconds into the 5-minute video. Before it arrives, you may find it pleasing to enjoy the continually changing view of the seemingly serene clouds of Jupiter, possibly with your lights low and sound up. The 41 frames that compose the video were captured in June as the robotic Juno spacecraft was making a close pass over our Solar System's largest planet. The time-lapse sequence actually occurred over four hours. Since arriving at Jupiter in 2016, Juno's numerous discoveries have included unexpectedly deep atmospheric jet streams, the most powerful auroras ever recorded, and water-bearing clouds bunched near Jupiter's equator.

APOD: 2020 July 29 - The Giants of Summer
Explanation: As Comet NEOWISE sweeps through northern summer skies, Jupiter and Saturn are shining brightly, near opposition. With Jupiter opposite the Sun on July 14 and Saturn on July 21, the giant planets are still near their closest to planet Earth in 2020. Sharing the constellation Sagittarius they are up all night, and offer their best and brightest views at the telescope. Both captured on July 22 from a balcony in Paris these two sharp telescopic images don't disappoint, showing off what the giant planets are famous for, Saturn's bright rings and Jupiter's Great Red Spot. These giants of the Solar System are worth following during 2020. On December 21, skygazers can watch the once-in-20-year great conjunction of Jupiter and Saturn.

APOD: 2020 July 6 - M43: Dust, Gas, and Stars in the Orion Nebula
Explanation: Unspeakable beauty and unimaginable bedlam can be found together in the Orion Nebula Arguably the most famous of all astronomy nebulas, the Great Nebula in Orion is an immense interstellar molecular cloud only 1500 light-years away. In the featured deep image shown in assigned colors, the part of the nebula's center known as M43 is shown as taken by the Hubble Space Telescope. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. The entire Orion Nebula, including both M42 and M43 spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2020 June 28 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 355 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. About 43 years ago, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

APOD: 2020 May 1 - A View Toward M106
Explanation: Big, bright, beautiful spiral, Messier 106 dominates this cosmic vista. The nearly two degree wide telescopic field of view looks toward the well-trained constellation Canes Venatici, near the handle of the Big Dipper. Also known as NGC 4258, M106 is about 80,000 light-years across and 23.5 million light-years away, the largest member of the Canes II galaxy group. For a far far away galaxy, the distance to M106 is well-known in part because it can be directly measured by tracking this galaxy's remarkable maser, or microwave laser emission. Very rare but naturally occurring, the maser emission is produced by water molecules in molecular clouds orbiting its active galactic nucleus. Another prominent spiral galaxy on the scene, viewed nearly edge-on, is NGC 4217 below and right of M106. The distance to NGC 4217 is much less well-known, estimated to be about 60 million light-years, but the bright spiky stars are in the foreground, well inside our own Milky Way galaxy. Even the existence of galaxies beyond the Milky Way was questioned 100 years ago in astronomy's Great Debate.

APOD: 2020 April 30 - Andromeda Island Universe
Explanation: The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy some two and a half million light-years away. But without a telescope, even this immense spiral galaxy - spanning over 200,000 light years - appears as a faint, nebulous cloud in the constellation Andromeda. In contrast, a bright yellow nucleus, dark winding dust lanes, expansive blue spiral arms and star clusters are recorded in this stunning telescopic image. While even casual skygazers are now inspired by the knowledge that there are many distant galaxies like M31, astronomers debated this fundamental concept 100 years ago. Were these "spiral nebulae" simply outlying components of our own Milky Way Galaxy or were they instead "island universes", distant systems of stars comparable to the Milky Way itself? This question was central to the famous Shapley-Curtis debate of 1920, which was later resolved by observations of M31 in favor of Andromeda, island universe.

APOD: 2020 April 26 - Edwin Hubble Discovers the Universe
Explanation: How big is our universe? This very question, among others, was debated by two leading astronomers 100 years ago today in what has become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and so wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. The featured image may not be pretty, but the variable spot on it opened a door through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.

APOD: 2020 March 19 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core and reveals three subtle dark lanes forming the apparent shape of a propeller just below and slightly left of center. Distant background galaxies in the medium-wide field of view include NGC 6207 at the upper left.

APOD: 2020 February 15 - Carina Nebula Close Up
Explanation: A jewel of the southern sky, the Great Carina Nebula, also known as NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This gorgeous telescopic close-up reveals remarkable details of the region's central glowing filaments of interstellar gas and obscuring cosmic dust clouds in a field of view nearly 20 light-years across. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic and violently variable Eta Carinae, a star system with well over 100 times the mass of the Sun. In the processed composite of space and ground-based image data a dusty, two-lobed Homunculus Nebula appears to surround Eta Carinae itself just below and left of center. While Eta Carinae is likely on the verge of a supernova explosion, X-ray images indicate that the Great Carina Nebula has been a veritable supernova factory.

APOD: 2020 January 3 - Quadrantids over the Great Wall
Explanation: Named for a forgotten constellation, the Quadrantid Meteor Shower is an annual event for planet Earth's northern hemisphere skygazers The shower's radiant on the sky lies within the old, astronomically obsolete constellation Quadrans Muralis. That location is not far from the Big Dipper, at the boundaries of the modern constellations Bootes and Draco. With the radiant out of the frame at the upper right, Quadrantid meteors streak through this night skyscape composed of digital frames recorded in the hours around the shower's peak on January 4, 2013. The last quarter moon illuminates rugged terrain and a section of the Great Wall in Hebei Province, China. A likely source of the dust stream that produces Quadrantid meteors was identified in 2003 as an asteroid. As usual, in 2020 the shower is expected to peak briefly on the night of January 3/4. Meteor fans in North America can anticipate a good show to celebrate the new year in moonless skies before tomorrow's dawn.

APOD: 2019 October 30 - M42: Inside the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the featured deep image in assigned colors highlighted by emission in oxygen and hydrogen, wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2019 July 17 - Apollo 11: Descent to the Moon
Explanation: It had never been done before. But with the words "You're Go for landing", 50 years ago this Saturday, Apollo 11 astronauts Aldrin and Armstrong were cleared to make the first try. The next few minutes would contain more than a bit of drama, as an unexpected boulder field and an unacceptably sloping crater loomed below. With fuel dwindling, Armstrong coolly rocketed the lander above the lunar surface as he looked for a clear and flat place to land. With only seconds of fuel remaining, and with the help of Aldrin and mission control calling out data, Armstrong finally found a safe spot -- and put the Eagle down. Many people on Earth listening to the live audio felt great relief on hearing "The Eagle has landed", and great pride knowing that for the first time ever, human beings were on the Moon. Combined in the featured descent video are two audio feeds, a video feed similar to what the astronauts saw, captions of the dialog, and data including the tilt of the Eagle lander. The video concludes with the panorama of the lunar landscape visible outside the Eagle. A few hours later, hundreds of millions of people across planet Earth, drawn together as a single species, watched fellow humans walk on the Moon.

APOD: 2019 May 8 - Jupiter Marble from Juno
Explanation: What does Jupiter look like up close? Most images of Jupiter are taken from far away, either from Earth or from a great enough distance that nearly half the planet is visible. This shot, though, was composed from images taken relatively close in, where less than half of the planet was visible. From here, Jupiter still appears spherical but perspective distortion now makes it look more like a marble. Visible on Jupiter's cloud tops are a prominent dark horizontal belt containing a white oval cloud, and a white zone cloud, both of which circle the planet. The Great Red Spot looms on the upper right. The featured image was taken by the robotic Juno spacecraft in February during its 17th close pass of our Solar System's largest planet. Juno's mission, now extended into 2021, is to study Jupiter in new ways. Juno's data has already enabled discoveries that include Jupiter's magnetic field being surprisingly lumpy, and that some of Jupiter's cloud systems run about 3,000 kilometers into the planet.

APOD: 2019 May 7 - The Great Nebula in Carina
Explanation: What's happening in the center of the Carina Nebula? Stars are forming, dying, and leaving an impressive tapestry of dark dusty filaments. The entire Carina Nebula, cataloged as NGC 3372, spans over 300 light years and lies about 8,500 light-years away in the constellation of Carina. The nebula is composed predominantly of hydrogen gas, which emits the pervasive red glow seen in this highly detailed featured image. The blue glow in the center is created by a trace amount of glowing oxygen. Young and massive stars located in the nebula's center expel dust when they explode in supernovae. Eta Carinae, the most energetic star in the nebula's center, was one of the brightest stars in the sky in the 1830s, but then faded dramatically.

APOD: 2019 April 21 - Spiral Aurora over Icelandic Divide
Explanation: Admire the beauty but fear the beast. The beauty is the aurora overhead, here taking the form of great green spiral, seen between picturesque clouds with the bright Moon to the side and stars in the background. The beast is the wave of charged particles that creates the aurora but might, one day, impair civilization. In 1859, following notable auroras seen all across the globe, a pulse of charged particles from a coronal mass ejection (CME) associated with a solar flare impacted Earth's magnetosphere so forcefully that they created the Carrington Event. A relatively direct path between the Sun and the Earth might have been cleared by a preceding CME. What is sure is that the Carrington Event compressed the Earth's magnetic field so violently that currents were created in telegraph wires so great that many wires sparked and gave telegraph operators shocks. Were a Carrington-class event to impact the Earth today, speculation holds that damage might occur to global power grids and electronics on a scale never yet experienced. The featured aurora was imaged in 2016 over Thingvallavatn Lake in Iceland, a lake that partly fills a fault that divides Earth's large Eurasian and North American tectonic plates.

APOD: 2019 January 30 - Wide Field View of Great American Eclipse
Explanation: Only in the fleeting darkness of a total solar eclipse is the light of the solar corona easily visible. Normally overwhelmed by the bright solar disk, the expansive corona, the sun's outer atmosphere, is an alluring sight. But the subtle details and extreme ranges in the corona's brightness, although discernible to the eye, are notoriously difficult to photograph. Pictured here, however, using over 120 images and meticulous digital processing, is a detailed wide-angle image of the Sun's corona taken during the Great American Eclipse in 2017 August. Clearly visible are intricate layers and glowing caustics of an ever changing mixture of hot gas and magnetic fields. Hundreds of stars as faint as 11th magnitude are visible behind the Moon and Sun, with Mars appearing in red on the far right. The next total eclipse of the Sun will occur on July 2 and be visible during sunset from a thin swath across Chile and Argentina.

APOD: 2018 December 27 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, also known as NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This gorgeous telescopic close-up reveals remarkable details of the region's central glowing filaments of interstellar gas and obscuring cosmic dust clouds. The field of view is over 50 light-years across. The Carina Nebula is home to young, extremely massive stars, including the stars of open cluster Trumpler 14 (above and left of center) and the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the brightest star, centered here just below the dusty Keyhole Nebula (NGC 3324). While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that the Great Carina Nebula has been a veritable supernova factory.

APOD: 2018 May 20 - In the Heart of the Tarantula Nebula
Explanation: In the heart of monstrous Tarantula Nebula lies huge bubbles of energetic gas, long filaments of dark dust, and unusually massive stars. In the center of this heart, is a knot of stars so dense that it was once thought to be a single star. This star cluster, labeled as R136 or NGC 2070, is visible just above the center of the featured image and home to a great number of hot young stars. The energetic light from these stars continually ionizes nebula gas, while their energetic particle wind blows bubbles and defines intricate filaments. The representative-color picture, a digital synthesis of images from the NASA/ESA orbiting Hubble Space Telescope and ESO's ground-based New Technology Telescope, shows great details of the LMC nebula's tumultuous center. The Tarantula Nebula, also known as the 30 Doradus nebula, is one of the largest star-formation regions known, and has been creating unusually strong episodes of star formation every few million years.

APOD: 2018 April 30 - Total Solar Eclipse Corona in HDR
Explanation: How great was the Great American Eclipse? The featured HDR image shows it to be perhaps greater than we knew. On August 21 of last year, the Moon blocked the Sun for a few minutes along a narrow path across the USA. Although one of the most photographed events in human history, this image -- only recently completed after an extraordinary amount of digital processing -- shows one of the most detailed depictions of a solar corona ever taken. Composed of extremely hot gas, the solar corona is only visible to the unaided eye during a total solar eclipse. The featured image combined over 70 images of different time exposures. The series of complementary HDR images recovered enough detail to see motion of the solar corona. The images were taken in Unity, Oregon in the morning to get steady atmospheric seeing conditions. The next total solar eclipse visible on Earth will be in 2019 July, while the next one visible across North America and the USA will occur in 2024 April.

APOD: 2018 April 25 - Hubble's Jupiter and the Shrinking Great Red Spot
Explanation: What will become of Jupiter's Great Red Spot? Gas giant Jupiter is the solar system's largest world with about 320 times the mass of planet Earth. Jupiter is home to one of the largest and longest lasting storm systems known, the Great Red Spot (GRS), visible to the left. The GRS is so large it could swallow Earth, although it has been shrinking. Comparison with historical notes indicate that the storm spans only about one third of the surface area it had 150 years ago. NASA's Outer Planets Atmospheres Legacy (OPAL) program has been monitoring the storm more recently using the Hubble Space Telescope. The featured Hubble OPAL image shows Jupiter as it appeared in 2016, processed in a way that makes red hues appear quite vibrant. Modern GRS data indicate that the storm continues to constrict its surface area, but is also becoming slightly taller, vertically. No one knows the future of the GRS, including the possibility that if the shrinking trend continues, the GRS might one day even do what smaller spots on Jupiter have done -- disappear completely.

APOD: 2018 February 21 - Jupiter in Infrared from Hubble
Explanation: Jupiter looks a bit different in infrared light. To better understand Jupiter's cloud motions and to help NASA's robotic Juno spacecraft understand the planetary context of the small fields that it sees, the Hubble Space Telescope is being directed to regularly image the entire Jovian giant. The colors of Jupiter being monitored go beyond the normal human visual range to include both ultraviolet and infrared light. Featured here in 2016, three bands of near-infrared light have been digitally reassigned into a mapped color image. Jupiter appears different in infrared partly because the amount of sunlight reflected back is distinct, giving differing cloud heights and latitudes discrepant brightnesses. Nevertheless, many familiar features on Jupiter remain, including the light zones and dark belts that circle the planet near the equator, the Great Red Spot on the lower left, and the string-of-pearls storm systems south of the Great Red Spot. The poles glow because high altitude haze there is energized by charged particles from Jupiter's magnetosphere. Juno has now completed 10 of 12 planned science orbits of Jupiter and continues to record data that are helping humanity to understand not only Jupiter's weather but what lies beneath Jupiter's thick clouds.

APOD: 2017 December 14 - Jupiter Diving
Explanation: Take this simulated plunge and dive into the upper atmosphere of Jupiter, the Solar System's ruling gas giant. The awesome animation is based on image data from JunoCam, and the microwave radiometer on board the Jupiter-orbiting Juno spacecraft. Your view will start about 3,000 kilometers above the southern Jovian cloud tops, but you can track your progress on the display at the left. As altitude decreases, temperature increases while you dive deeper at the location of Jupiter's famous Great Red Spot. In fact, Juno data indicates the Great Red Spot, the Solar System's largest storm system, penetrates some 300 kilometers into the giant planet's atmosphere. For comparison, the deepest point for planet Earth's oceans is just under 11 kilometers down. Don't panic though, you'll fly back out again.

APOD: 2017 November 29 - M42: The Great Orion Nebula
Explanation: Few astronomical sights excite the imagination like the nearby stellar nursery known as the Orion Nebula. The Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud. Many of the filamentary structures visible in the featured image are actually shock waves - fronts where fast moving material encounters slow moving gas. The Orion Nebula spans about 40 light years and is located about 1500 light years away in the same spiral arm of our Galaxy as the Sun. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. The featured image, taken last month, shows a two-hour exposure of the nebula in three colors. The whole Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years.

APOD: 2017 September 8 - The Great Gig in the Sky
Explanation: There were no crowds on the beach at Phillips Lake, Oregon on August 21. But a few had come there to stand, for a moment, in the dark shadow of the Moon. From the beach, this unscripted mosaic photo records their much anticipated solar eclipse. In two vertical panels it catches the last few seconds of totality and the first instant of 3rd contact, just as the eclipse ends and sunlight faintly returns. Across the US those gathered along the path of totality also took pictures and shared their moment. And like those at Phillips Lake they may treasure the experience more than any planned or unplanned photograph of the total eclipse of the Sun.

APOD: 2017 September 5 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 352 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. Forty years ago today, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

APOD: 2017 September 1 - A First Glimpse of the Great American Eclipse
Explanation: Making landfall in Oregon, the Moon's dark umbral shadow toured the United States on August 21. Those gathered along its coast to coast path were witness to a total eclipse of the Sun, possibly the most widely shared celestial event in history. But first, the Moon's shadow touched the northern Pacific and raced eastward toward land. This dramatic snapshot was taken while crossing the shadow path 250 miles off the Oregon coast, 45,000 feet above the cloudy northern Pacific. Though from a shorter totality, it captures the eclipse before it could be seen from the US mainland. With the eclipsed Sun not far above, beautiful colors appear along the eastern horizon giving way to a clear, pitch-black, stratospheric sky in the shadow of the Moon.

APOD: 2017 July 15 - Close up of the Great Red Spot
Explanation: On July 11, the Juno spacecraft once again swung near to Jupiter's turbulent cloud tops in its looping 53 day orbit around the Solar System's ruling gas giant. About 11 minutes after perijove 7, its closest approach on this orbit, it passed directly above Jupiter's Great Red Spot. During the much anticipated fly over, it captured this close-up image data from a distance of less than 10,000 kilometers. The raw JunoCam data was subsequently processed by citizen scientists. Very long-lived but found to be shrinking, the Solar System's largest storm system was measure to be 16,350 kilometers wide on April 15. That's about 1.3 times the diameter of planet Earth.

APOD: 2017 July 3 - The Summer Triangle over the Great Wall
Explanation: Have you ever seen the Summer Triangle? The bright stars Vega, Deneb, and Altair form a large triangle on the sky that can be seen rising in the northern spring during the morning, and rising in the northern fall during the evening. During summer months, the triangle can be found nearly overhead near midnight as three of the brightest stars on the sky. Featured here, along with the arch of the central band of our Milky Way Galaxy, the Summer Triangle asterism was captured this spring over the Great Wall of China. This part of the Great Wall, a World Culture Heritage Site, was built during the 6th century on the Yan Mountains. At the summit is Wangjinglou Tower from which, on a clear night, the lights of Beijing are visible in the distance.

APOD: 2017 June 13 - The Great Nebula in Carina
Explanation: In one of the brightest parts of Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebulas. The Keyhole Nebula (NGC 3324), the bright structure just to the right of the image center, houses several of these massive stars and has itself changed its appearance. The entire Carina Nebula, captured here, spans over 300 light years and lies about 7,500 light-years away in the constellation of Carina. Eta Carinae, the most energetic star in the nebula, was one of the brightest stars in the sky in the 1830s, but then faded dramatically. While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that much of the Great Nebula in Carina has been a veritable supernova factory.

APOD: 2017 May 23 - Approaching Jupiter
Explanation: What would it look like to approach Jupiter? To help answer this, a team of 91 amateur astrophotographers took over 1,000 pictures of Jupiter from the Earth with the resulting images aligned and digitally merged into the featured time-lapse video. Image taking began in 2014 December and lasted just over three months. The resulting fictitious approach sequence has similarities to what was seen by NASA's robotic Juno spacecraft as it first approached the Jovian world last July. The video begins with Jupiter appearing as a small orb near the image center. As Jupiter nears from below, the planet looms ever larger while the rotation of its cloud bands becomes apparent. Jupiter's shrinking Great Red Spot rotates into view twice, at times showing unusual activity. Many white ovals are visible moving around the giant planet. The video ends as the imaginary spacecraft passes over Jupiter's North Pole.

APOD: 2017 May 12 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

APOD: 2017 March 8 - Dust, Gas, and Stars in the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, filaments of dark dust and glowing gas surround hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the featured deep image shown in assigned colors, part of the nebula's center is shown as taken by the Hubble Space Telescope. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42 and M43, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2017 February 28 - A White Oval Cloud on Jupiter from Juno
Explanation: This storm cloud on Jupiter is almost as large as the Earth. Known as a white oval, the swirling cloud is a high pressure system equivalent to an Earthly anticyclone. The cloud is one of a "string of pearls" ovals south of Jupiter's famous Great Red Spot. Possibly, the Great Red Spot is just a really large white oval that turned red. Surrounding clouds show interesting turbulence as they flow around and past the oval. The featured image was captured on February 2 as NASA's robotic spacecraft Juno made a new pass just above the cloud tops of the Jovian world. Over the next few years, Juno will continue to orbit and probe Jupiter, determine atmospheric water abundance, and attempt to determine if Jupiter has a solid surface beneath its thick clouds.

APOD: 2017 February 5 - Odysseus Crater on Tethys
Explanation: Some moons wouldn't survive the collision. Tethys, one of Saturn's larger moons at about 1000 kilometers in diameter, survived the collision, but today exhibits the resulting expansive impact crater Odysseus. Sometimes called the Great Basin, Odysseus occurs on the leading hemisphere of Tethys and shows its great age by the relative amount of smaller craters that occur inside its towering walls. The density of Tethys is similar to water-ice. The featured image was captured in November by the robotic Cassini spacecraft in orbit around Saturn as it swooped past the giant ice ball. Cassini has now started on its Grand Finale Tour which will take it inside Saturn's rings and culminate in September with a dive into Saturn's thick atmosphere.

APOD: 2016 December 7 - NGC 4696: Filaments around a Black Hole
Explanation: What's happening at the center of elliptical galaxy NGC 4696? There, long tendrils of gas and dust have been imaged in great detail as shown by this recently released image from the Hubble Space Telescope. These filaments appear to connect to the central region of the galaxy, a region thought occupied by a supermassive black hole. Speculation holds that this black hole pumps out energy that heats surrounding gas, pushes out cooler filaments of gas and dust, and shuts down star formation. Balanced by magnetic fields, these filaments then appear to spiral back in toward and eventually circle the central black hole. NGC 4696 is the largest galaxy in the Centaurus Cluster of Galaxies, located about 150 million light years from Earth. The featured image shows a region about 45,000 light years across.

APOD: 2016 November 10 - Great Rift Near the Center of the Milky Way
Explanation: Over 100 telescopic image panels in this stunning vertical mosaic span about 50 degrees across the night sky. They follow part of the Great Rift, the dark river of dust and molecular gas that stretches along the plane of our Milky Way Galaxy. Start at top center and you can follow the galactic equator down through brighter stars in constellations Aquila, Serpens Cauda, and Scutum. At the bottom is Sagittarius near the center of the Milky Way. Along the way you'll encounter many obscuring dark nebulae hundreds of light-years distant flanked by bands of Milky Way starlight, and the telltale reddish glow of starforming regions. Notable Messier objects include The Eagle (M16) and Omega (M17) nebulae, the Sagittarius Star Cloud (M24), the beautiful Trifid (M20) and the deep Lagoon (M8).

APOD: 2016 August 30 - Aurora over Icelandic Fault
Explanation: Admire the beauty but fear the beast. The beauty is the aurora overhead, here taking the form of great green spiral, seen between picturesque clouds with the bright Moon to the side and stars in the background. The beast is the wave of charged particles that creates the aurora but might, one day, impair civilization. Exactly this week in 1859, following notable auroras seen all across the globe, a pulse of charged particles from a coronal mass ejection (CME) associated with a solar flare impacted Earth's magnetosphere so forcefully that they created the Carrington Event. A relatively direct path between the Sun and the Earth might have been cleared by a preceding CME. What is sure is that the Carrington Event compressed the Earth's magnetic field so violently that currents were created in telegraph wires so great that many wires sparked and gave telegraph operators shocks. Were a Carrington-class event to impact the Earth today, speculation holds that damage might occur to global power grids and electronics on a scale never yet experienced. The featured aurora was imaged last week over Thingvallavatn Lake in Iceland, a lake that partly fills a fault that divides Earth's large Eurasian and North American tectonic plates.

APOD: 2016 July 27 - M13: A Great Globular Cluster of Stars
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The featured image in HDR, taken through a small telescope, spans an angular size just larger than a full Moon, whereas the inset image, taken by Hubble Space Telescope, zooms in on the central 0.04 degrees.

APOD: 2016 May 27 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, also known as NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This gorgeous telescopic close-up reveals remarkable details of the region's central glowing filaments of interstellar gas and obscuring cosmic dust clouds. The field of view is over 50 light-years across. The Carina Nebula is home to young, extremely massive stars, including the stars of open cluster Trumpler 14 (below and right of center) and the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the brightest star, seen here just above the dusty Keyhole Nebula (NGC 3324). While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that the Great Carina Nebula has been a veritable supernova factory.

APOD: 2016 May 22 - LL Orionis: When Cosmic Winds Collide
Explanation: What created this great arc in space? This arcing, graceful structure is actually a bow shock about half a light-year across, created as the wind from young star LL Orionis collides with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the lower right hand edge of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The complex stellar nursery in Orion shows a myriad of similar fluid shapes associated with star formation, including the bow shock surrounding a faint star at the upper right. Part of a mosaic covering the Great Nebula in Orion, this composite color image was recorded in 1995 by the Hubble Space Telescope.

APOD: 2016 May 1 - Contemplating the Sun
Explanation: Have you contemplated your home star recently? Featured here, a Sun partially eclipsed on the top left by the Moon is also seen eclipsed by earthlings contemplating the eclipse below. The spectacular menagerie of silhouettes was taken in 2012 from the Glen Canyon National Recreation Area near Page, Arizona, USA, where park rangers and astronomers expounded on the unusual event to interested gatherers. Also faintly visible on the Sun's disk, just to the lower right of the dark Moon's disk, is a group of sunspots. Although a partial solar eclipse by the Moon is indeed a good chance to contemplate the Sun, a great chance -- and one that is significantly more rare -- will occur next week when the Sun undergoes a partial eclipse by the planet Mercury.

APOD: 2016 March 23 - The Great Nebula in Carina
Explanation: In one of the brightest parts of Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebulas. The Keyhole Nebula (NGC 3324), the bright structure just above the image center, houses several of these massive stars and has itself changed its appearance. The entire Carina Nebula spans over 300 light years and lies about 7,500 light-years away in the constellation of Carina. Eta Carinae, the most energetic star in the nebula, was one of the brightest stars in the sky in the 1830s, but then faded dramatically. Eta Carinae is the brightest star near the image center, just left of the Keyhole Nebula. While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that much of the Great Carina Nebula has been a veritable supernova factory.

APOD: 2016 March 1 - NGC 3310: A Starburst Spiral Galaxy
Explanation: The party is still going on in spiral galaxy NGC 3310. Roughly 100 million years ago, NGC 3310 likely collided with a smaller galaxy causing the large spiral galaxy to light up with a tremendous burst of star formation. The changing gravity during the collision created density waves that compressed existing clouds of gas and triggered the star-forming party. The featured image from the Gemini North Telescope shows the galaxy in great detail, color-coded so that pink highlights gas while white and blue highlight stars. Some of the star clusters in the galaxy are quite young, indicating that starburst galaxies may remain in star-burst mode for quite some time. NGC 3310 spans about 50,000 light years, lies about 50 million light years away, and is visible with a small telescope towards the constellation of Ursa Major.

APOD: 2015 November 8 - A Quadruple Sky Over Great Salt Lake
Explanation: This was a sky to show the kids. All in all, three children, three planets, the Moon, a star, an airplane and a mom were all captured in one image near Great Salt Lake in Utah, USA in early September of 2005. Minus the airplane and the quadruple on the ground, this busy quadruple coincidence sky was visible last week all over the world. The easiest object to spot is the crescent Moon, which is easily the brightest sky orb in the featured image. Venus is the highest planet in the sky, with Jupiter to its right. The bright star Spica completes the quadruple just below Venus. The streak on the far right is an airplane. Mom is seated. Grandpa, appreciating the beauty of the moment, took the picture. This week, the pre-dawn sky shows a similar conjunction of planets.

APOD: 2015 November 4 - The Great Orion Nebula M42
Explanation: The Great Nebula in Orion, also known as M42, is one of the most famous nebulae in the sky. The star forming region's glowing gas clouds and hot young stars are on the right in this sharp and colorful image that includes the bluish reflection nebulae NGC 1977 and friends on the left. Located at the edge of an otherwise invisible giant molecular cloud complex, these eye-catching nebulas represent only a small fraction of this galactic neighborhood's wealth of interstellar material. Within the well-studied stellar nursery, astronomers have also identified what appear to be numerous infant planetary systems. The gorgeous skyscape spans nearly two degrees or about 45 light-years at the Orion Nebula's estimated distance of 1,500 light-years.

APOD: 2015 October 24 - Jupiter in 2015
Explanation: Two remarkable global maps of Jupiter's banded cloud tops can be compared by just sliding your cursor over this sharp projection (or follow this link) of image data from the Hubble Space Telescope. Both captured on January 19, during back-to-back 10 hour rotations of the ruling gas giant, the all-planet projections represent the first in a series of planned annual portraits by the Outer Planet Atmospheres Legacy program. Comparing the two highlights cloud movements and measures wind speeds in the planet's dynamic atmosphere. In fact, the Great Red Spot, the famous long-lived swirling storm boasting 300 mile per hour winds, is seen sporting a rotating, twisting filament. The images confirm that Great Red Spot is still shrinking, though still larger than planet Earth. Posing next to it (lower right) is Oval BA, also known as Red Spot Junior.

APOD: 2015 July 10 - Messier 43
Explanation: Often imaged but rarely mentioned, Messier 43 is a large star forming region in its own right. It's just part of the star forming complex of gas and dust that includes the larger, more famous neighboring Messier 42, the Great Orion Nebula. In fact, the Great Orion Nebula itself lies off the lower edge of this scene. The close-up of Messier 43 was made while testing the capabilities of a near-infrared instrument with one of the twin 6.5 meter Magellan telescopes at Las Campanas Observatory in the Chilean Andes. The composite image shifts the otherwise invisible infrared wavelengths to blue, green, and red colors. Peering into caverns of interstellar dust hidden from visible light, the near-infrared view can also be used to study cool, brown dwarf stars in the complex region. Along with its celebrity neighbor, Messier 43 lies about 1,500 light-years away, at the edge of Orion's giant molecular cloud. At that distance, this field of view spans about 5 light-years.

APOD: 2015 May 15 - Jupiter, Ganymede, Great Red Spot
Explanation: In this sharp snapshot, the Solar System's largest moon Ganymede poses next to Jupiter, the largest planet. Captured on March 10 with a small telescope from our fair planet Earth, the scene also includes Jupiter's Great Red Spot, the Solar System's largest storm. In fact, Ganymede is about 5,260 kilometers in diameter. That beats out all three of its other fellow Galilean satellites, along with Saturn's Moon Titan at 5,150 kilometers and Earth's own Moon at 3,480 kilometers. Though its been shrinking lately, the Great Red Spot's diameter is still around 16,500 kilometers. Jupiter, the Solar System's ruling gas giant, is about 143,000 kilometers in diameter at its equator. That's nearly 10 percent the diameter of the Sun.

APOD: 2015 April 18 - The Great Crater Hokusai
Explanation: One of the largest young craters on Mercury, 114 kilometer (71 mile) diameter Hokusai crater's bright rays are known to extend across much of the planet. But this mosaic of oblique views focuses on Hokusai close up, its sunlit central peaks, terraced crater walls, and frozen sea of impact melt on the crater's floor. The images were captured by the MESSENGER spacecraft. The first to orbit Mercury, since 2011 MESSENGER has conducted scientific explorations, including extensive imaging of the Solar System's innermost planet. Now running out of propellant and unable to counter orbital perturbations caused by the Sun's gravity, MESSENGER is predicted to impact the surface of Mercury on April 30.

APOD: 2015 March 13 - The Great Wall by Moonlight
Explanation: Last Friday, an almost Full Moon rose as the Sun set, over this mountainous landscape north of Beijing, China. Also near apogee, the farthest point in its elliptical orbit around planet Earth, it was this year's smallest and faintest Full Moon. The Jiankou section of the Great Wall of China meanders through the scene, the ancient Great Wall itself the subject of an older-than-the-space-age myth that it would be visible to the eye when standing on the lunar surface. But even from low Earth orbit, the large scale artifact of human civilization is very difficult to identify. At its farthest from our fair planet, the Moon shines brightly in the twilight sky though, posing in the faint, pinkish band known as the antitwilight arch or the belt of Venus.

APOD: 2014 May 18 - Jupiters Great Red Spot from Voyager 1
Explanation: What will become of Jupiter's Great Red Spot? Recorded as shrinking since the 1930s, the rate of the Great Red Spot's size appears to have accelerated just in the past few years. A hurricane larger than Earth, the Great Red Spot has been raging at least as long as telescopes could see it. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Although small eddies that feed into the storm system seem to play a role, a more full understanding of the gigantic storm cloud remains a topic of continued research, and may result in a better understanding of weather here on Earth. The above image is a digital enhancement of an image of Jupiter taken in 1979 by the Voyager 1 spacecraft as it zoomed by the Solar System's largest planet. NASA's Juno spacecraft is currently heading toward Jupiter and will arrive in 2016.

APOD: 2014 May 17 - Hubble's Jupiter and Shrinking Great Red Spot
Explanation: Gas giant Jupiter is the solar system's largest world with about 320 times the mass of planet Earth. It's also known for a giant swirling storm system, the Great Red Spot, featured in this sharp Hubble image from April 21. Nestled between Jupiter-girdling cloud bands, the Great Red Spot itself could still easily swallow Earth, but lately it has been shrinking. The most recent Hubble observations measure the spot to be about 10,250 miles (16,500 kilometers) across. That's the smallest ever measured by Hubble and particularly dramatic when compared to 14,500 miles measured by the Voyager 1 and 2 flybys in 1979, and historic telescopic observations from the 1800s indicating a width of about 25,500 miles on its long axis. Current indications are that the rate of shrinking is increasing for the long-lived Great Red Spot.

APOD: 2014 April 8 - M42: Inside the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image composite in assigned colors taken by the Hubble Space Telescope wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2014 February 20 - Comet Lovejoy over The Great Wall
Explanation: Fading now as it returns to the outer solar system Comet Lovejoy (C/2013 R1) still graces planet Earth's sky, a delicate apparition in binoculars or small telescopes. The comet, a relic of the solar system's formative years, is seen here rising in the morning twilight on January 12 among the stars of Ophiuchus, the Serpent Bearer. Posing near the comet is bright star Alpha Ophiuchi, also known as Rasalhague, from Arabic "the head of the serpent collector". Of course, the serpentine shape below is the ancient Great Wall of China, along the Panlongshan section northeast of Beijing. Panlongshan is translated as "a coiled dragon". A moving and fortuitous scene, it was captured with a digital camera and telephoto lens in two consecutive exposures. The exposures were merged to show a natural looking foreground and twilight sky.

APOD: 2013 October 28 - The Great Comet of 1680 Over Rotterdam
Explanation: Was there ever another comet like ISON? Although no two comets are exactly alike, one that appears to have had notable similarities was Comet Kirch, the Great Comet of 1680. Like approaching Comet ISON, Comet Kirch was a bright sungrazer, making a very close approach to the surface of the Sun. Neither comet, coincidently, is a member of the most common group of sungrazers -- the Kreutz group -- populated by remnants of a comet that disintegrated near the Sun hundreds of years ago. The long tail of Comet Kirch is depicted in the above painting by Lieve Versheier. As pictured, some members of the foreground crowd of Rotterdam in the Netherlands are holding cross-staffs, an angle measuring device that predated the sextant. No one knows how Comet ISON will develop, but like Comet Kirch, it is expected to be brightest when very near the Sun, in ISON's case during last few days of November.

APOD: 2013 October 20 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007. Currently, many sky enthusiasts are following the development of Comet ISON, a comet which might become the Great Comet of 2013.

APOD: 2013 October 15 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, also known as NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This gorgeous telescopic portrait reveals remarkable details of the region's glowing filaments of interstellar gas and obscuring cosmic dust clouds. Wider than the Full Moon in angular size, the field of view stretches over 300 light-years across the nebula. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the brightest star near the image center, just left of the dusty Keyhole Nebula (NGC 3324). While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that the Great Carina Nebula has been a veritable supernova factory.

APOD: 2013 October 13 - Hale Bopp: The Great Comet of 1997
Explanation: Sixteen years ago, Comet Hale-Bopp rounded the Sun and offered a dazzling spectacle in planet Earth's night. This stunning view, recorded shortly after the comet's 1997 perihelion passage, features the memorable tails of Hale-Bopp -- a whitish dust tail and blue ion tail. Here, the ion tail extends well over ten degrees across the northern sky, fading near the double star clusters in Perseus, while the head of the comet lies near Almach, a bright star in the constellation Andromeda. Do you remember Hale-Bopp? The photographer's sons do, pictured in the foreground at ages 12 and 15. In all, Hale-Bopp was reported as visible to the naked eye from roughly late May 1996 through September 1997. Currently, sky enthusiasts await Comet ISON's continued brightening in the coming weeks, unsure how interesting its first journey to the inner Solar System will be.

APOD: 2013 March 20 - M42: Inside the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image in assigned colors highlighted by emission in oxygen and hydrogen, wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2013 February 18 - The Great Russian Meteor of 2013
Explanation: What in heaven's blazes is that? Thousands of people living near the Ural Mountains in Russia saw last Friday morning one of the more spectacular meteors of modern times streak across the sky. Forceful sound waves arrived at the ground minutes later, knocking people over and breaking windows for hundreds of kilometers. The above video is a compilation of several car dashcams and includes real time footage of the meteor rampaging, smoke trails drifting, shadows quickly shifting, and even the meteor's light reflecting off the back of a bus. The fireball is thought to have been caused by a car-sized chunk of ice and rock crashing into the Earth's atmosphere. Since the event was captured from so many angles, the meteor's trajectory has become determined well enough to indicate from where it came and to where any resultant pieces might have landed. It is already certain that this meteor had nothing to do with the several-times larger asteroid 2012 DA14 which passed the Earth from a different direction later the same day. If pieces of the meteor are found, they might tell humanity more about the early Solar System, when the meteor was likely formed.

APOD: 2013 February 13 - Infrared Orion from WISE
Explanation: The Great Nebula in Orion is a intriguing place. Visible to the unaided eye, it appears as a small fuzzy patch in the constellation of Orion. But this image, an illusory-color composite of four colors of infrared light taken with the Earth orbiting WISE observatory, shows the Orion Nebula to be a bustling neighborhood or recently formed stars, hot gas, and dark dust. The power behind much of the Orion Nebula (M42) is the stars of the Trapezium star cluster, seen near the center of the above wide field image. The eerie green glow surrounding the bright stars pictured here is their own starlight reflected by intricate dust filaments that cover much of the region. The current Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years.

APOD: 2013 February 9 - The Great Meteor Procession of 1913
Explanation: One hundred years ago today the Great Meteor Procession of 1913 occurred, a sky event described by some as "magnificent" and "entrancing" and which left people feeling "spellbound" and "privileged". Because one had to be in a right location, outside, and under clear skies, only about 1,000 people noted seeing the procession. Lucky sky gazers -- particularly those near Toronto, Canada -- had their eyes drawn to an amazing train of bright meteors streaming across the sky, in groups, over the course of a few minutes. A current leading progenitor hypothesis is that a single large meteor once grazed the Earth's atmosphere and broke up. When the resulting pieces next encountered the Earth, they came in over south-central Canada, traveled thousands of kilometers as they crossed over the northeastern USA, and eventually fell into the central Atlantic ocean. Pictured above is a digital scan of a halftone hand-tinted image by the artist Gustav Hahn who was fortunate enough to witness the event first hand. Although nothing quite like the Great Meteor Procession of 1913 has been reported since, numerous bright fireballs -- themselves pretty spectacular -- have since been recorded, some even on video.

APOD: 2012 June 14 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

APOD: 2011 September 13 - Great Orion Nebulae
Explanation: The Great Nebula in Orion, also known as M42, is one of the most famous nebulas in the sky. The star forming region's glowing gas clouds and hot young stars are on the right in this sharp and colorful image that includes the smaller nebula M43 near center and dusty, bluish reflection nebulae NGC 1977 and friends on the left. Located at the edge of an otherwise invisible giant molecular cloud complex, these eye-catching nebulae represent only a small fraction of this galactic neighborhood's wealth of interstellar material. Within the well-studied stellar nursery, astronomers have also identified what appear to be numerous infant planetary systems. The gorgeous skyscape spans nearly two degrees or about 45 light-years at the Orion Nebula's estimated distance of 1,500 light-years.

APOD: 2011 June 9 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, also known as NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This gorgeous telescopic portrait reveals remarkable details of the region's glowing filaments of interstellar gas and obscuring cosmic dust clouds. Wider than the Full Moon in angular size, the field of view stretches nearly 100 light-years across the nebula. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the brightest star at the left, near the dusty Keyhole Nebula (NGC 3324). While Eta Carinae itself maybe on the verge of a supernova explosion, X-ray images indicate that the Great Carina Nebula has been a veritable supernova factory.

APOD: 2011 May 2 - Jupiter's Great Red Spot from Voyager 1
Explanation: It is a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter's Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious. A better understanding of the weather on Jupiter may help contribute to the better understanding of weather here on Earth. The above image is a recently completed digital enhancement of an image of Jupiter taken in 1979 by the Voyager 1 spacecraft as it zoomed by the Solar System's largest planet. At about 117 AU from Earth, Voyager 1 is currently the most distant human made object in the universe and expected to leave the entire solar heliosheath any time now.

APOD: 2010 December 4 - Sunset at the Spiral Jetty
Explanation: In dwindling twilight at an August day's end, these broad dark bands appeared in the sky for a moment, seen from Robert Smithson's Spiral Jetty on the eastern shore of Utah's Great Salt Lake. Outlined by rays of sunlight known as crepuscular rays, they are actually shadows cast by clouds near the distant western horizon, the setting Sun having disappeared from direct view behind them. The cloud shadows are parallel, but seem to converge in the distance because of perspective. Coiled in the salt-encrusted lake surface, Smithson's most famous earthwork provides a dramatic contrast to the converging lines. The Spiral Jetty was constructed in 1970, when the water level was unusually low and was completely submerged in a few years as the level rose. Now just above water again, it has spent much of its existence submerged in the briny lake.

APOD: 2010 November 19 - Nebulae in the Northern Cross
Explanation: Explore a beautiful and complex region of nebulae strewn along the plane of our Milky Way Galaxy in this widefield skyscape. The image emphasizes cosmic gas clouds in a 25 by 25 degree view centered on the Northern Cross, the famous asterism in the constellation Cygnus. Bright, hot, supergiant star Deneb at the top of the cross, Sadr near the center, and beautiful Albireo run diagonally through the scene. Popular telescopic tour destinations such as the North America and Pelican emission regions, the Butterfly Nebula (IC 1318), and the Crescent and Veil nebulae can be identified by placing your cursor over the image. Silhouetted by the glowing interstellar clouds and crowded star fields, the dark Northern Coal Sack also stands out, part of a series of obscuring dust clouds forming the Great Rift in the Milky Way. These Northern Cross nebulosities are all located about 2,000 light-years away. Along with the Sun, they lie within the Orion spiral arm of our galaxy.

APOD: 2010 August 17 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 6,400 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of the cross (Crux).

APOD: 2010 May 27 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

APOD: 2010 March 9 - Galaxies Beyond the Heart: Maffei 1 and 2
Explanation: The two galaxies on the far left were unknown until 1968. Although they would have appeared as two of the brighter galaxies on the night sky, the opaque dust of the central band of our Milky Way Galaxy had obscured them from being seen in visible light. The above image in infrared light taken by the recently launched Wide-Field Infrared Survey Explorer (WISE), however, finds these galaxies in great detail far behind -- but seemingly next to -- the photogenic Heart nebula (IC 1805). The spiral galaxy near the top is the easiest to spot and is known as Maffei 2. Just below and to its right is fuzzy-looking Maffei 1, the closest giant elliptical galaxy to Earth. The above false-colored image spans three full moons from top to bottom. The Maffei galaxies each span about 15,000 light years across and lie about 10 million light years away toward the constellation of the Queen of Ethiopia (Cassiopeia). On the image right, stars, gaseous filaments, and warm dust highlight a detailed infrared view of the Heart nebula.

APOD: 2010 January 12 - The Flame Nebula in Infrared
Explanation: What lights up the Flame Nebula? Fifteen hundred light years away towards the constellation of Orion lies a nebula which, from its glow and dark dust lanes, appears, on the left, like a billowing fire. But fire, the rapid acquisition of oxygen, is not what makes this Flame glow. Rather the bright star Alnitak, the easternmost star in the Belt of Orion visible just above the nebula, shines energetic light into the Flame that knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine. The above false-color picture of the Flame Nebula (NGC 2024) was taken in infrared light, where a young star cluster becomes visible. The Flame Nebula is part of the Orion Molecular Cloud Complex, a star-forming region that includes the famous Horsehead Nebula, visible above on the far right.

APOD: 2010 January 9 - Andromeda Island Universe
Explanation: The most distant object easily visible to the eye is M31, the great Andromeda Galaxy some two and a half million light-years away. But without a telescope, even this immense spiral galaxy - spanning over 200,000 light years - appears as a faint, nebulous cloud in the constellation Andromeda. In contrast, details of a bright yellow nucleus and dark winding dust lanes, are revealed in this digital telescopic image. Narrow band image data recording emission from hydrogen atoms, shows off the reddish star-forming regions dotting gorgeous blue spiral arms and young star clusters. While even casual skygazers are now inspired by the knowledge that there are many distant galaxies like M31, astronomers seriously debated this fundamental concept in the 20th century. Were these "spiral nebulae" simply outlying components of our own Milky Way Galaxy or were they instead "island universes" -- distant systems of stars comparable to the Milky Way itself? This question was central to the famous Shapley-Curtis debate of 1920, which was later resolved by observations of M31 in favor of Andromeda, island universe.

APOD: 2009 December 8 - Ice Moon Tethys from Saturn Orbiting Cassini
Explanation: What processes formed the unusual surface of Saturn's moon Tethys? To help find out, NASA sent the robotic Cassini spacecraft right past the enigmatic ice moon in 2005. Pictured above is one of the highest resolution images of an entire face of Tethys yet created. The pervasive white color of Tethys is thought to be created by fresh ice particles continually falling onto the moon from Saturn's diffuse E-ring -- particles expelled by Saturn's moon Enceladus. Some of the unusual cratering patterns on Tethys remain less well understood, however. Close inspection of the above image of Tethys' south pole will reveal a great rift running diagonally down from the middle: Ithaca Chasma. A leading theory for the creation of this great canyon is anchored in the tremendous moon-wide surface cracking that likely occurred when Tethys' internal oceans froze. If so, Tethys may once have harbored internal oceans, possibly similar to the underground oceans some hypothesize to exist under the surface of Enceladus today. Might ancient life be frozen down there?

APOD: 2009 December 6 - The Magnificent Tail of Comet McNaught
Explanation: Comet McNaught, the Great Comet of 2007, was the brightest comet of the last 40 years. Its spectacular tail spread across the sky and was breathtaking to behold from dark locations for many Southern Hemisphere observers. The head of the comet remained quite bright and was easily visible to even city observers without any optical aide. Part of the spectacular tail was visible just above the horizon after sunset for many northern observers as well. Comet C/2006 P1 (McNaught), which reached an estimated peak brightness of magnitude -6 (minus six), was caught by the comet's discoverer in the above image soon after sunset in 2007 January from Siding Spring Observatory in Australia. The robotic Ulysses spacecraft fortuitously flew through Comet McNaught's tail and found, unexpectedly, that the speed of the solar wind dropped significantly.

APOD: 2009 November 11 - Great Observatories Explore Galactic Center
Explanation: Where can a telescope take you? Four hundred years ago, a telescope took Galileo to the Moon to discover craters, to Saturn to discover rings, to Jupiter to discover moons, to Venus to discover phases, and to the Sun to discover spots. Today, in celebration of Galileo's telescopic achievements and as part of the International Year of Astronomy, NASA has used its entire fleet of Great Observatories, and the Internet, to bring the center of our Galaxy to you. Pictured above, in greater detail and in more colors than ever seen before, are the combined images of the Hubble Space Telescope in near-infrared light, the Spitzer Space Telescope in infrared light, and the Chandra X-ray Observatory in X-ray light. A menagerie of vast star fields is visible, along with dense star clusters, long filaments of gas and dust, expanding supernova remnants, and the energetic surroundings of what likely is our Galaxy's central black hole. Many of these features are labeled on a complementary annotated image. Of course, a telescope's magnification and light-gathering ability create only an image of what a human could see if visiting these places. To actually go requires rockets.

APOD: 2009 July 18 - Planets, Great Wall, and Solar Eclipse
Explanation: This dramatic skyscape was recorded during the August 2008 total solar eclipse. The Moon's silhouette surrounded by a glistening solar corona hangs above the Jiayuguan Fort along the western edge of the Great Wall of China. Lined-up along the ecliptic plane, all the planets of the inner solar system, Mercury, Venus, Mars, (and Earth!) can also be seen along with Saturn and bright star Regulus, as the Moon's shadow tracks across the landscape. Beyond the Moon's shadow, outside the total eclipse track, sunlight still brightens the sky over mountains on the horizon 30 - 50 kilometers away. Much anticipated, the 2009 July 22nd total solar eclipse will again be visible from China. Planets and bright stars will briefly appear in darkened daytime skies, though a total eclipse won't be seen from the Great Wall. Still, major cities and populated areas lie along the 2009 total eclipse track that begins in India and sweeps eastward across Asia and into the Pacific Ocean.

APOD: 2009 June 17 - M13: A Great Globular Cluster of Stars
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 remains unknown.

APOD: 2009 March 31 - In the Heart of the Tarantula Nebula
Explanation: In the heart of monstrous Tarantula Nebula lies huge bubbles of energetic gas, long filaments of dark dust, and unusually massive stars. In the center of this heart, is a knot of stars so dense that it was once thought to be a single star. This star cluster, labeled as R136 or NGC 2070, is visible just above the center of the above image and home to a great number of hot young stars. The energetic light from these stars continually ionizes nebula gas, while their energetic particle wind blows bubbles and defines intricate filaments. The above representative-color picture of this great LMC nebula details its tumultuous center. The Tarantula Nebula, also known as the 30 Doradus nebula, is one of the largest star-formation regions known, and has been creating unusually strong episodes of star formation every few million years.

APOD: 2009 March 2 - Earthgrazer: The Great Daylight Fireball of 1972
Explanation: What is that streaking across the sky? A bright earthgrazing meteor. In 1972, an unusually bright meteor from space was witnessed bouncing off Earth's atmosphere, much like a skipping stone can bounce off of a calm lake. The impressive event lasted several seconds, was visible in daylight, and reportedly visible all the way from Utah, USA to Alberta, Canada. Pictured above, the fireball was photographed streaking above Teton mountains behind Jackson Lake, Wyoming, USA. The Great Daylight Fireball of 1972 was possibly the size of a small truck, and would likely have created an impressive airburst were it to have struck Earth more directly. Earthgrazing meteors are rare but are more commonly seen when the radiant of a meteor shower is just rising or setting. At that time, meteors closer to the Earth than earthgrazers would more usually strike the Earth near the horizon, while meteors further than earthgrazers would miss the Earth entirely.

APOD: 2009 February 16 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, aka NGC 3372, spans over 300 light-years, one of our Galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This stunning telescopic view from the 2.2-meter ESO/MPG telescope La Silla Observatory in Chile reveals remarkable details of the region's glowing filaments of interstellar gas and dark cosmic dust clouds. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the bright star left of the central dark notch in this field and near the dusty Keyhole Nebula (NGC 3324).

APOD: 2009 January 6 - Jupiter Eclipsing Ganymede
Explanation: How hazy is Jupiter's upper atmosphere? To help find out, astronomers deployed the Hubble Space Telescope to watch Jupiter eclipse its moon Ganymede. Although Ganymede circles Jupiter once a week, a particularly useful occultation occurs more rarely. Such an occultation was captured in great visual detail in April 2007. When near Jupiter's limb, Ganymede reflects sunlight though Jupiter's upper atmosphere, allowing astronomers to search for haze by noting a slight dimming at different colors. One result of this investigation was the above spectacular image, where bands of clouds that circle Jupiter are clearly visible, as well as magnificent swirling storm systems such as the Great Red Spot. Ganymede, at the image bottom, also shows noticeable detail on its dark icy surface. Since Jupiter and Ganymede are so bright, many eclipses can be seen right here on Earth with a small telescope.

APOD: 2008 October 23 - Great Orion Nebulae
Explanation: The Great Nebula in Orion, also known as M42, is one of the most famous nebulae in the sky. The star forming region's glowing gas clouds and hot young stars are on the right in this sharp and colorful two frame mosaic that includes the smaller nebula M43 near center and dusty, bluish reflection nebulae NGC 1977 and friends on the left. Located at the edge of an otherwise invisible giant molecular cloud complex, these eye-catching nebulae represent only a small fraction of this galactic neighborhood's wealth of interstellar material. Within the well-studied stellar nursery, astronomers have also identified what appear to be numerous infant solar systems. The gorgeous skyscape spans nearly two degrees or about 45 light-years at the Orion Nebula's estimated distance of 1,500 light-years.

APOD: 2008 August 31 - Eclipse over the Great Wall
Explanation: Contrary to the famous myth, you can't see the Great Wall of China from the Moon ... even during a total solar eclipse. But on August 1 you could see the Moon eclipsing the Sun from the Great Wall. In fact, from this location near the Great Wall's western end, the Moon completely blocked the Sun's overwhelming disk revealing a shimmering solar corona and bright planets in the briefly darkened sky. A main pass, The Great Wall's Jiayuguan Fort, is also silhouetted in the foreground. The pass is the western-most of the wall's passes and the best preserved, initially built around 1372 during the Ming dynasty. The nearby city of Jiayuguan in Gansu Province was an important stop on the Silk Road.

APOD: 2008 August 26 - 47 Tuc: A Great Globular Cluster of Stars
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Light takes about 13,000 years to reach us from 47 Tuc which can be seen on the sky near the Small Magellanic Cloud in the southern constellation of Tucana. Also known as NGC 104, the dense cluster is made up of several million stars in a volume only about 120 light-years across. The cluster's red giant stars are particularly easy to see in this picture. The globular cluster is also home to exotic x-ray binary star systems.

APOD: 2008 July 24 - When Storms Collide
Explanation: These detailed Hubble Space Telescope close-ups feature Jupiter's ancient swirling storm system known as the Great Red Spot. They also follow the progress of two newer storm systems that have grown to take on a similar reddish hue: the smaller "Red Spot Jr." (bottom), and smaller still, a "baby red spot". Red Spot Jr. was seen to form in 2006, while the smaller spot was just identified earlier this year. For scale, the Great Red Spot has almost twice the diameter of planet Earth. Moving horizontally from left to right past the Great Red Spot, Red Spot Jr. clearly went below the larger storm, but the smaller spot was pulled in. Emerging on the right, the baby spot's stretched and now paler shape is indicated by the arrow in the frame from July 8. It is expected that the baby red spot will be pulled back and merge, becoming part of the giant storm system.

APOD: 2008 June 24 - Ithaca Chasma: The Great Rift on Saturn's Tethys
Explanation: What created the Great Rift on Saturn's moon Tethys? No one is sure. More formally named Ithaca Chasma, the long canyon running across the right of the above image extends about 2,000 kilometers long and spreads as much as 100 kilometers wide. The above image was captured by the Saturn-orbiting robotic Cassini spacecraft as it zoomed by the icy moon last month. Hypotheses for the formation of Ithaca Chasma include cracking of Tethy's outer crust as the moon cooled long ago, and that somehow the rift is related to the huge Great Basin impact crater named Odysseus, visible elsewhere on the unusual moon. Cassini has now been orbiting Saturn for about four years and is scheduled to continue to probe and photograph Saturn for at least two more years.

APOD: 2008 May 23 - Jupiter's Three Red Spots
Explanation: For about 300 years Jupiter's banded atmosphere has shown a remarkable feature to telescopic viewers, a large swirling storm system known as The Great Red Spot. In 2006, another red storm system appeared, actually seen to form as smaller whitish oval-shaped storms merged and then developed the curious reddish hue. Now, Jupiter has a third red spot, again produced from a smaller whitish storm. All three are seen in this image made from data recorded on May 9 and 10 with the Hubble Space Telescope's Wide Field and Planetary Camera 2. The spots extend above the surrounding clouds and their red color may be due to deeper material dredged up by the storms and exposed to ultraviolet light, but the exact chemical process is still unknown. For scale, the Great Red Spot has almost twice the diameter of planet Earth, making both new spots less than one Earth-diameter across. The newest red spot is on the far left (west), along the same band of clouds as the Great Red Spot and is drifting toward it. If the motion continues, the new spot will encounter the much larger storm system in August. Jupiter's recent outbreak of red spots is likely related to large scale climate change as the gas giant planet is getting warmer near the equator.

APOD: 2008 February 11 - Saturn's Moon Epimetheus from the Cassini Spacecraft
Explanation: How did Epimetheus form? No one is yet sure. To help answer that question, this small moon has recently been imaged again in great detail by the robot spacecraft Cassini now orbiting Saturn. Epimetheus sometimes orbits Saturn in front of Janus, another small satellite, but sometimes behind. The above image, taken last December, shows a surface covered with craters indicating great age. Epimetheus spans about 115 kilometers across. Epimetheus does not have enough surface gravity to restructure itself into a sphere. The flattened face of Epimetheus shown above might have been created by a single large impact.

APOD: 2007 November 15 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is modestly recognized as the Great Globular Cluster in Hercules. A system of stars numbering in the hundreds of thousands, it is one of the brightest globular star clusters in the northern sky. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core over 100 stars would be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. This stunning view of the cluster combines recent telescopic images of the cluster's dense core with digitized photographic plates recorded between 1987 and 1991 using the Samuel Oschin Telescope, a wide-field survey instrument at Palomar Observatory. The resulting composite highlights both inner and outer reaches of the giant star cluster. Among the distant background galaxies also visible, NGC 6207 is above and to the left of the Great Globular Cluster M13.

APOD: 2007 November 7 - The Sloan Great Wall: Largest Known Structure
Explanation: What is the largest structure known? The answer might depend on how one defines "structure." A grouping of galaxies known as the Sloan Great Wall was discovered in the Sloan Digital Sky Survey and is a leading candidate. The Sloan Great Wall can be seen in this digitally recast contour map of galaxies in the Two Degree Field galaxy survey. Galaxies within one billion light years, a redshift of about 0.1, are depicted. The labeled Sloan Great Wall spans over one billion light years, longer than any structure ever measured. Critics worry that the Sloan Great Wall should not itself be characterized as a coherent structure because it is not currently gravitationally bound together and parts of it might never become gravitationally bound. Regardless, the beauty of the local universe of galaxies is evident in the image where several huge superclusters of galaxies -- clusters of galaxy clusters -- can also be seen. These include the Shapley Supercluster of galaxies, part of the Pisces-Cetus Supercluster, and part of the Horologium-Reticulum Supercluster.

APOD: 2007 October 27 - The Great Carina Nebula
Explanation: A jewel of the southern sky, the Great Carina Nebula, aka NGC 3372, spans over 300 light-years, one of our galaxy's largest star forming regions. Like the smaller, more northerly Orion Nebula, the Carina Nebula is easily visible to the naked eye, though at a distance of 7,500 light-years it is some 5 times farther away. This stunning telescopic view reveals remarkable details of the region's glowing filaments of interstellar gas and dark cosmic dust clouds. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the bright star left of the central dark notch in this field and just below the dusty Keyhole Nebula (NGC 3324).

APOD: 2007 September 9 - The Great Basin on Saturns Tethys
Explanation: Some moons wouldn't survive the collision. Tethys, one of Saturn's larger moons at about 1000 kilometers in diameter, survived the collision, but sports today the expansive impact crater Odysseus. Sometimes called the Great Basin, Odysseus occurs on the leading hemisphere of Tethys and shows its great age by the relative amount of smaller craters that occur inside its towering walls. Another large crater, Melanthius, is visible near the moon's terminator. The density of Tethys is similar to water-ice. The above digitally enhanced image was captured in July by the robot Cassini spacecraft in orbit around Saturn as it swooped past the giant ice ball.

APOD: 2007 June 7 - Great Mountain Moonrise
Explanation: On May 31st, a gorgeous Full Moon rose over Uludag Mountain in Bursa Province, Turkey. This alluring telephoto view of the twilight scene is a composite of images taken roughly every two minutes beginning shortly after Sunset, following the rising Moon as it moves up and to the right. Of course, as the Moon rises it gets brighter and changes color, becoming less reddened as the sight-line through the dense atmosphere is steadily reduced. Each of the final two exposures also captured a rising planet Jupiter. Like the Full Moon, the bright, wandering planet is nearly opposite the Sun in Earth's sky and was caught on the lefthand side of the picture in two places, just above a small peak in the mountain side. Intriguingly, some considered this Full Moon a Blue Moon.

APOD: 2007 May 18 - M13: The Great Globular Cluster in Hercules
Explanation: In 1714, Edmond Halley noted that M13 "shows itself to the naked eye when the sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this deep color image. A distant background galaxy, NGC 6207 is also visible above and to the right of the Great Globular Cluster M13.

APOD: 2007 March 31 - Hale-Bopp: The Great Comet of 1997
Explanation: Ten short years ago, Comet Hale-Bopp rounded the Sun and offered a dazzling spectacle in planet Earth's night. This stunning view, recorded shortly after the comet's perihelion passage on April 1, 1997, features the memorable tails of Hale-Bopp -- a whitish dust tail and blue ion tail. Here, the ion tail extends well over ten degrees across the northern sky, fading near the double star clusters in Perseus, while the head of the comet lies near Almach, a bright star in the constellation Andromeda. Do you remember Hale-Bopp? The photographer's sons do, pictured in the foreground at ages 12 and 15. In all, Hale-Bopp was reported as visible to the naked eye from roughly late May 1996 through September 1997.

APOD: 2007 March 29 - Jupiter Moon Movie
Explanation: South is toward the top in this frame from a stunning movie featuring Jupiter and moons recorded last Thursday from the Central Coast of New South Wales, Australia. In fact, three jovian moons and two red spots are ultimately seen in the full video as they glide around the solar system's ruling gas giant. In the early frame above, Ganymede, the largest moon in the solar system, is off the lower right limb of the planet, while intriguing Europa is visible against Jupiter's cloud tops, also near the lower right. Jupiter's new red spot junior is just above the broad white band in the planet's southern (upper) hemisphere. In later frames, as planet and moons rotate (right to left), red spot junior moves behind Jupiter's left edge while the Great Red Spot itself comes into view from the right. Also finally erupting into view at the right, is Jupiter's volcanic moon, Io. To download the full 2 megabyte movie as an animated gif file, click on the picture.

APOD: 2006 November 20 - M42: Wisps of the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image, faint wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2006 August 13 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April 1997, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on 1997 March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during a total lunar eclipse.

APOD: 2006 May 5 - Jupiter and the Red Spots
Explanation: Jupiter's Great Red Spot is a swirling storm seen for over 300 years, since the beginning of telescopic observations. But in February 2006, planetary imager Christopher Go noticed it had been joined by Red Spot Jr - formed as smaller whitish oval-shaped storms merged and then developed the remarkable reddish hue. This sharp Hubble Space Telescope image showing the two salmon-colored Jovian storms was recorded in April. About half the size of the original Red Spot, Red Spot Jr. is similar in diameter to planet Earth. Seen here below and left of the ancient storm system, it trails the Great Red Spot by about an hour as the planet rotates from left to right. While astronomers still don't exactly understand why Jupiter's red spots are red, they do think the appearance of Red Spot Jr. provides evidence for climate change on the Solar System's ruling gas giant.

APOD: 2006 March 18 - Red Spot Jr.
Explanation: Jupiter's Great Red Spot is a swirling storm seen for over 300 years, since the beginning of telescopic observations of the Solar System's ruling gas giant. But over the last month it has been joined by Red Spot Jr. Thought to be similar to the Great Red Spot itself, this not-so-great red spot was actually seen to form as smaller whitish oval-shaped storms merged and then developed the remarkable reddish hue. This webcam image showing the two red tinted Jovian storms was recorded on the morning of March 12 from the Central Coast of New South Wales, Australia - part of a series showing Jupiter's rotation. Similar in diameter to planet Earth, Red Spot Jr. is expected to last for a while, and trails the Great Red Spot by about an hour as the planet rotates. Astronomers still don't exactly understand why Jupiter's red spots are red.

APOD: 2006 February 8 - The Great Basin on Tethys
Explanation: Some moons wouldn't survive the collision. Tethys, one of Saturn's larger moons at about 1000 kilometers in diameter, survived the collision, but sports today the expansive impact crater Odysseus. Sometimes called the Great Basin, Odysseus occurs on the leading hemisphere of Tethys and shows its great age by the relative amount of smaller craters that occur inside its towering walls. The density of Tethys is similar to water-ice. The above digitally enhanced image was captured late last year by the robot Cassini spacecraft in orbit around Saturn as it swooped past the giant ice ball.

APOD: 2005 September 18 - M42: Wisps of the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image, faint wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2005 September 13 - A Quadruple Sky Over Great Salt Lake
Explanation: This was a sky to show the kids. All in all, three children, three planets, the Moon, a star, an airplane and a mom were all captured in one image near Great Salt Lake in Utah, USA on September 6. Minus the airplane and the quadruple on the ground, this busy quadruple coincidence sky was visible last week all over the world. The easiest object to spot is the crescent Moon, which is easily the brightest sky orb in the above image. Venus is the highest planet in the sky, with Jupiter to its right. The bright star Spica completes the quadruple just below Venus. The streak on the far right is an airplane. Mom is seated. Grandpa, appreciating the beauty of the moment, took the picture.

APOD: 2005 August 24 - Epimetheus: A Small Moon of Saturn
Explanation: How did Epimetheus form? No one is yet sure. To help answer that question, this small moon has recently been imaged again in great detail by the robot spacecraft Cassini now orbiting Saturn. Epimetheus sometimes orbits Saturn in front of Janus, another small satellite, but sometimes behind. The above false-color image, taken during mid July, shows a surface covered with craters indicating great age. Epimetheus spans about 115 kilometers across. Epimetheus does not have enough surface gravity to restructure itself into a sphere.

APOD: 2005 May 30 - A Great White Spot on Rhea
Explanation: What caused this great white spot on the surface of Saturn's moon Rhea? The spot was first noticed last year by the robot Cassini spacecraft now orbiting Saturn. Cassini's flyby of Rhea in April imaged in the spot in great detail. Astronomers hypothesize that the light-colored spot is the result of a relatively recent impact on the surface of the icy moon. The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface. Rhea spans 1,500 kilometers across and is the second largest moon of Saturn after Titan. Rhea sports several other light colored surface features that are, as yet, not well understood.

APOD: 2004 September 27 - The Great Nebula in Orion
Explanation: The Great Nebula in Orion is a colorful place. Visible to the unaided eye, it appears as a small fuzzy patch in the constellation of Orion. Long exposure, digitally sharpened images like this, however, show the Orion Nebula to be a busy neighborhood of young stars, hot gas, and dark dust. The power behind much of the Orion Nebula (M42) is the Trapezium - four of the brightest stars in the nebula. Many of the filamentary structures visible are actually shock waves - fronts where fast moving material encounters slow moving gas. The Orion Nebula spans about 40 light years and is located about 1500 light years away in the same spiral arm of our Galaxy as the Sun.

APOD: 2004 May 17 - NGC 3372: The Great Nebula in Carina
Explanation: In one of the brightest parts of the Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebula. Eta Carina, the most energetic star in the nebula was one of the brightest stars in the sky in the 1830s, but then faded dramatically. The Keyhole Nebula, visible near the center, houses several of the most massive stars known and has also changed its appearance. The Carina Nebula spans over 300 light years and lies about 7000 light-years away in the constellation of Carina. The above image was taken from La Frontera in Alcohuaz, Chile. Eta Carina might explode in a dramatic supernova within the next thousand years, and has even flared in brightness over just the past decade.

APOD: 2004 May 11 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

APOD: 2004 February 2 - The Tarantula Nebula from Spitzer
Explanation: In the heart of monstrous Tarantula Nebula lies one of the most unusual star clusters. Known as NGC 2070 or R136, it is home to a great number of hot young stars. The energetic light from these stars continually ionizes nebula gas, while their energetic particle wind blows bubbles and defines intricate filaments. The new Spitzer Space Telescope took the above representative-color infrared image of this great LMC cluster. The image details the cluster's tumultuous center in gas, dust and young stars. The 30 Doradus nebula is one of the largest star-formation regions known, and has been creating unusually strong episodes of star formation every few million years. In the heart of this heart is a central knot of stars that is so dense it was once thought to be a single star.

APOD: 2004 January 21 - Adirondack Rock on Mars
Explanation: Is this a great pyramid on Mars? Actually, the pictured rock dubbed Adirondack has an irregular shape, is only about the size of a football, and has formed by natural processes. Still, its relatively large size and dust-free surface made it the first destination for the robotic Spirit rover currently roving Mars. Spirit, itself the size of a golf cart, will now attempt to determine the rock's composition and history by prodding it with its sophisticated mechanical arm. Spirit's arm, programmed remotely from Earth, has the capability to bend, grind, and photograph the rock in minute detail. Spirit's twin rover Opportunity is scheduled to land on the other side of Mars this coming weekend.

APOD: 2003 December 17 - A Proton Aurora
Explanation: What are auroras made out of? Triggered by solar activity, normal auroras are caused by collisions between fast-moving electrons and the oxygen and nitrogen in Earth's upper atmosphere. The electrons come from the magnetosphere, the region of space controlled by Earth's magnetic field. As the excited oxygen and nitrogen molecules return to their low energy state, they emit light, seen as the auroral glow. Sometimes, however, auroras can be caused by collisions with heavier protons, causing a more energetic display with strong ultraviolet emission. In addition, protons can temporarily capture an electron and emit light for themselves. Such a proton aurora is seen above, recorded by the IMAGE satellite. A special feature is the bright spot near picture center, embedded in a ring of auroral emission around the north magnetic pole of planet Earth. Most solar wind protons never reach the Earth to cause auroras because they are completely deflected away at a great distance by the Earth's magnetic field. The bright spot in the auroral ring indicates a particularly deep crack in the Earth's magnetic field where protons were able to flow along a temporarily connected region between the Sun and the Earth, relatively undeflected, until they impacted the Earth's ionosphere.

APOD: 2003 September 5 - SIRTF Streak
Explanation: Streaking skyward, a Boeing Delta 2-Heavy rocket carries NASA's Space InfraRed Telescope Facility (SIRTF) aloft during the early morning hours of August 25th. The dramatic scene was recorded in a time exposure from the pier in Jetty Park at the northern end of Cocoa Beach, Florida, about 2.5 miles from the Cape Canaveral launch site. SIRTF (sounds like "sir tiff") will explore the distant Universe in infrared light as the fourth and final satellite observatory in NASA's Great Observatories Program. The three other large astrophysics satellites were designed for higher energies in the electromagnetic spectrum, with the Hubble Space Telescope operating near visible wavelengths, the Compton Gamma Ray Observatory instruments sensitive to gamma rays, and the Chandra Observatory detecting cosmic x-rays. SIRTF has been launched into an Earth-trailing solar orbit to reduce its exposure to infrared radiation from our fair planet. Cooled by an on board supply of liquid helium, SIRTF's infrared detectors will operate at near absolute zero temperatures. Presently, SIRTF's systems are undergoing a 90-day check out.

APOD: 2003 July 15 - Mars Rising Through Arch Rock
Explanation: Mars is heading for its closest encounter with Earth in over 50,000 years. Although Mars and Earth continue in their normal orbits around the Sun, about every two years Earth and Mars are on the same part of their orbit as seen from the Sun. When this happens again in late August, Mars will be almost as near to the Sun as it ever gets, while simultaneously Earth will be almost as far from the Sun as it ever gets. This means that now is a great time to launch your space probe to Mars. Alternatively, these next few months are a great time to see a bright red Mars from your backyard. Mars is so close that global features should be visible even through a small telescope. Look for Mars to rise about 11 pm and to remain the brightest red object in the sky until sunrise. Mars will rise increasingly earlier until its closest approach in late August. Mars was captured above rising through the Arch Rock in Valley of Fire State Park, Nevada, USA.

APOD: 2003 March 19 - Jupiter's Great Dark Spot
Explanation: Seventeenth century astronomer Giovanni Domenico Cassini was an astute observer of Jupiter's Great Red Spot. So it seems only fitting that his namesake, the Cassini spacecraft, has enabled detailed observations of another planet-sized blemish -- Jupiter's Great Dark Spot. Unlike the Red Spot, the Great Dark Spot lies near Jupiter's north pole and seems to appear and disappear over periods of months rather than persisting for hundreds of years. Seen at ultraviolet wavelengths, the dark feature resides in the Jovian stratosphere confined by pole-encircling winds, analogous to planet Earth's antarctic ozone hole. This image of the Dark Spot is a single frame from a movie created with data recorded during the spacecraft's year 2000 flyby of Jupiter. Projected to show Jupiter's north polar region, no data are available for the blank central area, while the Great Dark Spot lies above and just left of center. The white circle marks 60 degrees latitude and the blue contour outlines a persistent Jovian auroral zone which may be related to the formation of the Great Dark Spot.

APOD: 2003 March 13 - WIRO at Jupiter
Explanation: Gazing out over the mountaintops from the Wyoming InfraRed Observatory (WIRO), astronomers recently recorded this bizarre looking image of the solar system's ruling planet, gas giant Jupiter. The false-color picture is a composite of images taken to test a sophisticated digital camera operating at liquid helium temperatures and sensitive to wavelengths about three times longer than visible red light. At those infrared wavelengths (near 2.1 microns) the molecular hydrogen and methane gas in Jupiter's dense lower atmosphere strongly absorb sunlight, so the normally bright, banded planet looks very dark. But particles and haze over the equator and poles rise above the absorbing layers into Jupiter's stratosphere and reflect the infrared sunlight. Also clearly extending into the Jovian stratosphere is the famous Great Red Spot seen here in yellow just under the equatorial band at the right. North is up in this view and Jupiter's rapid 10 hour rotation will soon carry the Great Red Spot behind the planet's right limb.

APOD: 2003 February 27 - When Moons and Shadows Dance
Explanation: It's no wonder Jupiter is a favorite target for even modest earthbound telescopes. The most massive planet in the solar system with four of the largest moons also boasts the famous Great Red Spot, a giant hurricane-like storm system over three hundred years old. Recorded on December 15, 2002 between 7:19 and 8:40 UT, over a thousand digital images were processed and stacked to create this spectacular 21 frame animation of the Jovian system. South is up and as the Great Red Spot tracks across the face of Jupiter, innermost Galilean moon Io enters the scene at the far right. Io occults (passes in front of) the edge of the more sedately orbiting Ganymede with Io's shadow moving quickly across the gas giant's cloud tops, just below the Red Spot. While the moon Callisto is outside the field of view, its large, dark shadow is also visible crossing the Jovian disk at the upper left. Viewed from Earth, the orbits of the Galilean moons presently lie nearly edge-on, offering many chances to observe similar dances of Jupiter's moons.

APOD: 2003 February 25 - M42: Wisps of the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image, faint wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2002 March 1 - Jupiter's Great X Ray Spot
Explanation: The Solar System's largest planet, gas giant Jupiter, is famous for its swirling Great Red Spot. In the right hand panel above, the familiar giant planet with storm system and cloud bands is shown in an optical image from the passing Cassini spacecraft. In the left hand panel, a false-color image from the orbiting Chandra Observatory presents a corresponding x-ray view of Jupiter. The Chandra image shows clearly, for the first time, x-ray spots and auroral x-ray emission from the poles. The x-ray spot dominating the emission from Jupiter's north pole (top) is perhaps as surprising for astronomers today as the Great Red Spot once was. Confounding previous theories, the x-ray spot is too far north to be associated with heavy electrically charged particles from the vicinity of volcanic moon Io. Chandra data also show that the spot's x-ray emission mysteriously pulsates over a period of about 45 minutes.

APOD: 2002 February 13 - The Great Nebula in Orion
Explanation: Few astronomical sights excite the imagination like the nearby stellar nursery known as the Orion Nebula. The Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. The above image has been contrast balanced to bring out Orion's detail in spectacular fashion. Visible simultaneously are the bright stars of the Trapezium in Orion's heart, the sweeping lanes of dark dust that cross the center, the pervasive red glowing hydrogen gas, and the blue tinted dust that reflects the light of newborn stars. The whole Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years.

APOD: 2002 February 5 - Giant Storm Systems Battle on Jupiter
Explanation: Two of the largest storm systems on Jupiter are colliding, and nobody is sure what will result. The larger storm is the famous Great Red Spot, while the smaller is a large white oval. Both are swirling cloud systems that circulate on Jupiter. The white oval is part of a belt of clouds that circles Jupiter faster than the Great Red Spot. The oval started being slowed by the Great Red Spot two weeks ago and the collision could last another month. The oval will likely survive but could possibly be disrupted or absorbed. The two storm systems went at it at least once before in 1975 causing the Spot's red color to fade for several years. The passing Voyager 2 robot spacecraft took the above picture of Jupiter's Great Red Spot in 1979. A different white oval was then visible below the Spot.

APOD: 2001 December 1 - Neptune's Great Dark Spot: Gone But Not Forgotten
Explanation: When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up. Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby. When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!

APOD: 2001 September 19 - SIRTF: Name This Satellite
Explanation: NASA is preparing to launch its next Great Observatory in 2002, but it does not yet have a proper name. Can you help? Currently referred to only as the Space Infrared Telescope Facility (SIRTF), NASA seeks to add something more significant. Previously, NASA named its Great Observatories for scientists of the recent past, including the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory. SIRTF will be the most powerful infrared telescope ever launched, imaging everything from nearby planetary disks to distant galaxies. To enter the contest, one must conform to all rules including the submission of an essay of 250 words or less. The contest ends on December 20.

APOD: 2001 August 21 - Dark Spots on Neptune
Explanation: Neptune has spots. The Solar System's outermost gas giant shows a nearly uniform blue hue created by small amounts of methane drifting in a thick atmosphere of nearly colorless hydrogen and helium. Dark spots do appear, however, that are anti-cyclones: large high-pressure systems that swirl in Neptune's cold cloud tops. Two dark spots are visible in the above picture taken by the robot Voyager 2 spacecraft in 1989: an Earth-sized Great Dark Spot located on the far left, and Dark Spot 2 located near bottom. A bright cloud dubbed Scooter accompanies the Great Dark Spot. Recent computer simulations indicate that scooters are methane clouds that might commonly be found near dark spots. Subsequent images of Neptune by the Hubble Space Telescope in 1994 indicated that both of these dark spots had dissipated, but another had been created.

APOD: 2001 June 18 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 7500 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of Crux.

APOD: 2000 December 12 - Jupiter Eyes Ganymede
Explanation: Who keeps an eye on the largest moon in the Solar System? This moon, visible on the lower right, is Ganymede, and the planet it orbits, Jupiter, seems to be keeping a watchful eye, as its Great Red Spot appears serendipitously nearby. This recently released enhanced-contrast image from the robot spacecraft Cassini captures new details of the incredible intricacies of Jupiter's complex cloud patterns. Features as small as 250 kilometers can be seen. Counter-clockwise rotating high-pressure white ovals that are similar to the Great Red Spot appear in the red band below the spot. Between these spots are darker low-pressure systems that rotate clockwise. The hydrogen and helium that compose most of Jupiter's clouds is nearly invisible - the trace chemicals that give Jupiter these colors remain unknown. The Cassini spacecraft is using Jupiter to pull it toward Saturn, where it is scheduled to arrive in 2004.

APOD: 2000 November 23 - Cassini At Jupiter: Red Spot Movie
Explanation: Everything is big on Jupiter, the solar system's reigning gas giant. For example, Jupiter's Great Red Spot is a hurricane-like storm system at least twice the diameter of planet Earth. Approaching Jupiter in early October the Cassini spacecraft recorded the images used in this excellent movie of the swirling storm system and planet-circling cloud bands. Seven mosaicked frames make up the movie sequence, each separated by one or two rotation periods (Jupiter rotates about once every 10 hours). The sequence is viewed as a simple cylindrical map projection spanning 50 degrees north to 50 degrees south of the Jovian equator. Can you see the small bright "clouds" which seem to suddenly appear west (left) of the Red Spot? Data from the Galileo spacecraft, orbiting Jupiter since 1996, suggest that these features are large lightning storms. Saturn-bound, the Cassini spacecraft will take a few months to fly by Jupiter, coordinating Jovian explorations with Galileo and picking up speed for the final leg of its interplanetary journey.

APOD: 2000 September 26 - Approaching Jupiter
Explanation: In 1979 the Voyager 1 spacecraft compiled this view as it approached the gas giant Jupiter. Snapping a picture every time the Great Red Spot was properly aligned, the above time-lapse sequence shows not only spot rotation but also the swirling of neighboring clouds. Since Jupiter takes about 10 hours to rotate, this short sequence actually covers several days. Voyager 1 shot past Jupiter rapidly taking pictures on which many discoveries would be made, including previously unknown cloud patterns, rings, moons, and active volcanoes on Jupiter's moon Io. Voyager is moving so fast that it will one day leave our Solar System.

APOD: 2000 March 1 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

APOD: 2000 January 4 - Galaxies Cluster Toward the Great Attractor
Explanation: Galaxies dot the sky like jewels in the direction of a mass so large it is known simply as the Great Attractor. The galaxies pictured above are part of a cluster of galaxies called ACO 3627 near the center of the Great Attractor. Previously, this cluster of galaxies, also known as the Norma Cluster, was largely unstudied because dust in the disk of our own Galaxy obscured much of its light. The Great Attractor is a diffuse mass concentration fully 250 million light-years away, but so large it pulls our own Milky Way Galaxy and millions of other galaxies towards it. Many of the galaxies in ACO 3627 are slowly heading towards collisions with each other.

APOD: December 26, 1999 - West Of The Great Red Spot
Explanation: The turbulent region West of Jupiter's Great Red Spot is highlighted in this picture constructed from data recorded by the Galileo spacecraft. The image is color coded to show cloud height and thickness; white clouds are high and thick, light blue clouds are high and thin, and reddish clouds are low. The edge of the Red Spot itself appears blue here (lower right) and spans about 10,000 kilometers along the curving limb of the planet (north is up). Westward winds, deflected north by the circulation within the Great Red Spot, collide with Eastward winds at higher latitudes and generate the roiling, Turbulent structures. The largest eddies near the Northwestern edge of the Red Spot are bright, suggesting upward convection and high altitude cloud formation are taking place there.

APOD: October 27, 1999 - In the Heart of the Tarantula Nebula
Explanation: In the heart of monstrous Tarantula Nebula lies one of the most unusual star clusters. Known as NGC 2070 or R136, it is home to a great number of hot young stars. The energetic light from these stars continually ionizes nebula gas, while their energetic particle wind blows bubbles and defines intricate filaments. The above representative-color picture of this great LMC cluster details its tumultuous center in gas, dust and young stars. The 30 Doradus nebula is one of the largest star-formation regions known, and has been creating unusually strong episodes of star formation every few million years. In the heart of this heart is a central knot of stars that is so dense it was once thought to be a single star.

APOD: August 6, 1999 - Hubble Tracks Jupiters Great Red Spot
Explanation: It is a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter's Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious. A better understanding of the weather on Jupiter may help contribute to the better understanding of weather here on Earth. In the pictures on the left, the Hubble Space Telescope has captured Jupiter's Great Red Spot in various states over the past several years.

APOD: July 19, 1999 - NGC 3372: The Great Nebula in Carina
Explanation: In one of the brightest parts of the Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebula. Eta Carina, the most energetic star in the nebula was one of the brightest stars in the sky in the 1830s, but then faded dramatically. The Keyhole Nebula, visible near the center, houses several of the most massive stars known and has also changed its appearance. The Carina Nebula is about 7000 light-years away in the constellation of Carina. The CTIO Curtis-Schmidt Telescope in Chile, South America took the above photograph. Eta Carina might explode in a dramatic supernova within the next thousand years, and has even flared in brightness over just the past two years.

APOD: July 18, 1999 - Jupiter from Voyager
Explanation: This picture of the planet Jupiter was taken by the Voyager 1 spacecraft as it passed the planet in 1979. Jupiter, a gas giant planet with no solid surface, is the largest planet in the Solar System and is made mostly of the hydrogen and helium. Clearly visible in the above photo is the Great Red Spot, a giant, hurricane-like storm system that rotates with the clouds of Jupiter. It is so large three complete Earths could fit inside it. Astronomers have recorded this giant storm on Jupiter for over 300 years.

APOD: July 10, 1999 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: May 22, 1999 - M42: A Mosaic of Orion's Great Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulae. Here, 15 pictures from the Hubble Space Telescope have been mosaicked to cover the inner 2.5 light years of the nebula and illustrate its diverse nature. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Most of the filamentary structures visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Shocks are particularly apparent near the bright stars in the lower left of the picture. The Orion Nebula is about 1500 light years distant, located in the same spiral arm of our Galaxy as the Sun.

APOD: March 14, 1999 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April 1997, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on 1997 March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during a lunar eclipse.

APOD: October 15, 1998 - A Great Day For SOHO
Explanation: The last 10 days have been great days for SOHO, the space-based SOlar and Heliospheric Observatory. Contact was completely lost with this international research spacecraft over 3 months ago but recovery teams have reacquired control of SOHO and, beginning October 5th, have been successfully switching on its scientific instruments. This October 13th view of the Sun in the light of ionized Helium atoms was recorded by the restored EIT instrument. It shows bright active regions and lofty prominences above the solar limb. North is toward the left rather than the top as the spacecraft's orientation has not yet been fully adjusted. (For a full Sun / full resolution view, click on the picture!) With the solar cycle approaching a maximum in the coming years, excitement continues to build as it becomes very likely that SOHO will be able to resume its unprecedented exploration of solar phenomena.

APOD: August 19, 1998 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first steps beyond the ordinary visible to the casual sky gazer. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

APOD: June 27, 1998 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: April 20, 1998 - Name This Satellite
Explanation: Can you name this satellite? In December, NASA's third Great Observatory is planned for launch. The two NASA Great Observatories currently in orbit are the Hubble Space Telescope and the Compton Gamma-Ray Observatory, both now named for famous scientists. But after whom should the Advanced X-ray Astrophysics Facility (AXAF) be named? If your submitted suggestion conforms with contest rules and is chosen, you will have named the most powerful X-ray satellite ever built, and may even win a prize. AXAF is the size of a bus, has strange mirrors polished to atomic smoothness, and will produce X-ray images five times clearer of objects twice as faint as any previous X-ray satellite. This should allow AXAF the ability to see X-rays emitted near small black holes, from distant active galaxies, and inside huge clusters of galaxies. Astronomers now hope for an uneventful launch, routine operations, and spectacular discoveries.

APOD: January 27, 1998 - The Great Nebula in Orion
Explanation: The Great Nebula in Orion can be found just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. This fuzzy patch contains one of the closest stellar nurseries, lying at a distance of about 1500 light years. In the above picture, the red region on the left consists of nebulae designated M42 and M43 and contains the bright Trapezium open cluster. The blue region on the right is a nebula primarily reflecting the light from internal bright stars. Recent observations of the Orion Nebula by the Hubble Space Telescope have located solar-system sized star-forming regions.

APOD: December 26, 1997 - West Of The Great Red Spot
Explanation: The turbulent region West of Jupiter's Great Red Spot is highlighted in this recent picture constructed from data recorded by the Galileo spacecraft. The image is color coded to show cloud height and thickness; white clouds are high and thick, light blue clouds are high and thin, and reddish clouds are low. The edge of the Red Spot itself appears blue here (lower right) and spans about 6,600 miles along the curving limb of the planet (north is up). Westward winds, deflected north by the circulation within the Great Red Spot, collide with Eastward winds at higher latitudes and generate the roiling, turbulent structures. The largest eddies near the Northwestern edge of the Red Spot are bright, suggesting upward convection and high altitude cloud formation are taking place there.

APOD: December 15, 1997 - A Farewell to Tails
Explanation: As 1997 fades, so does the Great Comet of 1997: Comet Hale-Bopp. Discovered even before the Great Comet of 1996, Comet Hale-Bopp became the brightest comet since 1976. Many will remember Comet Hale-Bopp as a comet with a coma so bright it could be seen by eye even when near the Moon. Others will remember spectacular photographs that appeared in magazines and on the web. Amateurs, inspired by the beauty of the comet, took most of these photographs. In particular, today APOD salutes Wally Pacholka, who took the above famous photograph. Mr. Pacholka reports that he repeatedly drove 150 miles to a national park, stayed up half the night, and took hundreds of photos while carefully waving a flashlight to momentarily illuminate the foreground. His equipment consisted only of a standard 35-mm camera which, for pointing accuracy, he piggybacked on a telescope bought at age 12 with money earned from a paper route.

APOD: November 25, 1997 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during the lunar eclipse.

APOD: August 8, 1997 - White Oval Clouds on Jupiter
Explanation: What are those white ovals all over Jupiter? Storms! Jupiter's clouds can swirl rapidly in raised high-pressure storm systems that circle the planet. The above pictured white ovals are located near the Great Red Spot, and have persisted on Jupiter since the 1930s. The Great Red Spot has persisted for at least 300 years. Currently, no one knows why ovals last as long as they do. White ovals are confined to circular belts around Jupiter, but can interact to cause nearby chaotic cloud regions.

APOD: May 29, 1997 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: May 11, 1997 - M42: A Mosaic of Orion's Great Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulae. Here, 15 pictures from the Hubble Space Telescope have been mosaicked to cover the inner 2.5 light years of the nebula and illustrate its diverse nature. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Most of the filamentary structures visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Shocks are particularly apparent near the bright stars in the lower left of the picture. The Orion Nebula is about 1500 light years distant, located in the same spiral arm of our Galaxy as the Sun.

APOD: March 25, 1997 - Hale-Bopp Brightest Comet This Century
Explanation: A comet as bright as Comet Hale-Bopp is very rare indeed. No comet has emitted or reflected this much light since possibly the Great Comet of 1811. However, since Comet Hale-Bopp is across the inner Solar System from us, it does not appear as bright as Comet West did in 1975. The Great Comet of 1996, Comet Hyakutake, was relatively dim but also appeared bright since it passed close to the Earth. Above, Comet Hale-Bopp was photographed high over the town of Las Palmas of the Spanish Canary Islands, on March 11^th.

APOD: March 13, 1997 - Hale-Bopp Brightest Comet This Decade
Explanation: The Great Comet of 1997 is now brighter than the Great Comet of 1996 ever was. In fact, it is brighter than almost every star in the sky. Yet Comet Hale-Bopp is still about two weeks away from maximum light. Comet Hale-Bopp is now well north of the plane of the Earth's orbit and on the same side of the sky as the Sun. Therefore, Comet Hale-Bopp is visible from Earth's Northern Hemisphere both just after sunset and just before sunrise. The above picture of Comet Hale-Bopp was taken last week in Italy. Many Milky Way stars and nebulae are visible. To the left is a rock face partly illuminated by artificial light.

APOD: March 11, 1997 - Jupiter: The Great Yellow Spot
Explanation: What happened to Jupiter's Great Red Spot? Operating at a chilly 55 degrees Kelvin, the Galileo Spacecraft's Near Infrared Mapping Spectrometer (NIMS) recorded this composite image of Jupiter's Great Red Spot in late June 1996. Red, green, and blue colors were chosen to represent three different infrared wavelengths detected by the NIMS instrument. The resulting yellowish green appearance of the massive Jovian storm system - a cold, high pressure area 2 to 3 Earth diameters wide - indicates that it lies high above the surrounding cloud features. Blue corresponds to regions where the clouds are relatively thin and the features lie at greater depths.

APOD: November 11, 1996 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlie it's name: The Jewel Box. The bright central star Kappa Crucis is red, in contrast to the many blue stars that surround it. The cluster contains just over 100 stars, and might be no older than 10 million years. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 7500 light-years away, so the light that we see today was emitted from the clusters before even the Great Pyramids in Egypt were built.

APOD: August 27, 1996 - Galileo Zooms in on Jupiter's Red Spot
Explanation: What does the largest storm system ever recorded look like close-up? This storm system is Jupiter's Great Red Spot and it was captured recently in detail by the robot spacecraft Galileo now in orbit around Jupiter. Using real images from three color filters, the Galileo team was able to compute what a person would see if able to float just above this ancient rotating cloud system. But don't get too close - remember that Jupiter's Great Red Spot is a cold, high pressure area more than twice as wide as planet Earth.

APOD: August 2, 1996 - Galileo, Cassini, and the Great Red Spot
Explanation: Imagine a hurricane that lasted for 300 years! Jupiter's Great Red Spot indeed seems to be a giant hurricane-like storm system rotating with the Jovian clouds. Observed in 1655 by Italian-French astronomer Jean-Dominique Cassini it is seen here over 300 years later - still going strong - in a mosaic of recent Galileo spacecraft images. The Great Red Spot is a cold, high pressure area 2-3 times wider than planet Earth. Its outer edge rotates in a counter clockwise direction about once every six days. Jupiter's own rapid rotation period is a brief 10 hours. The Solar System's largest gas giant planet, it is presently well placed for evening viewing. (APOD thanks to Alan Radecki for assembling a preliminary mosaic from the Galileo imagery!)

APOD: May 8, 1996 - Neptune's Great Dark Spot: Gone But Not Forgotten
Explanation: When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up. Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby. When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!

APOD: February 18, 1996 - Abell 3627 in the Great Attractor
Explanation: Are these galaxies near the center of the largest gravitationally bound concentration of mass yet known? Previously, the cluster of galaxies known as Abell 3627 was largely unstudied because dust in the disk of our own Galaxy obscured much of its light. Several galaxies from Abell 3627 appear above as fuzzy blue patches behind many stars in our Galaxy. Recent observations by Renee Kraan-Korteweg (Paris Observatory) and collaborators, however, indicate that this cluster of galaxies is near the center of the huge nearby conglomeration of mass known as the Great Attractor. Evidence for this was uncovered in new accurate measurements of the large extent and nearby distance of Abell 3627.

APOD: February 17, 1996 - Edwin Hubble Discovers the Universe
Explanation: No person in history has had greater impact in determining the extent of our universe than Edwin Hubble. From proving that other galaxies existed to proving that galaxies move apart from one another, Hubble's work defined our place in the cosmos. Hubble lived from 1889 to 1953 and is shown above posing with the 48-inch telescope on Palomar Mountain and his famous pipe. In memory of his great work, the Orbiting Space Telescope was named after him. Today a great controversy rages on the rate of the universe's expansion, parameterized by a quantity known as Hubble's constant. A real live debate on this subject will take place in Washington, DC this April.

APOD: February 8, 1996 - Hyakutake: The Great Comet of 1996?
Explanation: Get ready for one of the most impressive but least anticipated light shows in modern astronomical history. Next month, newly discovered Comet Hyakutake will pass closer to the Earth than any recent comet. Unknown before its discovery by Yuji Hyakutake on 30 January 1996, the fuzzy spot in the above photograph is a comet now predicted to become bright enough to see without a telescope. Although comets act in such diverse ways that predictions are frequently inaccurate, even conservative estimates indicate that this comet is likely to impress. For example, even if Comet Hyakutake remains physically unchanged, its close pass near the Earth in late March 1996 should cause it to appear to brighten to about 3rd magnitude - still bright enough to see with the unaided eye. In the next two months, though, the comet will continue to approach the Sun and hence should become brighter still. Optimistic predictions include that Comet Hyakutake will change physically, develop a larger coma and tail, brighten dramatically, move noticeably in the sky during a single night, and may ultimately become known as the "The Great Comet of 1996." Move over Hale-Bopp!

APOD: November 21, 1995 - M42: Orion Nebula Mosaic
Explanation: The Great Nebula in Orion is one of the most interesting of all astronomical nebulae known. Here fifteen pictures from the Hubble Space Telescope have been merged to show the great expanse and diverse nature of the nebula. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Much of the filamentary structure visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Some shock waves are visible near one of the bright stars in the lower left of the picture. The Orion Nebula is located in the same spiral arm of our Galaxy as is our Sun. It takes light about 1500 years to reach us from there.

APOD: July 26, 1995 - M15: A Great Globular Cluster
Explanation: A globular cluster is a system of about one million stars that together orbit a galaxy. One of the brightest globular clusters in our Milky Way galaxy is the pictured M15, the fifteenth object on Messier's list of diffuse objects on our sky. Most stars in globular clusters are older and redder than our Sun, which is about 5 billion years old.

APOD: July 3, 1995 - The Great Nebula in Orion
Explanation: The Great Nebula in Orion, M42, can be found on the night sky just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. This nebula is one of the closest stellar nurseries - where young stars are being formed even now. Clumps of gas (mostly hydrogen and helium) and dust in the nebula are squeezed together by their own gravity until they collapse and form stars. Some stars we can see here partially obscured by the nebula, are only about 100,000 years old - just babies compared to the 5 billion (5,000,000,000) years of our Sun.