Astronomy Picture of the Day Search Results for "Saturn"

APOD: 2024 June 23 – The Colors of Saturn from Cassini
Explanation: What creates Saturn's colors? The featured picture of Saturn only slightly exaggerates what a human would see if hovering close to the giant ringed world. The image was taken in 2005 by the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Here Saturn's majestic rings appear directly only as a curved line, appearing brown, in part from its infrared glow. The rings best show their complex structure in the dark shadows they create across the upper part of the planet. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why some of Saturn's clouds are colored gold.

APOD: 2023 August 25 - A Season of Saturn
Explanation: Ringed planet Saturn will be at its 2023 opposition, opposite the Sun in Earth's skies, on August 27. While that puts the sixth planet from the Sun at its brightest and well-placed for viewing, its beautiful ring system isn't visible to the unaided eye. Still, this sequence of six telescopic images taken a year apart follows both Saturn and rings as seen from inner planet Earth. The gas giant's ring plane tilts from most open in 2018 to approaching edge-on in 2023 (top to bottom). That's summer to nearly the autumn equinox for Saturn's northern hemisphere. In the sharp planetary portraits Saturn's northern hexagon and a large storm system are clearly visible in 2018. In 2023 ice moon Tethys is transiting, casting its shadow across southern hemisphere cloud bands while Saturn's cold blue south pole is emerging from almost a decade of winter darkness.

APOD: 2023 April 30 – Saturn's Moon Helene in Color
Explanation: Although its colors may be subtle, Saturn's moon Helene is an enigma in any light. The moon was imaged in unprecedented detail in 2012 as the robotic Cassini spacecraft orbiting Saturn swooped to within a single Earth diameter of the diminutive moon. Although conventional craters and hills appear, the above image also shows terrain that appears unusually smooth and streaked. Planetary astronomers are inspecting these detailed images of Helene to glean clues about the origin and evolution of the 30-km across floating iceberg. Helene is also unusual because it circles Saturn just ahead of the large moon Dione, making it one of only four known Saturnian moons to occupy a gravitational dimple known as a stable Lagrange point.

APOD: 2022 December 23 - Cassini Looks Out from Saturn
Explanation: This is what Saturn looks like from inside the rings. In 2017, for the first time, NASA directed the Cassini spacecraft to swoop between Saturn and its rings. During the dive, the robotic spacecraft took hundreds of images showing unprecedented detail for structures in Saturn's atmosphere. Looking back out, however, the spacecraft was also able to capture impressive vistas. In the featured image, taken a few hours before closest approach, Saturn's unusual northern hexagon is seen surrounding the North Pole. Saturn's B ring is the closest visible, while the dark Cassini Division separates B from the outer A. A close inspection will find the two small moons that shepherd the F-ring, the farthest ring discernable. A few months after this image was taken -- and after more than a decade of exploration and discovery -- the Cassini spacecraft ran low on fuel and was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2022 November 26 - Saturn at Night
Explanation: Saturn is still bright in planet Earth's night skies. Telescopic views of the distant gas giant and its beautiful rings often make it a star at star parties. But this stunning view of Saturn's rings and night side just isn't possible from telescopes closer to the Sun than the outer planet. They can only bring Saturn's day into view. In fact, this image of Saturn's slender sunlit crescent with night's shadow cast across its broad and complex ring system was captured by the Cassini spacecraft. A robot spacecraft from planet Earth, Cassini called Saturn orbit home for 13 years before it was directed to dive into the atmosphere of the gas giant on September 15, 2017. This magnificent mosaic is composed of frames recorded by Cassini's wide-angle camera only two days before its grand final plunge. Saturn's night will not be seen again until another spaceship from Earth calls.

APOD: 2022 July 9 - Saturn and ISS
Explanation: Soaring high in skies around planet Earth, bright planet Saturn was a star of June's morning planet parade. But very briefly on June 24 it posed with a bright object in low Earth orbit, the International Space Station. On that date from a school parking lot in Temecula, California the ringed-planet and International Space Station were both caught in this single high-speed video frame. Though Saturn was shining at +0.5 stellar magnitude the space station was an even brighter -3 on the magnitude scale. That difference in brightness is faithfully represented in the video capture frame. In the challenging image, the orbiting ISS was at a range of 602 kilometers. Saturn was about 1.4 billion kilometers from the school parking lot.

APOD: 2022 April 9 - Mars-Saturn Conjunction
Explanation: Fainter stars in the zodiacal constellation Capricornus are scattered near the plane of the ecliptic in this field of view. The two brightest ones at center aren't stars at all though, but the planets Mars and Saturn. Taken on the morning of April 4, the telescopic snapshot captured their tantalizing close conjunction in a predawn sky, the pair of planets separated by only about 1/3 of a degree. That's easily less than the apparent width of a Full Moon. Can you tell which planet is which? If you guessed Mars is the redder one , you'd be right. Above Mars, slightly fainter Saturn still shines with a paler yellowish tinge in reflected sunlight. Even at the low magnification, Saturn's largest and brightest moon Titan can be spotted hugging the planet very closely on the left.

APOD: 2022 January 23 - Saturn, Tethys, Rings, and Shadows
Explanation: Seen from ice moon Tethys, rings and shadows would display fantastic views of the Saturnian system. Haven't dropped in on Tethys lately? Then this gorgeous ringscape from the Cassini spacecraft will have to do for now. Caught in sunlight just below and left of picture center in 2005, Tethys itself is about 1,000 kilometers in diameter and orbits not quite five saturn-radii from the center of the gas giant planet. At that distance (around 300,000 kilometers) it is well outside Saturn's main bright rings, but Tethys is still one of five major moons that find themselves within the boundaries of the faint and tenuous outer E ring. Discovered in the 1980s, two very small moons Telesto and Calypso are locked in stable along Tethys' orbit. Telesto precedes and Calypso follows Tethys as the trio circles Saturn.

APOD: 2022 January 4 - Moons Beyond Rings at Saturn
Explanation: What's happened to that moon of Saturn? Nothing -- Saturn's moon Rhea is just partly hidden behind Saturn's rings. In 2010, the robotic Cassini spacecraft then orbiting Saturn took this narrow-angle view looking across the Solar System's most famous rings. Rings visible in the foreground include the thin F ring on the outside and the much wider A and B rings just interior to it. Although it seems to be hovering over the rings, Saturn's moon Janus is actually far behind them. Janus is one of Saturn's smaller moons and measures only about 180 kilometers across. Farther out from the camera is the heavily cratered Rhea, a much larger moon measuring 1,500 kilometers across. The top of Rhea is visible only through gaps in the rings. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2021 September 19 - Rings and Seasons of Saturn
Explanation: On Saturn, the rings tell you the season. On Earth, Wednesday marks an equinox, the time when the Earth's equator tilts directly toward the Sun. Since Saturn's grand rings orbit along the planet's equator, these rings appear most prominent -- from the direction of the Sun -- when the spin axis of Saturn points toward the Sun. Conversely, when Saturn's spin axis points to the side, an equinox occurs and the edge-on rings are hard to see from not only the Sun -- but Earth. In the featured montage, images of Saturn between the years of 2004 and 2015 have been superposed to show the giant planet passing from southern summer toward northern summer. Saturn was as close as it can get to planet Earth last month, and this month the ringed giant is still bright and visible throughout much of the night

APOD: 2021 September 11 - Saturn at Night
Explanation: Still bright in planet Earth's night skies, good telescopic views of Saturn and its beautiful rings often make it a star at star parties. But this stunning view of Saturn's rings and night side just isn't possible from telescopes closer to the Sun than the outer planet. They can only bring Saturn's day into view. In fact, this image of Saturn's slender sunlit crescent with night's shadow cast across its broad and complex ring system was captured by the Cassini spacecraft. A robot spacecraft from planet Earth, Cassini called Saturn orbit home for 13 years before it was directed to dive into the atmosphere of the gas giant on September 15, 2017. This magnificent mosaic is composed of frames recorded by Cassini's wide-angle camera only two days before its grand final plunge. Saturn's night will not be seen again until another spaceship from Earth calls.

APOD: 2021 July 6 - Saturn and Six Moons
Explanation: How many moons does Saturn have? So far 82 have been confirmed, the smallest being only a fraction of a kilometer across. Six of its largest satellites can be seen here in a composite image with 13 short exposure of the bright planet, and 13 long exposures of the brightest of its faint moons, taken over two weeks last month. Larger than Earth's Moon and even slightly larger than Mercury,Saturn's largest moon Titan has a diameter of 5,150 kilometers and was captured making nearly a complete orbit around its ringed parent planet. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, in contrast with several newly discovered moons announced in 2019. The trail on the far right belongs to Iapetus, Saturn's third largest moon. The radius of painted Iapetus' orbit is so large that only a portion of it was captured here. Saturn leads Jupiter across the night sky this month, rising soon after sunset toward the southeast, and remaining visible until dawn.

APOD: 2021 June 27 - The Dancing Auroras of Saturn
Explanation: What drives auroras on Saturn? To help find out, scientists have sorted through hundreds of infrared images of Saturn taken by the Cassini spacecraft for other purposes, trying to find enough aurora images to correlate changes and make movies. Once made, some movies clearly show that Saturnian auroras can change not only with the angle of the Sun, but also as the planet rotates. Furthermore, some auroral changes appear related to waves in Saturn's magnetosphere likely caused by Saturn's moons. Pictured here, a false-colored image taken in 2007 shows Saturn in three bands of infrared light. The rings reflect relatively blue sunlight, while the planet itself glows in comparatively low energy red. A band of southern aurora in visible in green. In has recently been found that auroras heat Saturn's upper atmosphere. Understanding Saturn's auroras is a path toward a better understanding of Earth's auroras.

APOD: 2021 April 4 - In, Through, and Beyond Saturn's Rings
Explanation: Four moons are visible on the featured image -- can you find them all? First -- and farthest in the background -- is Titan, the largest moon of Saturn and one of the larger moons in the Solar System. The dark feature across the top of this perpetually cloudy world is the north polar hood. The next most obvious moon is bright Dione, visible in the foreground, complete with craters and long ice cliffs. Jutting in from the left are several of Saturn's expansive rings, including Saturn's A ring featuring the dark Encke Gap. On the far right, just outside the rings, is Pandora, a moon only 80-kilometers across that helps shepherd Saturn's F ring. The fourth moon? If you look closely inside Saturn's rings, in the Encke Gap, you will find a speck that is actually Pan. Although one of Saturn's smallest moons at 35-kilometers across, Pan is massive enough to help keep the Encke gap relatively free of ring particles. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2021 January 9 - Titan: Moon over Saturn
Explanation: Like Earth's moon, Saturn's largest moon Titan is locked in synchronous rotation. This mosaic of images recorded by the Cassini spacecraft in May of 2012 shows its anti-Saturn side, the side always facing away from the ringed gas giant. The only moon in the solar system with a dense atmosphere, Titan is the only solar system world besides Earth known to have standing bodies of liquid on its surface and an earthlike cycle of liquid rain and evaporation. Its high altitude layer of atmospheric haze is evident in the Cassini view of the 5,000 kilometer diameter moon over Saturn's rings and cloud tops. Near center is the dark dune-filled region known as Shangri-La. The Cassini-delivered Huygens probe rests below and left of center, after the most distant landing for a spacecraft from 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 August 8 - Crescent Saturn
Explanation: From Earth, Saturn never shows a crescent phase. But when viewed from a spacecraft the majestic giant planet can show just a sunlit slice. This image of crescent Saturn in natural color was taken by the robotic Cassini spacecraft in 2007. It captures Saturn's rings from the side of the ring plane opposite the Sun -- the unilluminated side -- another vista not visible from Earth. Visible are subtle colors of cloud bands, the complex shadows of the rings on the planet, and the shadow of the planet on the rings. The moons Mimas, at 2 o'clock, and Janus 4 o'clock, can be seen as specks of light, but the real challenge is to find Pandora (8 o'clock). From Earth, Saturn's disk is nearly full now and opposite the Sun. Along with bright fellow giant planet Jupiter it rises in the early evening.

APOD: 2020 May 27 - Earth and Moon through Saturn's Rings
Explanation: What are those dots between Saturn's rings? Our Earth and Moon. Just over three years ago, because the Sun was temporarily blocked by the body of Saturn, the robotic Cassini spacecraft was able to look toward the inner Solar System. There, it spotted our Earth and Moon -- just pin-pricks of light lying about 1.4 billion kilometers distant. Toward the right of the featured image is Saturn's A ring, with the broad Encke Gap on the far right and the narrower Keeler Gap toward the center. On the far left is Saturn's continually changing F Ring. From this perspective, the light seen from Saturn's rings was scattered mostly forward , and so appeared backlit. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2020 April 19 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and hundreds of thousands more since entering orbit. Some of Cassini's early images have been digitally tweaked, cropped, and compiled into the featured inspiring video which is part of a larger developing IMAX movie project named In Saturn's Rings. In the concluding sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2020 March 30 - The Colors of Saturn from Cassini
Explanation: What creates Saturn's colors? The featured picture of Saturn only slightly exaggerates what a human would see if hovering close to the giant ringed world. The image was taken in 2005 by the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Here Saturn's majestic rings appear directly only as a curved line, appearing brown, in part, from its infrared glow. The rings best show their complex structure in the dark shadows they create across the upper part of the planet. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why some of Saturn's clouds are colored gold.

APOD: 2019 December 29 - Cassini Spacecraft Crosses Saturn's Ring Plane
Explanation: If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn frequently crossed Saturn's ring plane during its mission to Saturn, from 2004 to 2017. A series of plane crossing images from 2005 February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured here, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold. Details of Saturn's rings can be seen in the high dark shadows across the top of this image, taken back in 2005. The moons Dione and Enceladus appear as bumps in the rings.

APOD: 2019 November 9 - Saturn the Giant
Explanation: On May 25, 1961 U.S. president John Kennedy announced the goal of landing astronauts on the Moon by the end of the decade. By November 9, 1967 this Saturn V rocket was ready for launch and the first full test of its capabilities on the Apollo 4 mission. Its development directed by rocket pioneer Wernher Von Braun, the three stage Saturn V stood over 36 stories tall. It had a cluster of five first stage engines fueled by liquid oxygen and kerosene which together were capable of producing 7.9 million pounds of thrust. Giant Saturn V rockets ultimately hurled nine Apollo missions to the Moon and back again with six landing on the lunar surface. The first landing mission, Apollo 11, achieved Kennedy's goal on July 20, 1969.

APOD: 2019 November 3 - Daphnis and the Rings of Saturn
Explanation: What's happening to the rings of Saturn? A little moon making big waves. The moon is 8-kilometer Daphnis and it is making waves in the Keeler Gap of Saturn's rings using just its gravity -- as it bobs up and down, in and out. The featured image is a colored and more detailed version of a previously released images taken in 2017 by the robotic Cassini spacecraft during one of its Grand Finale orbits. Daphnis can be seen on the far right, sporting ridges likely accumulated from ring particles. Daphnis was discovered in Cassini images in 2005 and raised mounds of ring particles so high in 2009 -- during Saturn's equinox when the ring plane pointed directly at the Sun -- that they cast notable shadows.

APOD: 2019 October 17 - Moons of Saturn
Explanation: On July 29, 2011 the Cassini spacecraft's narrow-angle camera took this snapshot and captured 5 of Saturn's moons, from just above the ringplane. Left to right are small moons Janus and Pandora respectively 179 and 81 kilometers across, shiny 504 kilometer diameter Enceladus, and Mimas, 396 kilometers across, seen just next to Rhea. Cut off by the right edge of the frame, Rhea is Saturn's second largest moon at 1,528 kilometers across. So how many moons does Saturn have? Twenty new found outer satellites bring its total to 82 known moons, and since Jupiter's moon total stands at 79, Saturn is the Solar System's new moon king. The newly announced Saturnian satellites are all very small, 5 kilometers or so in diameter, and most are in retrograde orbits inclined to Saturn's ringplane. You can help name Saturn's new moons, but you should understand the rules. Hint: A knowledge of Norse, Inuit, and Gallic mythology will help.

APOD: 2019 September 20 - Saturn at Night
Explanation: Still bright in planet Earth's night skies, good telescopic views of Saturn and its beautiful rings often make it a star at star parties. But this stunning view of Saturn's rings and night side just isn't possible from telescopes closer to the Sun than the outer planet. They can only bring Saturn's day into view. In fact, this image of Saturn's slender sunlit crescent with night's shadow cast across its broad and complex ring system was captured by the Cassini spacecraft. A robot spacecraft from planet Earth, Cassini called Saturn orbit home for 13 years before it was directed to dive into the atmosphere of the gas giant on September 15, 2017. This magnificent mosaic is composed of frames recorded by Cassini's wide-angle camera only two days before its grand final plunge. Saturn's night will not be seen again until another spaceship from Earth calls.

APOD: 2019 September 15 - A Long Storm System on Saturn
Explanation: It was one of the largest and longest lived storms ever recorded in our Solar System. First seen in late 2010, the above cloud formation in the northern hemisphere of Saturn started larger than the Earth and soon spread completely around the planet. The storm was tracked not only from Earth but from up close by the robotic Cassini spacecraft then orbiting Saturn. Pictured here in false colored infrared in February, orange colors indicate clouds deep in the atmosphere, while light colors highlight clouds higher up. The rings of Saturn are seen nearly edge-on as the thin blue horizontal line. The warped dark bands are the shadows of the rings cast onto the cloud tops by the Sun to the upper left. A source of radio noise from lightning, the intense storm was thought to relate to seasonal changes when spring emerges in the north of Saturn. After raging for over six months, the iconic storm circled the entire planet and then tried to absorb its own tail -- which surprisingly caused it to fade away.

APOD: 2019 August 14 - Saturn Behind the Moon
Explanation: What's that next to the Moon? Saturn. In its monthly trip around the Earth -- and hence Earth's sky -- our Moon passed nearly in front of Sun-orbiting Saturn earlier this week. Actually the Moon passed directly in front of Saturn from the viewpoints of a wide swath of Earth's Southern Hemisphere. The featured image from Sydney, Australia captured the pair a few minutes before the eclipse. The image was a single shot lasting only 1/500th of a second, later processed to better highlight both the Moon and Saturn. Since Saturn is nearly opposite the Sun, it can be seen nearly the entire night, starting at sunset, toward the south and east. The gibbous Moon was also nearly opposite the Sun, and so also visible nearly the entire night -- it will be full tomorrow night. The Moon will occult Saturn again during every lap it makes around the Earth this year.

APOD: 2019 July 7 - Crescent Saturn
Explanation: Saturn never shows a crescent phase -- from Earth. But when viewed from beyond, the majestic giant planet can show an unfamiliar diminutive sliver. This image of crescent Saturn in natural color was taken by the robotic Cassini spacecraft in 2007. The featured image captures Saturn's majestic rings from the side of the ring plane opposite the Sun -- the unilluminated side -- another vista not visible from Earth. Pictured are many of Saturn's photogenic wonders, including the subtle colors of cloud bands, the complex shadows of the rings on the planet, and the shadow of the planet on the rings. A careful eye will find the moons Mimas (2 o'clock) and Janus (4 o'clock), but the real challenge is to find Pandora (8 o'clock). Saturn is now nearly opposite from the Sun in the Earth's sky and so can be seen in the evening starting just after sunset for the rest of the night.

APOD: 2019 May 5 - Saturn, Titan, Rings, and Haze
Explanation: This is not a solar eclipse. Pictured here is a busy vista of moons and rings taken at Saturn. The large circular object in the center of the image is Titan, the largest moon of Saturn and one of the most intriguing objects in the entire Solar System. The dark spot in the center is the main solid part of the moon. The bright surrounding ring is atmospheric haze above Titan, gas that is scattering sunlight to a camera operating onboard the robotic Cassini spacecraft. Cutting horizontally across the image are the rings of Saturn, seen nearly edge on. At the lower right of Titan is Enceladus, a small moon of Saturn. Since the image was taken pointing nearly at the Sun, the surfaces of Titan and Enceladus appear in silhouette, and the rings of Saturn appear similar to a photographic negative. Now if you look really really closely at Enceladus, you can see a hint of icy jets shooting out toward the bottom of the image. It is these jets that inspired future proposals to land on Enceladus, burrow into the ice, and search for signs of extraterrestrial life.

APOD: 2019 April 9 - Moon Occults Saturn
Explanation: Sometimes Saturn disappears. It doesn't really go away, though, it just disappears from view when our Moon moves in front. Such a Saturnian eclipse was visible along a small swath of Earth -- from Brazil to Sri Lanka -- near the end of last month. The featured color image is a digital fusion of the clearest images captured by successive videos of the event taken in red, green, and blue, and taken separately for Saturn and the comparative bright Moon. The exposures were taken from South Africa just before occultation -- and also just before sunrise. When Saturn re-appeared on the other side of the Moon almost two hours later, the Sun had risen. This year, eclipses of Saturn by the Moon occur almost monthly, but, unfortunately, are visible only to those with the right location and with clear and dark skies.

APOD: 2018 September 29 - 55 Nights with Saturn
Explanation: For 55 consecutive nights Mediterranean skies were at least partly clear this summer, from the 1st of July to the 24th of August 2018. An exposure from each night was incorporated in this composited telephoto and telescopic image to follow bright planet Saturn as it wandered through the generous evening skies. Through August, the outer planet's seasonal apparent retrograde motion slowed and drifted to the right, framed by a starry background. That brought it near the line-of-sight to the central Milky Way, and the beautiful Lagoon (M8) and Trifid (M20) nebulae. Of course Saturn's largest moon Titan was also along for the ride. Swinging around the gas giant in a 16 day long orbit, Titan's resulting wave-like motion is easier to spot when the almost-too-bright Saturn is digitally edited from the scene.

APOD: 2018 September 7 - Saturn's North Polar Hexagon
Explanation: In full view, the amazing six-sided jet stream known as Saturn's north polar hexagon is shown in this colorful Cassini image. Extending to 70 degrees north latitude, the false-color video frame is map-projected, based on infrared, visible, and ultraviolet image data recorded by the Saturn-orbiting spacecraft in late 2012. First found in the outbound Voyager flyby images from the 1980s, the bizarre, long-lived feature tied to the planet's rotation is about 30,000 kilometers across. At its center lies the ringed gas giant's hurricane-like north polar storm. A new long term study of Cassini data has found a remarkable higher-altitude vortex, exactly matching the outlines of the north polar hexagon, that formed as summer approached the planet's northern hemisphere. It appears to reach hundreds of kilometers above these deeper cloud tops, into Saturn's stratosphere.

APOD: 2018 April 24 - Play Saturn's Rings Like a Harp
Explanation: Sure, you've seen Saturn's rings -- but have you ever heard them? Well then please take this opportunity to play Saturn's rings like a harp. In the featured sonification, increasing brighter regions of Saturn's central B-ring play as increasingly higher pitched notes. With a computer browser, click anywhere on the image to begin, and pluck consecutive strings by sliding the spacecraft icon's magnetometer boom across the strings. Both manual and automatic modes are possible. The featured natural-color image was taken by the late Cassini spacecraft in 2017 July as it grazed Saturn's rings and took the highest-resolution ring images ever. The reason why the mostly water-ice rings have a tan hue -- instead of white -- is currently a topic of research. Played in minor harmony, a different false-color version of the same image appears where regions with a greater abundance of water ice appears more red.

APOD: 2018 April 2 - Moons, Rings, Shadows, Clouds: Saturn (Cassini)
Explanation: While cruising around Saturn, be on the lookout for picturesque juxtapositions of moons, rings, and shadows. One quite picturesque arrangement occurred in 2005 and was captured by the then Saturn-orbiting Cassini spacecraft. In the featured image, moons Tethys and Mimas are visible on either side of Saturn's thin rings, which are seen nearly edge-on. Across the top of Saturn are dark shadows of the wide rings, exhibiting their impressive complexity. The violet-light image brings up the texture of the backdrop: Saturn's clouds. Cassini orbited Saturn from 2004 until September of last year, when the robotic spacecraft was directed to dive into Saturn to keep it from contaminating any moons.

APOD: 2017 September 26 - Cassinis Last Ring Portrait at Saturn
Explanation: How should Cassini say farewell to Saturn? Three days before plunging into Saturn's sunny side, the robotic Cassini spacecraft swooped far behind Saturn's night side with cameras blazing. Thirty-six of these images have been merged -- by an alert and adept citizen scientist -- into a last full-ring portrait of Cassini's home planet for the past 13 years. The Sun is just above the frame, causing Saturn to cast a dark shadow onto its enormous rings. This shadow position cannot be imaged from Earth and will not be visible again until another Earth-launched spaceship visits the ringed giant. Data and images from Cassini's mission-ending dive into Saturn's atmosphere on September 15 continue to be analyzed.

APOD: 2017 September 11 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and hundreds of thousands more since entering orbit. Some of Cassini's early images have been digitally tweaked, cropped, and compiled into the featured inspiring video which is part of a larger developing IMAX movie project named In Saturn's Rings. In the concluding sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends. The Cassini spacecraft itself, low on fuel, is scheduled to end on Friday when it will be directed to approach so close to Saturn that it falls in and melts.

APOD: 2017 September 4 - Saturn's Rings from the Inside Out
Explanation: What do Saturn's rings look like from Saturn? Images from the robotic spacecraft Cassini are providing humanity with this unprecedented vantage point as it nears the completion of its mission. Previous to Cassini's Grand Finale orbits, all images of Saturn's majestic ring system were taken from outside of the rings looking in. Pictured in the inset is the remarkable video, while the spacecraft's positions are depicted in the surrounding animation. Details of the complex rings are evident as the short time-lapse sequence begins, while the paper-thin thickness of the rings becomes apparent near the video's end. The featured images were taken on August 20. Cassini has only a few more orbits around Saturn left before it is directed to dive into the giant planet on September 15.

APOD: 2017 August 29 - Saturn in Blue and Gold
Explanation: Why is Saturn partly blue? The featured picture of Saturn approximates what a human would see if hovering close to the giant ringed world. The image was taken in 2006 March by the robot Cassini spacecraft now orbiting Saturn. Here Saturn's majestic rings appear directly only as a thin vertical line. The rings show their complex structure in the dark shadows they create on the image left. Saturn's fountain moon Enceladus, only about 500 kilometers across, is seen as the bump in the plane of the rings. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why Saturn's clouds are colored gold. Next month, Cassini will end its mission with a final dramatic dive into Saturn's atmosphere.

APOD: 2017 August 8 - Density Waves in Saturn's Rings from Cassini
Explanation: What causes the patterns in Saturn's rings? The Cassini spacecraft, soon ending its 13 years orbiting Saturn, has sent back another spectacular image of Saturn's immense ring system in unprecedented detail. The physical cause for some of Saturn's ring structures is not always understood. The cause for the beautifully geometric type of ring structure shown here in ring of Saturn, however, is surely a density wave. A small moon systematically perturbing the orbits of ring particles circling Saturn at slightly different distances causes such a density wave bunching. Also visible on the lower right of the image is a bending wave, a vertical wave in ring particles also caused by the gravity of a nearby moon. Cassini's final orbits are allowing a series of novel scientific measurements and images of the Solar System's most grand ring system.

APOD: 2017 July 28 - Noodle Mosaic of Saturn
Explanation: On April 26 the Cassini spacecraft swooped toward Saturn on the first of its Grand Finale dives between Saturn and rings. In this long, thin, noodle mosaic, a rapid series of 137 low resolution images captured by Cassini's wide-angle camera track its progress across the gas giant's swirling cloud tops. The mosaic projection maps the arc along Saturn's atmospheric curve on to a flat image plane. At top, the first mosaic panel is centered at 90 degrees north, about 72,400 kilometers above Saturn's dark north polar vortex. As the mosaic progresses it narrows, the pixel scale shrinking from 8.7 kilometers to 1 kilometer per pixel. For the last panel, the spacecraft is 8,374 kilometers above a region 18 degrees north of Saturn's equator. Frame orientation changes near the bottom as Cassini rotates to maneuver its large, dish-shaped, high-gain antenna forward, providing a shield before crossing Saturn's ring plane.

APOD: 2017 June 18 - Views from Cassini at Saturn
Explanation: What has the Cassini orbiter seen at Saturn? The featured music video shows some of the early highlights. In the first time-lapse sequence (00:07), a vertical line appears that is really Saturn's thin rings seen nearly edge-on. Soon some of Saturn's moon shoot past. The next sequence (00:11) features Saturn's unusually wavy F-ring that is constrained by the two shepherd moons that are also continually perturbing it. Soon much of Saturn's extensive ring system flashes by, sometimes juxtaposed to the grandeur of the immense planet itself. Cloud patterns on Titan (00:39) and Saturn (00:41) are highlighted. Clips from flybys of several of Saturn's moon are then shown, including Phoebe, Mimas, Epimetheus, and Iapetus. In other sequences, moons of Saturn appear to pass each other as they orbit Saturn. Background star fields seen by Cassini are sometimes intruded upon by bright passing moons. The robotic Cassini spacecraft has been revolutionizing humanity's knowledge of Saturn and its moons since 2004. In September, Cassini's mission will be brought to a dramatic conclusion as the spacecraft will be directed to dive into ringed giant.

APOD: 2017 June 17 - Saturn near Opposition
Explanation: Saturn reached its 2017 opposition on June 16. Of course, opposition means opposite the Sun in Earth's sky and near opposition Saturn is up all night, at its closest and brightest for the year. This remarkably sharp image of the ringed planet was taken only days before, on June 11, with a 1-meter telescope from the mountain top Pic du Midi observatory. North is at the top with the giant planet's north polar storm and curious hexagon clearly seen bathed in sunlight. But Saturn's spectacular ring system is also shown in stunning detail. The narrow Encke division is visible around the entire outer A ring, small ringlets can be traced within the fainter inner C ring, and Saturn's southern hemisphere can be glimpsed through the wider Cassini division. Near opposition Saturn's rings also appear exceptionally bright, known as the opposition surge or Seeliger Effect. Directly illuminated from Earth's perspective, the ring's icy particles cast no shadows and strongly backscatter sunlight creating the dramatic increase in brightness. Still, the best views of the ringed planet are currently from the Saturn-orbiting Cassini spacecraft. Diving close, Cassini's Grand Finale orbit number 9 is in progress.

APOD: 2017 June 10 - Saturn in the Milky Way
Explanation: Saturn is near opposition in planet Earth's sky. Rising at sunset and shining brightly throughout the night, it also lies near a line-of-sight to crowded starfields, nebulae, and obscuring dust clouds along the Milky Way. Whitish Saturn is up and left of center in this gorgeous central Milky Way skyscape, a two panel mosaic recorded earlier this month. You can find the bright planet above the bowl of the dusty Pipe nebula, and just beyond the end of a dark river to Antares, alpha star of the constellation Scorpius. For now the best views of the ringed giant planet are from the Saturn-orbiting Cassini spacecraft, though. Diving close, Cassini's Grand Finale orbit number 8 is in progress.

APOD: 2017 April 30 - Cassini Looks Out from Saturn
Explanation: This is what Saturn looks like from inside the rings. Last week, for the first time, NASA directed the Cassini spacecraft to swoop between Saturn and its rings. During the dive, the robotic spacecraft took hundreds of images showing unprecedented detail for structures in Saturn's atmosphere. Looking back out, however, the spacecraft was also able to capture impressive vistas. In the featured image taken a few hours before closest approach, Saturn's unusual northern hexagon is seen surrounding the North Pole. Saturn's B ring is the closest visible, while the dark Cassini Division separates B from the outer A. A close inspection will find the two small moons that shepherd the F-ring, the farthest ring discernable. This image is raw and will be officially verified, calibrated and released at a later date. Cassini remains on schedule to end its mission by plunging into Saturn's atmosphere on September 15.

APOD: 2017 April 3 - Saturn in Infrared from Cassini
Explanation: Many details of Saturn appear clearly in infrared light. Bands of clouds show great structure, including long stretching storms. Also quite striking in infrared is the unusual hexagonal cloud pattern surrounding Saturn's North Pole. Each side of the dark hexagon spans roughly the width of our Earth. The hexagon's existence was not predicted, and its origin and likely stability remains a topic of research. Saturn's famous rings circle the planet and cast shadows below the equator. The featured image was taken by the robotic Cassini spacecraft in 2014 in several infrared colors -- but only processed recently. In September, Cassini's mission will be brought to a dramatic conclusion as the spacecraft will be directed to dive into ringed giant.

APOD: 2017 March 13 - Saturn's Moon Pan from Cassini
Explanation: Why does Saturn's moon Pan look so odd? Images taken last week from the robotic Cassini spacecraft orbiting Saturn have resolved the moon in unprecedented detail. The surprising images reveal a moon that looks something like a walnut with a slab through its middle. Other visible features on Pan include rolling terrain, long ridges, and a few craters. Spanning 30-kilometer across, Pan orbits inside the 300-kilometer wide Encke Gap of Saturn's expansive A-ring, a gap known since the late 1800s. Next month, Cassini will be directed to pass near Saturn's massive moon Titan so it can be pulled into a final series of orbits that will take it, on occasion, completely inside Saturn's rings and prepare it to dive into Saturn's atmosphere.

APOD: 2017 February 22 - Daphnis and the Rings of Saturn
Explanation: What's happening to the rings of Saturn? Nothing much, just a little moon making waves. The moon is 8-kilometer Daphnis and it is making waves in the Keeler Gap of Saturn's rings using just its gravity -- as it bobs up and down, in and out. The featured image is a wide-field version of a previously released image taken last month by the robotic Cassini spacecraft during one of its new Grand Finale orbits. Daphnis can be seen on the far right, sporting ridges likely accumulated from ring particles. Daphnis was discovered in Cassini images in 2005 and raised mounds of ring particles so high in 2009 -- during Saturn's equinox when the ring plane pointed directly at the Sun -- that they cast notable shadows.

APOD: 2017 January 25 - Cassini's Grand Finale Tour at Saturn
Explanation: Cassini is being prepared to dive into Saturn. The robotic spacecraft that has been orbiting and exploring Saturn for over a decade will end its mission in September with a spectacular atmospheric plunge. Pictured here is a diagram of Cassini's remaining orbits, each taking about one week. Cassini is scheduled to complete a few months of orbits that will take it just outside Saturn's outermost ring F. Then, in April, Titan will give Cassini a gravitational pull into Proximal orbits, the last of which, on September 15, will impact Saturn and cause the spacecraft to implode and melt. Cassini's Grand Finale orbits are designed to record data and first-ever views from inside the rings -- between the rings and planet -- as well as some small moons interspersed in the rings. Cassini's demise is designed to protect any life that may occur around Saturn or its moons from contamination by Cassini itself.

APOD: 2017 January 3 - Pandora Close up at Saturn
Explanation: What do the craters of Saturn's small moon Pandora look like up close? To help find out, NASA sent the robotic Cassini spacecraft, now orbiting Saturn, past the unusual moon two weeks ago. The highest resolution image of Pandora ever taken was then captured from about 40,000 kilometers out and is featured here. Structures as small as 300 meters can be discerned on 80-kilometer wide Pandora. Craters on Pandora appear to be covered over by some sort of material, providing a more smooth appearance than sponge-like Hyperion, another small moon of Saturn. Curious grooves and ridges also appear to cross the surface of the small moon. Pandora is partly interesting because, along with its companion moon Prometheus, it helps shepherd the particles of Saturn's F ring into a distinct ring.

APOD: 2016 December 12 - Over Saturn's Turbulent North Pole
Explanation: The Cassini spacecraft's Grand Finale at Saturn has begun. The Grand Finale will allow Cassini to explore Saturn and some of Saturn's moons and rings in unprecedented detail. The first phase started two weeks ago when a close flyby of Titan changed Cassini's orbit into one that passes near Saturn's poles and just outside of Saturn's outermost F-ring. Featured here is an image taken during the first of Cassini's 20 week-long F-ring orbits around Saturn. Visible are the central polar vortex on the upper left, a hexagonal cloud boundary through the image center, and numerous light-colored turbulent storm systems. In 2017 April, Cassini will again use the gravity of Titan to begin a new series of 22 Proximal orbits -- trajectories that will take Cassini inside of Saturn's rings for the first time. Cassini's new science adventure is scheduled to end on 2017 September 17, though, when the robotic spacecraft will be directed into a dramatic mission-ending dive into Saturn's atmosphere.

APOD: 2016 October 26 - Propeller Shadows on Saturn's Rings
Explanation: What created these unusually long shadows on Saturn's rings? The dark shadows -- visible near the middle of the image -- extend opposite the Sun and, given their length, stem from objects having heights up to a few kilometers. The long shadows were unexpected given that the usual thickness of Saturn's A and B rings is only about 10 meters. After considering the choppy but elongated shapes apparent near the B-ring edge, however, a leading theory has emerged that some kilometer-sized moonlets exist there that have enough gravity to create even larger vertical deflections of nearby small ring particles. The resulting ring waves are called propellers, named for how they appear individually. It is these coherent groups of smaller ring particles that are hypothesized to be casting the long shadows. The featured image was taken by the robotic Cassini spacecraft currently orbiting Saturn. The image was captured in 2009, near Saturn's equinox, when sunlight streamed directly over the ring plane and caused the longest shadows to be cast.

APOD: 2016 September 25 - Saturn from Above
Explanation: This image of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the ringed planet is visible from the Earth. In fact, this image mosaic was taken earlier this year by the robotic Cassini spacecraft now orbiting Saturn, just before filming a 44-hour video of Saturn rotating. The beautiful rings of Saturn are seen in full expanse, while cloud details are visible including the polar hexagon surrounding the north pole. The Cassini mission is now in its final year as the spacecraft is scheduled to be programmed to dive into Saturn's atmosphere next September.

APOD: 2016 September 15 - Retrograde Mars and Saturn
Explanation: Wandering Mars and Saturn have spent much of this year remarkably close in planet Earth's night sky. In a sequence of exposures spanning mid-December 2015 through the beginning of this week, this composited skyview follows their time together, including both near opposition, just north of bright star Antares near the Milky Way's central bulge. In the corresponding video, Saturn's apparent movement is seen to be back and forth along the flattened, compact loop, while Mars traces the wider, reversing S-shaped track from upper right to lower left through the frame. To connect the dots and dates just slide your cursor over the picture (or follow this link). It looks that way, but Mars and Saturn don't actually reverse direction along their orbits. Instead, their apparent backwards or retrograde motion with respect to the background stars is a reflection of the orbital motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own relatively close-in orbit.

APOD: 2016 August 1 - Behind Saturn
Explanation: What's behind Saturn? The first answer is the camera itself, perched on the Cassini spacecraft currently orbiting behind the planet with the most grand ring system in our Solar System. The unusual perspective places Cassini on the far side of Saturn from the Sun so that more than half of Saturn appears dark -- a perspective that no Earth-based observer could achieve. Behind Saturn, in the context of the featured infrared image, is Saturn's moon Tethys, visible as the small speck above the unusual hexagonal cloud pattern that encompasses Saturn's North Pole. Tethys actually orbits Saturn right in the ring plane, which places the 1000-km moon much farther from Cassini than the planet itself. Cassini has been studying Saturn and its moons for 12 years, but, unfortunately, its amazing mission will soon come to an end. In order to protect life that may exist on or inside Saturn's moons, the robotic spacecraft will be directed to crash into Saturn's thick atmosphere next September.

APOD: 2016 March 13 - Neon Saturn
Explanation: If seen in the right light, Saturn glows like a neon sign. Although Saturn has comparatively little of the element neon, a composite image false-colored in three bands of infrared light highlights features of the giant ringed planet like a glowing sign. At the most blue band of the infrared light featured, false-colored blue in the above image, Saturn itself appears dark but Saturn's thin rings brightly reflect light from our Sun. Conversely, Saturn's B ring is so thick that little reflected light makes it through, creating a dark band between Saturn's A and C rings. At the most red band of the infrared, false-colored red above, Saturn emits a surprisingly detailed thermal glow, indicating planet-wide bands, huge hurricane-like storms, and a strange hexagon-shaped cloud system around the North Pole. In the middle infrared band, false-colored green, the sunlit side of Saturn's atmosphere reflects brightly. The above image was obtained in 2007 by the robotic Cassini spacecraft orbiting about 1.6 million kilometers out from Saturn.

APOD: 2015 August 24 - Dione, Rings, Shadows, Saturn
Explanation: What's happening in this strange juxtaposition of moon and planet? First and foremost, Saturn's moon Dione was captured here in a dramatic panorama by the robotic Cassini spacecraft currently orbiting the giant planet. The bright and cratered moon itself spans about 1100-km, with the large multi-ringed crater Evander visible on the lower right. Since the rings of Saturn are seen here nearly edge-on, they are directly visible only as a thin horizontal line that passes behind Dione. Arcing across the bottom of the image, however, are shadows of Saturn's rings, showing some of the rich texture that could not be seen directly. In the background, few cloud features are visible on Saturn. The featured image was taken during the last planned flyby of Dione by Cassini, as the spacecraft is scheduled to dive into Saturn's atmosphere during 2017.

APOD: 2015 June 21 - Rings and Seasons of Saturn
Explanation: On Saturn, the rings tell you the season. On Earth, today marks a solstice, the time when the Earth's spin axis tilts directly toward the Sun. On Earth's northern hemisphere, today is the Summer Solstice, the day of maximum daylight. Since Saturn's grand rings orbit along the planet's equator, these rings appear most prominent -- from the direction of the Sun -- when the Saturn's spin axis points toward the Sun. Conversely, when Saturn's spin axis points to the side, an equinox occurs and the edge-on rings are hard to see. In the featured montage, images of Saturn over the past 11 years have been superposed to show the giant planet passing from southern summer toward northern summer. Although Saturn will only reach its northern summer solstice in 2017 May, the image of Saturn most analogous to today's Earth solstice is the bottommost one.

APOD: 2015 May 29 - Saturn at Opposition
Explanation: Telescopic observers on Earth have been treated to spectacular views of Saturn lately as the ringed planet reached its 2015 opposition on May 23 at 0200 UT. Of course opposition means opposite the Sun in Earth's sky. So near opposition Saturn is up all night, at its closest and brightest for the year. These sharp images taken within hours of the Sun-Earth-Saturn alignment also show the strong brightening of Saturn's rings known as the opposition surge or the Seeliger Effect. Directly illuminated, the ring's icy particles cast no shadows and strongly backscatter sunlight toward planet Earth, creating the dramatic surge in brightness. Saturn currently stands in the sky not far from bright Antares, alpha star of the constellation Scorpius.

APOD: 2015 April 5 - Saturn, Tethys, Rings, and Shadows
Explanation: Seen from ice moon Tethys, rings and shadows would display fantastic views of the Saturnian system. Haven't dropped in on Tethys lately? Then this gorgeous ringscape from the Cassini spacecraft will have to do for now. Caught in sunlight just below and left of picture center in 2005, Tethys itself is about 1,000 kilometers in diameter and orbits not quite five saturn-radii from the center of the gas giant planet. At that distance (around 300,000 kilometers) it is well outside Saturn's main bright rings, but Tethys is still one of five major moons that find themselves within the boundaries of the faint and tenuous outer E ring. Discovered in the 1980s, two very small moons Telesto and Calypso are locked in stable locations along Tethys' orbit. Telesto precedes and Calypso follows Tethys as the trio circles Saturn.

APOD: 2015 January 4 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few years ago. As striking as the above image is, it is but a single frame from a 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center.

APOD: 2014 September 21 - Saturn at Equinox
Explanation: How would Saturn look if its ring plane pointed right at the Sun? Before August 2009, nobody knew. Every 15 years, as seen from Earth, Saturn's rings point toward the Earth and appear to disappear. The disappearing rings are no longer a mystery -- Saturn's rings are known to be so thin and the Earth is so near the Sun that when the rings point toward the Sun, they also point nearly edge-on at the Earth. Fortunately, in this third millennium, humanity is advanced enough to have a spacecraft that can see the rings during equinox from the side. In August 2009, that Saturn-orbiting spacecraft, Cassini, was able to snap a series of unprecedented pictures of Saturn's rings during equinox. A digital composite of 75 such images is shown above. The rings appear unusually dark, and a very thin ring shadow line can be made out on Saturn's cloud-tops. Objects sticking out of the ring plane are brightly illuminated and cast long shadows. Inspection of these images is helping humanity to understand the specific sizes of Saturn's ring particles and the general dynamics of orbital motion. This week, Earth undergoes an equinox.

APOD: 2014 July 16 - The Moon Eclipses Saturn
Explanation: What happened to half of Saturn? Nothing other than Earth's Moon getting in the way. As pictured above on the far right, Saturn is partly eclipsed by a dark edge of a Moon itself only partly illuminated by the Sun. This year the orbits of the Moon and Saturn have led to an unusually high number of alignments of the ringed giant behind Earth's largest satellite. Technically termed an occultation, the above image captured one such photogenic juxtaposition from Buenos Aires, Argentina that occurred early last week. Visible to the unaided eye but best viewed with binoculars, there are still four more eclipses of Saturn by our Moon left in 2014. The next one will be on August 4 and visible from Australia, while the one after will occur on August 31 and be visible from western Africa at night but simultaneously from much of eastern North America during the day.

APOD: 2014 April 13 - Saturn in Blue and Gold
Explanation: Why is Saturn partly blue? The above picture of Saturn approximates what a human would see if hovering close to the giant ringed world. The above picture was taken in 2006 March by the robot Cassini spacecraft now orbiting Saturn. Here Saturn's majestic rings appear directly only as a thin vertical line. The rings show their complex structure in the dark shadows they create on the image left. Saturn's fountain moon Enceladus, only about 500 kilometers across, is seen as the bump in the plane of the rings. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why Saturn's clouds are colored gold.

APOD: 2013 November 13 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn drifted in giant planet's shadow earlier this year and looked back toward the eclipsed Sun. Cassini saw a unique and celebrated view. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, Saturn's expansive ring system appears as majestic as always even from this odd angle. Ring particles, many glowing only as irregular crescents, slightly scatter sunlight toward Cassini in this natural color image. Several moons and ring features are also discernible. Appearing quite prominently is Saturn's E ring, the ring created by the unusual ice-fountains of the moon Enceladus and the outermost ring visible above. To the upper left, far in the distance, are the planets Mars and Venus. To the lower right, however, is perhaps the most wondrous spectacle of all: the almost invisible, nearly ignorable, pale blue dot of Earth.

APOD: 2013 October 21 - Saturn from Above
Explanation: This image of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the ringed planet is visible from the Earth. In fact, this image mosaic was taken earlier this month by the robotic Cassini spacecraft now orbiting Saturn. The beautiful rings of Saturn are seen in full expanse, while cloud details are visible including the polar hexagon surrounding the north pole, and an extended light-colored storm system.

APOD: 2013 August 24 - Earth Waves at Saturn
Explanation: This friendly photo collage is constructed from more than 1,400 images shared by denizens of planet Earth as part of the Cassini Mission's July 19th Wave at Saturn event. The base picture of Earth corresponds to the view from the Saturn-orbiting Cassini spacecraft on that date, when its own cameras recorded images including planet Earth as a pale blue dot in the background. Of course, Saturn was 9.65 Astronomical Units away at the time, so it took light from all the waving Earth dwellers just over 80 minutes to travel there. Want to smile? Download and zoom in to the full-resolution (28MB jpg file) collage image available here.

APOD: 2013 July 29 - Saturn, Titan, Rings, and Haze
Explanation: This is not a solar eclipse. Pictured above is a busy vista of moons and rings taken at Saturn. The large circular object in the center of the image is Titan, the largest moon of Saturn and one of the most intriguing objects in the entire Solar System. The dark spot in the center is the main solid part of the moon. The bright surrounding ring is atmospheric haze above Titan, gas that is scattering sunlight to a camera operating onboard the robotic Cassini spacecraft. Cutting horizontally across the image are the rings of Saturn, seen nearly edge on. At the lower right of Titan is Enceladus, a small moon of Saturn. Since the image was taken pointing nearly at the Sun, the surfaces of Titan and Enceladus appear in silhouette, and the rings of Saturn appear similar to a photographic negative. Now if you look really really closely at Enceladus, you can see a hint of icy jets shooting out toward the bottom of the image. It is these jets that inspired future proposals to land on Enceladus, burrow into the ice, and search for signs of extraterrestrial life.

APOD: 2013 July 22 - Earth and Moon from Saturn
Explanation: You are here. Everyone you've ever known is here. Every human who has ever lived -- is here. Pictured above is the Earth-Moon system as captured by the Cassini mission orbiting Saturn in the outer Solar System. Earth is the brighter and bluer of the two spots near the center, while the Moon is visible to its lower right. Images of Earth from Saturn were taken on Friday. Quickly released unprocessed images were released Saturday showing several streaks that are not stars but rather cosmic rays that struck the digital camera while it was taking the image. The above processed image was released earlier today. At nearly the same time, many humans on Earth were snapping their own pictures of Saturn.

APOD: 2013 July 21 - The Seasons of Saturn
Explanation: Since Saturn's axis is tilted as it orbits the Sun, Saturn has seasons, like those of planet Earth ... but Saturn's seasons last for over seven years. So what season is it on Saturn now? Orbiting the equator, the tilt of the rings of Saturn provides quite a graphic seasonal display. Each year until 2016, Saturn's rings will be increasingly apparent after appearing nearly edge-on in 2009. The ringed planet is also well placed in evening skies providing a grand view as summer comes to Saturn's northern hemisphere and winter to the south. The Hubble Space Telescope took the above sequence of images about a year apart, starting on the left in 1996 and ending on the right in 2000. Although they look solid, Saturn's Rings are likely less than 50 meters thick and consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2013 July 19 - Take a Picture of Saturn
Explanation: Take a picture of Saturn in the sky tonight. You could capture a view like this one. Recorded just last month looking toward the south, planet Earth and ruins of the ancient temple of Athena at Assos, Turkey are in the foreground. The Moon rises at the far left of the frame and Saturn is the bright "star" at the upper right, near Virgo's alpha star Spica (picture with labels). If you do take a picture of Saturn or wave at Saturn and take a picture, you can share it online and submit it to the Saturn Mosaic Project. Why take a picture tonight? Because the Cassini spacecraft will be orbiting Saturn and taking a picture of you.

APOD: 2013 April 28 - A Raging Storm System on Saturn
Explanation: It was one of the largest and longest lived storms ever recorded in our Solar System.   First seen in late 2010, the above cloud formation in the northern hemisphere of Saturn started larger than the Earth and soon spread completely around the planet. The storm was tracked not only from Earth but from up close by the robotic Cassini spacecraft currently orbiting Saturn. Pictured above in false colored infrared in February, orange colors indicate clouds deep in the atmosphere, while light colors highlight clouds higher up. The rings of Saturn are seen nearly edge-on as the thin blue horizontal line. The warped dark bands are the shadows of the rings cast onto the cloud tops by the Sun to the upper left. A source of radio noise from lightning, the intense storm was thought to relate to seasonal changes when spring emerges in the north of Saturn. After raging for over six months, the iconic storm circled the entire planet and then tried to absorb its own tail -- which surprisingly caused it to fade away.

APOD: 2013 April 7 - The Moon's Saturn
Explanation: Just days after sharing the western evening sky with Venus in 2007, the Moon moved on to Saturn - actually passing in front of the ringed planet Saturn when viewed in skies over Europe, northern Africa, and western Asia. Because the Moon and bright planets wander through the sky near the ecliptic plane, such occultation events are not uncommon, but they are dramatic, especially in telescopic views. For example, in this sharp image Saturn is captured emerging from behind the Moon, giving the illusion that it lies just beyond the Moon's bright edge. Of course, the Moon is a mere 400 thousand kilometers away, compared to Saturn's distance of 1.4 billion kilometers. Taken with a digital camera and 20 inch diameter telescope at the Weikersheim Observatory in southern Germany, the picture is a single exposure adjusted to reduce the difference in brightness between Saturn and the cratered lunar surface.

APOD: 2013 February 20 - Saturn's Hexagon and Rings
Explanation: Why would clouds form a hexagon on Saturn? Nobody is sure. Originally discovered during the Voyager flybys of Saturn in the 1980s, nobody has ever seen anything like it anywhere else in the Solar System. If Saturn's South Pole wasn't strange enough with its rotating vortex, Saturn's North Pole might be considered even stranger. The bizarre cloud pattern is shown above in great detail by a recent image taken by the Saturn-orbiting Cassini spacecraft. This and similar images show the stability of the hexagon even 20+ years after Voyager. Movies of Saturn's North Pole show the cloud structure maintaining its hexagonal structure while rotating. Unlike individual clouds appearing like a hexagon on Earth, the Saturn cloud pattern appears to have six well defined sides of nearly equal length. Four Earths could fit inside the hexagon. Imaged from the side, the dark shadow of the Jovian planet is seen eclipsing part of its grand system of rings, partly visible on the upper right.

APOD: 2012 December 31 - Saturn's Rings from the Dark Side
Explanation: What do Saturn's rings look like from the dark side? From Earth, we usually see Saturn's rings from the same side of the ring plane that the Sun illuminates them -- one might call this the bright side. Geometrically, in the above picture taken in August by the robot Cassini spacecraft now orbiting Saturn, the Sun is behind the camera but on the other side of the ring plane. Such a vantage point gives a breathtaking views of the most splendid ring system in the Solar System. Strangely, the rings have similarities to a photographic negative of a front view. For example, the dark band in the middle is actually the normally bright B-ring. The ring brightness as recorded from different angles indicates ring thickness and particle density of ring particles. At the top left of the frame is Saturn's moon Tethys, which although harder to find, contains much more mass than the entire ring system.

APOD: 2012 September 16 - Saturn: Bright Tethys and Ancient Rings
Explanation: How old are Saturn's rings? No one is quite sure. One possibility is that the rings formed relatively recently in our Solar System's history, perhaps only about 100 million years ago when a moon-sized object broke up near Saturn. Evidence for a young ring age includes a basic stability analysis for rings, and the fact that the rings are so bright and relatively unaffected by numerous small dark meteor impacts. More recent evidence, however, raises the possibility that some of Saturn's rings may be billions of years old and so almost as old as Saturn itself. Inspection of images by the Saturn-orbiting Cassini spacecraft indicates that some of Saturn's ring particles temporarily bunch and collide, effectively recycling ring particles by bringing fresh bright ices to the surface. Seen here, Saturn's rings were imaged in their true colors by the robotic Cassini in late October. Icy bright Tethys, a moon of Saturn likely brightened by a sandblasting rain of ice from sister moon Enceladus, is visible in front of the darker rings.

APOD: 2012 July 3 - In the Shadow of Saturn's Rings
Explanation: Humanity's robot orbiting Saturn has recorded yet another amazing view. That robot, of course, is the spacecraft Cassini, while the new amazing view includes a bright moon, thin rings, oddly broken clouds, and warped shadows. Titan, Saturn's largest moon, appears above as a featureless tan as it is continually shrouded in thick clouds. The rings of Saturn are seen as a thin line because they are so flat and imaged nearly edge on. Details of Saturn's rings are therefore best visible in the dark ring shadows seen across the giant planet's cloud tops. Since the ring particles orbit in the same plane as Titan, they appear to skewer the foreground moon. In the upper hemisphere of Saturn, the clouds show many details, including dips in long bright bands indicating disturbances in a high altitude jet stream. Recent precise measurements of how much Titan flexes as it orbits Saturn hint that vast oceans of water might exist deep underground.

APOD: 2012 May 31 - Lantern Saturn
Explanation: Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels. The infrared glow also shines from the broad shadows of Saturn's rings sweeping across the planet's upper hemisphere.

APOD: 2012 May 21 - A Close Pass of Saturn's Moon Dione
Explanation: What's that past Dione? When making its closest pass yet of Saturn's moon Dione late last year, the robotic Cassini spacecraft snapped this far-ranging picture featuring Dione, Saturn's rings, and the two small moons Epimetheus and Prometheus. The above image captures part of the heavily cratered snow-white surface of the 1,100 kilometer wide Dione, the thinness of Saturn's rings, and the comparative darkness of the smaller moon Epimetheus. The image was taken when Cassini was only about 100,000 kilometers from the large icy moon. Future events in Cassini's continuing exploration of Saturn and its moons include tomorrow's flyby of Titan and imaging the distant Earth passing behind Saturn in June.

APOD: 2012 May 2 - Saturn's Moon Helene in Color
Explanation: Although its colors may be subtle, Saturn's moon Helene is an enigma in any light. The moon was imaged in unprecedented detail last June as the robotic Cassini spacecraft orbiting Saturn swooped to within a single Earth diameter of the diminutive moon. Although conventional craters and hills appear, the above image also shows terrain that appears unusually smooth and streaked. Planetary astronomers are inspecting these detailed images of Helene to glean clues about the origin and evolution of the 30-km across floating iceberg. Helene is also unusual because it circles Saturn just ahead of the large moon Dione, making it one of only four known Saturnian moons to occupy a gravitational well known as a stable Lagrange point.

APOD: 2012 April 14 - Six Moons of Saturn
Explanation: How many moons does Saturn have? So far 62 have been discovered, the smallest only a fraction of a kilometer across. Six of its largest satellites can be seen here, though, in a sharp Saturnian family portrait taken on March 9. Larger than Earth's Moon and even slightly larger than Mercury, Titan has a diameter of 5,150 kilometers and starts the line-up at the lower left. Continuing to the right across the frame are Mimas, Tethys, [Saturn], Enceladus, Dione, and Rhea at far right. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, while most recently the satellite provisionally designated S/2009 S1 was found by the Cassini Imaging Science Team in 2009. Tonight, Saturn reaches opposition in planet Earth's sky, offering the best telescopic views of the ringed planet and moons.

APOD: 2012 January 22 - Saturn's Hexagon Comes to Light
Explanation: Believe it or not, this is the North Pole of Saturn. It is unclear how an unusual hexagonal cloud system that surrounds Saturn's north pole was created, keeps its shape, or how long it will last. Originally discovered during the Voyager flybys of Saturn in the 1980s, nobody has ever seen anything like it elsewhere in the Solar System. Although its infrared glow was visible previously to the Cassini spacecraft now orbiting Saturn, in 2009 the mysterious hexagonal vortex became fully illuminated by sunlight for the first time during the Cassini's visit. Since then, Cassini has imaged the rotating hexagon in visible light enough times to create a time-lapse movie. The pole center was not well imaged and has been excluded. This movie shows many unexpected cloud motions, such as waves emanating from the corners of the hexagon. Planetary scientists are sure to continue to study this most unusual cloud formation for quite some time.

APOD: 2011 December 26 - A Raging Storm System on Saturn
Explanation: It is one of the largest and longest lived storms ever recorded in our Solar System. First seen late last year, the above cloud formation in the northern hemisphere of Saturn started larger than the Earth and soon spread completely around the planet. The storm has been tracked not only from Earth but from up close by the robotic Cassini spacecraft currently orbiting Saturn. Pictured above in false colored infrared in February, orange colors indicate clouds deep in the atmosphere, while light colors highlight clouds higher up. The rings of Saturn are seen nearly edge-on as the thin blue horizontal line. The warped dark bands are the shadows of the rings cast onto the cloud tops by the Sun to the upper left. A source of radio noise from lightning, the intense storm may relate to seasonal changes as spring slowly emerges in the north of Saturn.

APOD: 2011 October 26 - In, Through, and Beyond Saturn's Rings
Explanation: A fourth moon is visible on the above image if you look hard enough. First -- and farthest in the background -- is Titan, the largest moon of Saturn and one of the larger moons in the Solar System. The dark feature across the top of this perpetually cloudy world is the north polar hood. The next most obvious moon is bright Dione, visible in the foreground, complete with craters and long ice cliffs. Jutting in from the left are several of Saturn's expansive rings, including Saturn's A ring featuring the dark Encke Gap. On the far right, just outside the rings, is Pandora, a moon only 80-kilometers across that helps shepherd Saturn's F ring. The fourth moon? If you look closely in the Encke Gap you'll find a speck that is actually Pan. Although one of Saturn's smallest moons at 35-kilometers across, Pan is massive enough to help keep the Encke gap relatively free of ring particles.

APOD: 2011 October 12 - Saturn: Shadows of a Seasonal Sundial
Explanation: Saturn's rings form one of the larger sundials known. This sundial, however, determines only the season of Saturn, not the time of day. In 2009, during Saturn's last equinox, Saturn's thin rings threw almost no shadows onto Saturn, since the ring plane pointed directly toward the Sun. As Saturn continued in its orbit around the Sun, however, the ring shadows become increasingly wider and cast further south. These shadows are not easily visible from the Earth because from our vantage point near the Sun, the rings always block the shadows. The above image was taken in August by the robotic Cassini spacecraft currently orbiting Saturn. The rings themselves appear as a vertical bar on the image right. The Sun, far to the upper right, shines through the rings and casts captivatingly complex shadows on south Saturn, on the image left. Cassini has been exploring Saturn, its rings, and its moons since 2004, and is expected to continue until at least the maximum elongation of Saturn's shadows occurs in 2017.

APOD: 2011 September 4 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn drifted in giant planet's shadow for about 12 hours in 2006 and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn, slightly scattering sunlight, in this exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the image. Seen in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus and the outermost ring visible above. Far in the distance, at the left, just above the bright main rings, is the almost ignorable pale blue dot of Earth.

APOD: 2011 July 8 - Saturn Storm Panoramas
Explanation: These tantalizing panoramas follow a remarkable giant storm encircling the northern hemisphere of ringed planet Saturn. Still active, the roiling storm clouds were captured in near-infrared images recorded by the Cassini spacecraft on February 26 and stitched into the high resolution, false-color mosaics. Seen late last year as a prominent bright spot by amateur astronomers when Saturn rose in predawn skies, the powerful storm has grown to enormous proportions. Its north-south extent is nearly 15,000 kilometers and it now stretches completely around the gas giant's northern hemisphere some 300,000 kilometers. Taken about one Saturn day (11 hours) apart, the panoramas show the head of the storm at the left and cover about 150 degrees in longitude. Also a source of radio noise from lightning, the intense storm may be related to seasonal changes as Saturn experiences northern hemisphere spring.

APOD: 2011 June 13 - Views from Cassini at Saturn
Explanation: What has the Cassini orbiter seen since arriving at Saturn? The above music video shows some of the highlights. In the first time-lapse sequence (00:07), a vertical line appears that is really Saturn's thin rings seen nearly edge-on. Soon some of Saturn's moon shoot past. The next sequence (00:11) features Saturn's unusually wavy F-ring that is constrained by the two shepherd moons that are also continually perturbing it. Soon much of Saturn's extensive ring system flashes by, sometimes juxtaposed to the grandeur of the immense planet itself. Cloud patterns on Titan (00:39) and Saturn (00:41) are highlighted. Clips from flyby's of several of Saturn's moon are then shown, including Phoebe, Mimas, Epimetheus, and Iapetus. In other sequences, moons of Saturn appear to pass each other as they orbit Saturn. Background star fields seen by Cassini are sometimes intruded upon by bright passing moons. The robotic Cassini spacecraft has been revolutionizing humanity's knowledge of Saturn and its moons since 2004.

APOD: 2011 March 15 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and thousands more since entering orbit. Recently, some of these images have been digitally tweaked, cropped, and compiled into the above inspiring video which is part of a larger developing IMAX movie project named Outside In. In the last sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends.

APOD: 2011 March 8 - Titan, Rings, and Saturn from Cassini
Explanation: How thin are the rings of Saturn? Brightness measurements from different angles have shown Saturn's rings to be about one kilometer thick, making them many times thinner, in relative proportion, than a razor blade. This thinness sometimes appears in dramatic fashion during an image taken nearly along the ring plane. The robot Cassini spacecraft now orbiting Saturn has now captured another shot that dramatically highlights the ring's thinness. The above image was taken in mid January in infrared and polarized light. Titan looms just over the thin rings, while dark ring shadows on Saturn show the Sun to be above the ring plane. Close inspection of the image will show the smaller moon Enceladus on the far right. Cassini, humanity's first mission to orbit Saturn, currently has operations planned until 2017.

APOD: 2011 January 19 - Saturn Storm
Explanation: Late last year, a new, remarkably bright storm erupted in Saturn's northern hemisphere. Amateur astronomers first spotted it in early December, with the ringed gas giant rising in planet Earth's predawn sky. Orbiting Saturn, the Cassini spacecraft was able to record this close-up of the complex disturbance from a distance of 1.8 million kilometers on December 24th. Over time, the storm has evolved, spreading substantially in longitude, and now stretches far around the planet. Saturn's thin rings are also seen slicing across this space-based view, casting broad shadows on the planet's southern hemisphere.

APOD: 2010 October 12 - Saturn: Light, Dark, and Strange
Explanation: What's creating those dark bands on Saturn? Sometimes it takes a little sleuthing to figure out the how and why of a picture taken by the Saturn-orbiting Cassini spacecraft. Let's see. That large orb on the left must be Saturn itself. Those arcs on the right are surely the rings. The dark band running diagonally must be the shadow of Saturn on the rings. That leaves the unusual dark bands superposed on Saturn's disk -- are they the shadows of the rings? A punctilious detective would conclude that they are not. If one looks carefully, the rings arc from behind the planet on the lower left, around to the right, and therefore must pass on the camera side of the planet on the upper left. So the rings themselves cause the dark streaks on Saturn. These rings segments appear dark because they are in the shadow of Saturn. The night part of Saturn shows a faint glow because of sunlight reflected from other parts of the rings. Got it? Unfortunately, if it weren't for the tile floor, tomorrow's picture would be even harder to understand.

APOD: 2010 September 27 - The Dancing Auroras of Saturn
Explanation: What drives auroras on Saturn? To help find out, scientists have sorted through hundreds of infrared images of Saturn taken by the Cassini spacecraft for other purposes, trying to find enough aurora images to correlate changes and make movies. Once made, some movies clearly show that Saturnian auroras can change not only with the angle of the Sun, but also as the planet rotates. Furthermore, some auroral changes appear related to waves in Saturn's magnetosphere likely caused by Saturn's moons. Pictured above, a false-colored image taken in 2007 shows Saturn in three bands of infrared light. The rings reflect relatively blue sunlight, while the planet itself glows in comparatively low energy red. A band of southern aurora in visible in green. Inspection of many more Saturnian images may well lead to an even better understanding of both Saturn's and Earth's auroras.

APOD: 2010 August 2 - Prometheus Creating Saturn Ring Streamers
Explanation: What's causing those strange dark streaks in the rings of Saturn? Prometheus. Specifically, an orbital dance involving Saturn's moon Prometheus keeps creating unusual light and dark streamers in the F-Ring of Saturn. Now Prometheus orbits Saturn just inside the thin F-ring, but ventures into its inner edge about every 15 hours. Prometheus' gravity then pulls the closest ring particles toward the 80-km moon. The result is not only a stream of bright ring particles but also a dark ribbon where ring particles used to be. Since Prometheus orbits faster than the ring particles, the icy moon pulls out a new streamer every pass. Above, several streamers or kinks are visible at once. The above photograph was taken in June by the robotic Cassini Spacecraft orbiting Saturn. The oblong moon Prometheus is visible on the far left.

APOD: 2010 July 12 - Moons Beyond the Rings of Saturn
Explanation: What's happened to that moon of Saturn? Nothing -- Saturn's moon Rhea is just partly hidden behind Saturn's rings. In April, the robotic Cassini spacecraft now orbiting Saturn took this narrow-angle view looking across the Solar System's most famous rings. Rings visible in the foreground include the thin F ring on the outside and the much wider A and B rings just interior to it. Although it seems to be hovering over the rings, Saturn's moon Janus is actually far behind them. Janus is one of Saturn's smaller moons and measures only about 180 kilometers across. Farther out from the camera is the heavily cratered Rhea, a much larger moon measuring 1,500 kilometers across. The top of Rhea is visible only through gaps in the rings. The Cassini mission around Saturn has been extended to 2017 to better study the complex planetary system as its season changes from equinox to solstice.

APOD: 2010 May 31 - Moons and Rings Before Saturn
Explanation: While cruising around Saturn, be on the lookout for picturesque juxtapositions of moons and rings. Another striking alignment occurred last March in the view of humanity's Saturn-orbiting Cassini spacecraft. Rhea, one of Saturn's larger moons, was caught passing Epimetheus, one of Saturn's smaller moons. Epimetheus, as pictured above, is actually well behind the heavily cratered Rhea. Further back, several of the complex rings of Saturn can be seen crossing the image horizontally. Behind both the moons and rings is giant Saturn itself, showing expansive but featureless clouds in the green light where the above image was taken. The Cassini mission around Saturn has now been extended to 2017 to better study the complex planetary system as its season changes from equinox to solstice.

APOD: 2010 February 17 - An Unusually Smooth Surface on Saturn's Calypso
Explanation: Why is this moon of Saturn so smooth? This past weekend, humanity's Saturn-orbiting Cassini spacecraft passed as close to Saturn's small moon Calypso as it ever has, and imaged the small moon in unprecedented detail. Pictured above is an early return, raw, unprocessed image of the 20-km long irregularly shaped moon. Like its sister moon Telesto and the shepherd moon Pandora, Calypso has shown itself to be unusually smooth, much smoother than most of Saturn's larger moons. A leading hypothesis for Calypso's smoothness is that much of the moon's surface is actually a relatively loose jumble of rubble -- making Calypso a rubble-pile moon. The loose nature of the small ice pieces allows them to fill in many small craters and other surface features. Calypso orbits Saturn always behind Saturn's much larger moon Tethys, whereas Telesto's orbit always precedes Tethys. Calypso's extremely white surface -- not unlike fresh snow -- may result from the continuous accumulation of fresh ice particles falling in from Saturn's E ring.

APOD: 2010 February 15 - Cassini Spacecraft Crosses Saturn's Ring Plane
Explanation: If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from 2005 February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold. Since Saturn just passed its equinox, today the ring plane is pointed close to the Sun and the rings could not cast the high dark shadows seen across the top of this image, taken back in 2005. Moons appear as bumps in the rings.

APOD: 2009 November 10 - Saturn After Equinox
Explanation: The other side of Saturn's ring plane is now directly illuminated by the Sun. For the previous 15 years, the southern side of Saturn and its rings were directly illuminated, but since Saturn's equinox in August, the orientation has reversed. Pictured above last month, the robotic Cassini spacecraft orbiting Saturn has captured the giant planet and its majestic rings soon after equinox. Imaged from nearly behind, Saturn and its moon Tethys each show a crescent phase to Cassini that is not visible from Earth. As the rings continue to point nearly toward the Sun, only a thin shadow of Saturn's rings is visible across the center of the planet. Close inspection of Saturn's rings, however, shows superposed bright features identified as spokes that are thought to be groups of very small electrically charged ice particles. Understanding the nature and dynamics of spokes is not fully understood and remains a topic of research.

APOD: 2009 October 13 - Giant Dust Ring Discovered Around Saturn
Explanation: What has created a large dust ring around Saturn? At over 200 times the radius of Saturn and over 50 times the radius of Saturn's expansive E ring, the newly discovered dust ring is the largest planetary ring yet imaged. The ring was found in infrared light by the Earth-trailing Spitzer Space Telescope. A leading hypothesis for its origin is impact material ejected from Saturn's moon Phoebe, which orbits right through the dust ring's middle. An additional possibility is that the dust ring supplies the mysterious material that coats part of Saturn's moon Iapetus, which orbits near the dust ring's inner edge. Pictured above in the inset, part of the dust ring appears as false-color orange in front of numerous background stars.

APOD: 2009 September 30 - Saturn at Equinox
Explanation: How would Saturn look if its ring plane pointed right at the Sun? Before last month, nobody knew. Every 15 years, as seen from Earth, Saturn's rings point toward the Earth and appear to disappear. The disappearing rings are no longer a mystery -- Saturn's rings are known to be so thin and the Earth is so near the Sun that when the rings point toward the Sun, they also point nearly edge-on at the Earth. Fortunately, in this third millennium, humanity is advanced enough to have a spacecraft that can see the rings during equinox from the side. Last month, that Saturn-orbiting spacecraft, Cassini, was able to snap a series of unprecedented pictures of Saturn's rings during equinox. A digital composite of 75 such images is shown above. The rings appear unusually dark, and a very thin ring shadow line can be made out on Saturn's cloud-tops. Objects sticking out of the ring plane are brightly illuminated and cast long shadows. Inspection of these images may help humanity understand the specific sizes of Saturn's ring particles and the general dynamics of orbital motion.

APOD: 2009 September 4 - 6 Years of Saturn
Explanation: Today, planet Earth passes through the plane of Saturn's rings. From the perspective of earthbound astronomers, Saturn's rings will be edge-on. The problem is, Saturn itself is now very close to the Sun, low on horizon after sunset, so good telescopic images will be difficult to come by. Still, this composite of Saturn views taken from 2004 - 2009 (lower right to upper left) illustrates the change in ring tilt over the last six years and includes a nearly edge-on ring view, based on images captured earlier this year. While Saturn's south pole is clearly seen in the sequence, particularly at the lower right, it will be hidden in the coming years. Saturn's north pole will be increasingly visible, along with the tilting rings, as the planet emerges this fall in the predawn sky.

APOD: 2009 September 1 - Shadows of Saturn at Equinox
Explanation: Unusual shadows and dark rings appeared around Saturn near its equinox last month. At that time -- early August -- Saturn's ring plane pointed directly at the Sun. Visible above, Saturn's moon Tethys casts a shadow visible only on the far right. Saturn's own shadow blacks out a large swath of rings on the right. The night side of Saturn glows with ringshine -- sunlight reflected by ring particles back onto Saturn. Images near equinox at Saturn are giving astronomers a chance to search for unexpected shadows that may illuminate previously unknown features of Saturn's complex ring system. Cassini, the robotic spacecraft orbiting Saturn that took this image, is not expected to survive to the next Saturnian equinox in 15 years.

APOD: 2009 August 25 - Equinox at Saturn
Explanation: What would Saturn's rings look like if the ring plane pointed directly at the Sun? That situation occurred earlier this month when equinox occurred on Saturn. Since the Earth is nearly in the same direction as the Sun from Saturn, the rings appeared to disappear from Earth. From the robotic Cassini spacecraft orbiting Saturn, however, the unusually illuminated ring plane could be viewed from on high. Pictured above, Saturn's rings, darker than ever seen before, were captured just a few hours before equinox on 2009 August 10. The reason for the unusual brightness of an inner ring is currently unknown, but possibly related to particle sizes there being larger than the 10 meter average thickness of the rest of Saturn's rings. Short light streaks in the frame are artificial image artifacts and have nothing to do with Saturn's ring plane. Planetary scientists will be studying ring images taken near equinox to help better understand the dynamics and particle size distribution of the Solar System's most extensive ring system.

APOD: 2009 June 28 - Fresh Tiger Stripes on Saturn's Enceladus
Explanation: Do underground oceans vent through the tiger stripes on Saturn's moon Enceladus? Long features dubbed tiger stripes are known to be spewing ice from the moon's icy interior into space, creating a cloud of fine ice particles over the moon's South Pole and creating Saturn's mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft now orbiting Saturn. Pictured above, a high resolution image of Enceladus is shown from a close flyby. The unusual surface features dubbed tiger stripes are visible in false-color blue. Why Enceladus is active remains a mystery, as the neighboring moon Mimas, approximately the same size, appears quite dead. Most recently, an analysis of dust captured by Cassini found evidence for sodium as expected in a deep salty ocean. Conversely however, recent Earth-based observations of ice ejected by Enceladus into Saturn's E-Ring showed no evidence of the expected sodium. Such research is particularly interesting since such an ocean would be a candidate to contain life.

APOD: 2009 April 27 - Prometheus Creating Saturn Ring Streamers
Explanation: What's causing those strange dark streaks in the rings of Saturn? Prometheus. Specifically, an orbital dance involving Saturn's moon Prometheus keeps creating unusual light and dark streamers in the F-Ring of Saturn. Now Prometheus orbits Saturn just inside the thin F-ring, but ventures into its inner edge about every 15 hours. Prometheus' gravity then pulls the closest ring particles toward the 100-km moon. The result is not only a stream of bright ring particles but also a dark ribbon where ring particles used to be. Since Prometheus orbits faster than the ring particles, the icy moon pulls out a new streamer every pass. Sometimes, several streamers or kinks are visible at once. The above photograph taken in mid-January by the robotic Cassini Spacecraft orbiting Saturn. The oblong moon Prometheus is visible on the far left of the image.

APOD: 2009 April 15 - Jagged Shadows May Indicate Saturn Ring Particles
Explanation: What's causing unusual jagged shadows on Saturn's rings? No one is yet sure. As Saturn nears equinox, its rings increasingly show only their thin edge to the Earth and Sun. As a result, Saturn's moons now commonly cast long shadows onto the rings. An example of this is the elongated vertical shadow of Mimas seen on the above right. The series of shorter, jagged shadows that run diagonally, however, are more unusual. Now Saturn's rings have been known to be made of particles for hundreds of years, but these particles have so far escaped direct imaging. It is therefore particularly exciting that a preliminary hypothesis holds that these jagged shadows are silhouettes of transient groups of ring particles temporarily held close by their own gravity. Future work will surely continue, as the robotic Cassini spacecraft orbiting Saturn that took the above image will continue to photograph Saturn's magnificent rings right through Saturn's equinox this August.

APOD: 2009 March 19 - Saturn: Moons in Transit
Explanation: Every 14 to 15 years, Saturn's rings are tilted edge-on to our line of sight. As the bright, beautiful rings seem to grow narrower it becomes increasingly difficult to see them, even with large telescopes. But it does provide the opportunity to watch multiple transits of Saturn's moons. During a transit, a sunlit moon and its shadow glide across the cloudy face of the gas giant. Recorded on February 24, this Hubble image is part of a sequence showing the transit of four of Saturn's moons. From left to right are Enceladus and shadow, Dione and shadow, and Saturn's largest moon Titan. Small moon Mimas is just touching Saturn's disk near the ring plane at the far right. The shadows of Titan and Mimas have both moved off the right side of the disk. Saturn itself has an equatorial diameter of about 120,000 kilometers.

APOD: 2009 March 4 - Saturn in View
Explanation: Very good telescopic views of Saturn can be expected in the coming days as the ringed planet nears opposition on March 8th, its closest approach to Earth in 2009. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs almost yearly for Saturn. But while Saturn itself grows larger in telescopic images, Saturn's rings seem to be vanishing as their tilt to our line-of-sight decreases. In fact, the rings will be nearly invisible, edge-on from our perspective, by September 4. Recorded on February 28, this sharp image was made with the 1 meter telescope at Pic Du Midi, a mountain top observatory in the French Pyrenees. The rings are seen to be tilted nearly edge-on, but remarkable details are visible in the gas giant's cloud bands. The icy moon Tethys appears just beyond the rings at the lower left.

APOD: 2009 February 27 - Lulin and Saturn near Opposition
Explanation: Tracking through the constellation Leo on February 23rd, bright planet Saturn and Comet Lulin were both near opposition -- opposite the Sun in planet Earth's sky. They also passed within only 2 degrees of each other creating a dramatic celestial photo-op. Comet Lulin was near its closest approach to planet Earth at the time, at a distance of some 61 million kilometers, but was orbiting in the opposite direction. As a result it swept remarkably rapidly across the background of stars. This telephoto image captures both bright Saturn and greenish Lulin in the same field in a scene not too different from binocular views. Don't recognize ringed Saturn? The rings are presently tilted nearly edge-on to our view and the brighter planet is overexposed to record details of the fainter comet. At the upper right, Saturn is marked by multiple diffraction spikes created by the aperture blades in the telephoto lens.

APOD: 2009 January 11 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn, slightly scattering sunlight, in this exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the image. Seen in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus and the outermost ring visible above. Far in the distance, at the left, just above the bright main rings, is the almost ignorable pale blue dot of Earth.

APOD: 2008 October 27 - Beneath the South Pole of Saturn
Explanation: What clouds lurk beneath Saturn's unusual South Pole? To help find out, the robotic Cassini spacecraft currently orbiting Saturn imaged the nether region of the gigantic ringed orb in infrared light. There thick clouds appear dark as they mask much of the infrared light emitted from warmer regions below, while relatively thin clouds appear much lighter. Bands of clouds circle Saturn at several latitudes, while dark ovals indicate many dark swirling storm systems. Surprisingly, a haze of upper level clouds visible towards Saturn's equator disappears near the pole, including over Saturn's strange polar vortex. Cassini entered orbit around Saturn in 2004, and recorded the above image last year.

APOD: 2008 October 20 - Moons, Rings, and Unexpected Colors on Saturn
Explanation: Why would Saturn show such strange colors? The robotic Cassini spacecraft currently orbiting Saturn has beamed back images showing that the northern hemisphere our Solar System's most spectacularly ringed planet has changed noticeably since Cassini arrived in 2004, now sporting unusual and unexpected colors. No one is sure why. Although the cause for many of Saturn's colors is unknown, the recent change in colors is thought to be related to the changing seasons. Pictured above, the unusual colors are visible just north of the dark ring shadows. The razor-thin plane of ring particles is visible nearly edge-on across the bottom of the image. The cloudy moon Titan looms large just above the rings, while close observation will reveal three other moons. Cassini arrived at Saturn in 2004, sending back data and images that have not only led to a deeper understanding of the Jovian world's atmosphere, moons, and rings, but also raised new mysteries.

APOD: 2008 September 10 - The Anthe Arc around Saturn
Explanation: What created this unusual partial ring around Saturn? Discovered last year, the arc was captured in clear detail only two months ago by the Saturn-orbiting Cassini spacecraft. Since the arc occupies the same orbit as the small moon Anthe, a leading hypothesis holds that the arc was created by, and is replenished by, meteor impacts on Anthe. Similar arcs have been previously discovered, including an arc associated with the small Saturnian moon Methone, one arc related to Saturn's G ring, and several arcs orbiting Neptune. Pictured above, Anthe, only two kilometers across, is seen as the bright point near the top of the Anthe arc. The Anthe arc was imaged by the robotic space probe as it swooped to within 1.5 million kilometers of the small moon.

APOD: 2008 July 20 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few years ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Although Cassini has now concluded its primary mission, its past successes and opportunistic location have prompted NASA to start a two-year Equinox Mission, further exploring not only Saturn's enigmatic moons Titan and Enceladus, but Saturn herself as her grand rings tilt right at the Sun in August 2009.

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 June 9 - Saturn's Rings from the Other Side
Explanation: What do Saturn's rings look like from the other side? From Earth, we usually see Saturn's rings from the same side of the ring plane that the Sun illuminates them. Geometrically, in the above picture taken in April by the robot Cassini spacecraft now orbiting Saturn, the Sun is behind the camera but on the other side of the ring plane. This vantage point, specifically 17 degrees above the ring plane, gives a breathtaking views of the most splendid ring system in the Solar System. Strangely, the rings have similarities to a photographic negative of a front view. The ring brightness as recorded from different angles indicates ring thickness and particle density of ring particles. Elsewhere, ring shadows can be seen on the sunlit face of Saturn, shown sporting numerous cloud structures in nearly true color.

APOD: 2008 May 5 - A Persistent Electrical Storm on Saturn
Explanation: How do large storms evolve on Saturn? On Earth, a hurricane can persist for weeks, while the Great Red Spot on Jupiter has been in existence for over 150 years. On Saturn, a storm system has now set a new endurance record, now being discernable for greater than three months. Electrical signals were detected from the storm in late November of 2007, while the above image was taken in early March 2008. The storm has roughly the width of planet Earth. Planetary scientists hypothesize that the storm runs deep into Saturn's cloud tops. The above image is shown in exaggerated colors combining violet and green light with light normally too red for humans to see. Visible on the upper right are shadows of Saturn's expansive ring system. Careful inspection will reveal Saturn's small moon Janus just below a ring shadow. Understanding weather on other planets helps atmospheric scientists better understand our Earth's weather. Observers of our Solar System's huge ringed world will be tracking the storm to see how it evolves and how long it will ultimately last.

APOD: 2008 March 24 - Saturn and Titan from Cassini
Explanation: Spectacular vistas of Saturn and its moon continue to be recorded by the Cassini spacecraft. Launched from Earth in 1997, robotic Cassini entered orbit around Saturn in 2004 and has revolutionized much of humanity's knowledge of Saturn, its expansive and complex rings, and it many old and battered moons. Soon after reaching Saturn, Cassini released the Huygen's probe which landed on Titan, Saturn's largest moon, and send back unprecedented pictures from below Titan's opaque cloud decks. Recent radar images of Titan from Cassini indicate flat regions that are likely lakes of liquid methane, indicating a complex weather system where it likely rains chemicals similar to gasoline. Pictured above, magnificent Saturn and enigmatic Titan were imaged together in true color by Cassini earlier this year.

APOD: 2007 December 29 - Saturn's Infrared Glow
Explanation: Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels.

APOD: 2007 December 17 - Saturn's Ancient Rings
Explanation: How old are Saturn's rings? No one is quite sure. One possibility is that the rings formed relatively recently in our Solar System's history, perhaps only about 100 million years ago when a moon-sized object broke up near Saturn. Evidence for a young ring age includes a basic stability analysis for rings, and the fact that the rings are so bright and relatively unaffected by numerous small dark meteor impacts. New evidence, however, raises the possibility that some of Saturn's rings may be billions of years old and so almost as old as Saturn itself. Inspection of images by the Saturn-orbiting Cassini spacecraft indicates that some of Saturn's ring particles temporarily bunch and collide, effectively recycling ring particles by bringing fresh bright ices to the surface. Seen here, Saturn's rings were imaged in their true colors by the robotic Cassini in late October. Icy bright Tethys, a moon of Saturn likely brightened by a sandblasting rain of ice from sister moon Enceladus, is visible in front of the darker rings.

APOD: 2007 October 23 - Crescent Saturn
Explanation: Saturn never shows a crescent phase -- from Earth. But when viewed from beyond, the majestic giant planet can show an unfamiliar diminutive sliver. This image of crescent Saturn in natural color was taken by the robotic Cassini spacecraft in May. The image captures Saturn's majestic rings from the side of the ring plane opposite the Sun -- the unilluminated side -- another vista not visible from Earth. Pictured are many of Saturn's photogenic wonders, including the subtle colors of cloud bands, the complex shadows of the rings on the planet, the shadow of the planet on the rings, and the moons Mimas (2 o'clock), Janus (4 o'clock), and Pandora (8 o'clock). As Saturn moves towards equinox in 2009, the ring shadows are becoming smaller and moving toward the equator. During equinox, the rings will be nearly invisible from Earth and project only an extremely thin shadow line onto the planet.

APOD: 2007 June 27 - Neon Saturn
Explanation: If seen in the right light, Saturn glows like a neon sign. Although Saturn has comparatively little of the element neon, a composite image false-colored in three bands of infrared light highlights features of the giant ringed planet like a glowing sign. At the most blue band of the infrared light featured, false-colored blue in the above image, Saturn itself appears dark but Saturn's thin rings brightly reflect light from our Sun. Conversely, Saturn's B ring is so thick that little reflected light makes it through, creating a dark band between Saturn's A and C rings. At the most red band of the infrared, false-colored red above, Saturn emits a surprisingly detailed thermal glow, indicating planet-wide bands, huge hurricane-like storms, and a strange hexagon-shaped cloud system around the North Pole. In the middle infrared band, false-colored green, the sunlit side of Saturn's atmosphere reflects brightly. The above image was obtained in late February by the robotic Cassini spacecraft orbiting about 1.6 million kilometers out from Saturn.

APOD: 2007 May 26 - The Moon's Saturn
Explanation: On May 22nd, just days after sharing the western evening sky with Venus, the Moon moved on to Saturn - actually passing in front of the ringed planet when viewed in skies over Europe, northern Africa, and western Asia. Because the Moon and bright planets wander through the sky near the ecliptic plane, such occultation events are not uncommon, but they are dramatic, especially in telescopic views. For example, in this sharp image Saturn is captured emerging from behind the Moon, giving the illusion that it lies just beyond the Moon's bright edge. Of course, the Moon is a mere 400 thousand kilometers away, compared to Saturn's distance of 1.4 billion kilometers. Taken with a digital camera and 20 inch diameter telescope at the Weikersheim Observatory in southern Germany, the picture is a single exposure adjusted to reduce the difference in brightness between Saturn and the cratered lunar surface.

APOD: 2007 April 10 - Saturn from Below
Explanation: Swooping below Saturn, the Cassini spacecraft spied several strange wonders. Visible in the distance are some of the many complex rings that orbit the Solar System's second largest planet. In the foreground looms the gigantic world itself, covered with white dots that are clouds high in Saturn's thick atmosphere. Saturn's atmosphere is so thick that only clouds are visible. At the very South Pole of Saturn lies a huge vortex that is a hurricane-like storm showing no sign of dissipating. The robotic Cassini spacecraft took the above image in January from about one million kilometers out, resolving details about 50 kilometers across.

APOD: 2007 April 7 - Three Years of Saturn
Explanation: Using an image recorded just last month as a base, this composite illustration tracks the motion of bright Saturn as it wanders through planet Earth's night sky. Starting at the upper right, Saturn's position is shown about every two weeks beginning in August 2005 and projected through September 2008. Over the three year period, Saturn actually appears to reverse its general eastward (leftward) drift, tracing out three flattened curves. The periodic backwards or retrograde motion with respect to the background stars is a reflection of the motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own closer-in orbit. The Beehive star cluster in Cancer lies near the track at the upper right. Stars along the "backward question mark" at the head of Leo are in the left half of the frame. Saturn's position this month is near the right hand limit of the middle curve. Click on the picture to download and view the gif animation.

APOD: 2007 April 6 - Four Years of Saturn
Explanation: Saturn and its magnificent ring system can offer even casual astronomers the most memorable of telescopic sights. Wandering between Leo and Cancer this month, a bright Saturn is well placed for viewing in evening skies. But from our earthbound perspective, the tilt of Saturn's rings does change with time. In 1995 and 1996 the broad rings were edge-on and nearly invisible, gradually opening to a spectacular maximum tilt of about 27 degrees by 2003. This frame from a series of Saturn images beginning a year later, in 2004, and ending just last month shows the steady decrease in apparent tilt as the rings head toward another edge-on presentation in 2009. Saturn's south pole is toward the bottom. Click on the picture to view the sharp, color gif movie.

APOD: 2007 April 3 - A Mysterious Hexagonal Cloud System on Saturn
Explanation: Why would clouds form a hexagon on Saturn? Nobody is yet sure. Originally discovered during the Voyager flybys of Saturn in the 1980s, nobody has ever seen anything like it anywhere else in the Solar System. If Saturn's South Pole wasn't strange enough with its rotating vortex, Saturn's North Pole might now be considered even stranger. The bizarre cloud pattern is shown above in a recent infrared image taken by the Saturn-orbiting Cassini spacecraft. The images show the stability of the hexagon even 20 years after Voyager. Movies of Saturn's North Pole show the cloud structure maintaining its hexagonal structure while rotating. Unlike individual clouds appearing like a hexagon on Earth, the Saturn cloud pattern appears to have six well defined sides of nearly equal length. Four Earths could fit inside the hexagon. Although full explanations are not yet available, planetary scientists are sure to continue to study this most unusual cloud formation for quite some time.

APOD: 2007 March 6 - Saturn from Above
Explanation: This image of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this image mosaic was taken in January by the robotic Cassini spacecraft now orbiting Saturn. The beautiful rings of Saturn are seen in full expanse, while cloud details are visible near the night-day terminator divide.

APOD: 2007 January 31 - Movie: Cassini Crosses Saturn's Ring Plane
Explanation: What would the rings of Saturn look like if you passed right through the ring plane? To find out, NASA aimed cameras from the Cassini spacecraft right at Saturn's rings as the robotic explorer passed from the sunlit side of the rings to the shadowed side. Resulting images from a vantage point outside the rings and most moons, but inside the orbit of Titan, have been gathered together in the above time-lapse movie. The dramatic movie demonstrates that ring particle density and reflectivity makes some parts of the shadowed side nearly the photographic negative of the sunlit side, but nearly empty regions remain continually dark. Visible also are Saturn-orbiting moons Enceladus, Mimas, Janus, Epimetheus, Prometheus, and Pandora. The extreme thinness of Saturn's rings can be appreciated from frames taken near the crossing time.

APOD: 2006 November 27 - Mysterious Spokes in Saturn's Rings
Explanation: What causes the mysterious spokes in Saturn's rings? Visible on the left of the above image as ghostlike impressions, spokes were first discovered by the Voyager spacecraft that buzzed by Saturn in the early 1980s. Their existence was unexpected, and no genesis hypothesis has ever become accepted. Oddly, the spokes were conspicuously absent from initial images sent back by the robot Cassini spacecraft now orbiting Saturn. Analyses of archived Voyager images have led to the conclusions that the transient spokes, which may form and dissipate over a few hours, are composed of electrically charged sheets of small dust-sized particles. Some recent images from Cassini like that shown above have now finally shown the enigmatic spokes superposed on Saturn's B ring. Hypotheses for spoke creation include small meteors impacting the rings and electron beams from Saturnian atmospheric lightning spraying the rings. Observations of the puzzling spokes, as well as creative origin speculations, are ongoing.

APOD: 2006 November 13 - A Hurricane Over the South Pole of Saturn
Explanation: What's happening at the south pole of Saturn? To find out, scientists sent the robot Cassini probe now orbiting Saturn directly over the lower spin axis of the ringed giant. Cassini found there a spectacular massive swirling storm system with a well developed eye-wall, similar to a hurricane here on Earth. One image of the storm is shown above, while several frames from the overpass have been made into a movie that shows the huge vortex rotating. The storm is slightly larger than the entire Earth and carries winds that reach 550 kilometers per hour, twice the velocity of a Category 5 hurricane. This pole vortex on Saturn might have been raging for billions of years and is not expected to drift off the pole.

APOD: 2006 November 7 - Janus: Potato Shaped Moon of Saturn
Explanation: Janus is one of the stranger moons of Saturn. First, Janus travels in an unusual orbit around Saturn where it periodically trades places with its sister moon Epimetheus, which typically orbits about 50 kilometers away. Janus, although slightly larger than Epimetheus, is potato-shaped and has a largest diameter of about 190 kilometers. Next, Janus is covered with large craters but strangely appears to lack small craters. One possible reason for this is a fine dust that might cover the small moon, a surface also hypothesized for Pandora and Telesto. Pictured above, Janus was captured in front of the cloud tops of Saturn in late September.

APOD: 2006 October 16 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the above image. Visible in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus, and the outermost ring visible above. Far in the distance, visible on the image left just above the bright main rings, is the almost ignorable pale blue dot of Earth.

APOD: 2006 October 12 - Saturn's Infrared Glow
Explanation: Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels. The thermal infrared glow is also apparent in the broad bands of ring shadows draped across the northern hemisphere of Saturn.

APOD: 2006 September 27- Earth from Saturn
Explanation: What's that pale blue dot in this image taken from Saturn? Earth. The robotic Cassini spacecraft looked back toward its old home world earlier this month as it orbited Saturn. Using Saturn itself to block the bright Sun, Cassini imaged a faint dot on the right of the above photograph. That dot is expanded on the image inset, where a slight elongation in the direction of Earth's Moon is visible. Vast water oceans make Earth's reflection of sunlight somewhat blue. Earth is home to over six billion humans and over one octillion Prochlorococcus.

APOD: 2006 September 12 - Saturn at Night
Explanation: This is what Saturn looks like at night. In contrast to the human-made lights that cause the nighttime side of Earth to glow faintly, Saturn's faint nighttime glow is primarily caused by sunlight reflecting off of its own majestic rings. The above image of Saturn at night was captured in July by the Cassini spacecraft now orbiting Saturn. The above image was taken when the Sun was far in front of the spacecraft. From this vantage point, the northern hemisphere of nighttime Saturn, visible on the left, appears eerily dark. Sunlit rings are visible ahead, but are abruptly cut off by Saturn's shadow. In Saturn's southern hemisphere, visible on the right, the dim reflected glow from the sunlit rings is most apparent. Imprinted on this diffuse glow, though, are thin black stripes not discernable to any Earth telescope -- the silhouetted C ring of Saturn. Cassini has been orbiting Saturn since 2004 and its mission is scheduled to continue until 2008.

APOD: 2006 July 11 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few months ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Cassini has now passed the official half-way mark of its mission around Saturn, but is well situated to complete another two years investigating this complex and surprising system.

APOD: 2006 June 27 - The Moving Moons of Saturn
Explanation: The moons of Saturn never stop. A space traveler orbiting the ringed giant planet would witness a continuing silent dance where Saturn's multiple moons pass near each other in numerous combinations. Like a miniature Solar System, the innermost moons orbit Saturn the fastest. The above movie was centered on Saturn's moon Rhea, so that the moons Mimas and Enceladus appear to glide by. At 1,500 kilometers across, Rhea is over three times larger than the comparably sized Mimas and Enceladus. The Sun illuminates the scene from the lower right, giving all of the moons the same crescent phase. The above time lapse movie was created by the Saturn-orbiting robotic Cassini spacecraft over a period of about 40 minutes.

APOD: 2006 June 17 - Saturn, Mars, and the Beehive Cluster
Explanation: Grab a pair of binoculars and check out Saturn and Mars in the early evening sky tonight! Looking west shortly after sunset, your view could be similar to this one - recorded on June 14. But while this picture shows the two bright planets (Saturn at left) separated by around 1.5 degrees and neatly flanking M44, the Beehive Star Cluster, tonight should find those planets even closer together. In fact, Saturn and Mars are scheduled to achieve their closest alignment near sunset, approaching to within about half a degree. The Beehive will still stand out in the distant starry background. Still got those binoculars in hand? You might as well look for Mercury and Jupiter too.

APOD: 2006 May 30 - Ancient Craters on Saturn's Rhea
Explanation: Saturn's ragged moon Rhea has one of the oldest surfaces known. Estimated as changing little in the past billion years, Rhea shows craters so old they no longer appear round – their edges have become compromised by more recent cratering. Like Earth's Moon, Rhea's rotation is locked on Saturn, and the above image shows part of Rhea's surface that always faces Saturn. Rhea's leading surface is more highly cratered than its trailing surface. Rhea is composed mostly of water-ice but is thought to have a small rocky core. The above image was taken by the robot Cassini spacecraft now orbiting Saturn. Cassini swooped past Rhea two months ago and captured the above image from about 100,000 kilometers away. Rhea spans 1,500 kilometers making it Saturn's second largest moon after Titan. Several surface features on Rhea remain unexplained including large light patches.

APOD: 2006 May 3 - Saturn in Blue and Gold
Explanation: Why is Saturn partly blue? The above picture of Saturn approximates what a human would see if hovering close to the giant ringed world. The above picture was taken in mid-March by the robot Cassini spacecraft now orbiting Saturn. Here Saturn's majestic rings appear directly only as a thin vertical line. The rings show their complex structure in the dark shadows they create on the image left. Saturn's fountain moon Enceladus, only about 500 kilometers across, is seen as the bump in the plane of the rings. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why Saturn's clouds are colored gold.

APOD: 2006 April 5 - Slightly Beneath Saturn's Ring Plane
Explanation: When orbiting Saturn, be sure to watch for breathtaking superpositions of moons, rings, and shadows. One such picturesque vista was visible recently to the robot Cassini spacecraft now orbiting Saturn. In late February, Cassini captured Rhea, the second largest moon of Saturn, while looking up from slightly beneath Saturn's expansive ring plane. Signature dark gaps are visible in the nearly edge-on rings. A shadow of Saturn's F ring cuts across the cratered ice-moon. Cassini is scheduled to continue sending back images from the orbit of Saturn until at least 2008.

APOD: 2006 March 22 - Enceladus Near Saturn
Explanation: Some images of Saturn appear surreal. Earlier this year, the robot spacecraft Cassini now orbiting Saturn took this surreal image of the gas giant Saturn, its majestic rings, and its enigmatic world Enceladus all in one frame. Enceladus, recently found to emit jets of ice from possible underground seas, appears white as its surface is covered with relatively clean water-ice. Below Enceladus are the rings of Saturn, seen nearly edge on. Compared to Enceladus, Saturn's rings show their comparatively high density of dirt with their golden-brown color in this natural color image. The planet Saturn, in the background, appears relatively featureless with the exception of thin ring shadows visible on the upper left. The terminator between night and day is seen vertically across the face of this distant world.

APOD: 2006 February 23 - Saturn Storm by Ringshine
Explanation: Imaged on the night side of Saturn by the Cassini spacecraft, these swirling storm clouds are illuminated by ringshine - sunlight reflected from the gas giant's magnificent ring system. The storm (top) was actually spotted last month by amateur astronomers as it rotated across Saturn's day side and spans about 3,500 kilometers. When the storm was on the same side of Saturn as the Cassini spacecraft, bursts of radio noise were detected, suggesting lightning discharges connected with the storm were responsible for the radio emission. While no lightning is seen directly in this Cassini image, scientists note that this storm appears along the planet's southern hemisphere storm alley in approximately the same location as Saturn's Dragon Storm, reported early last year. Though the new storm is larger and seems to be more powerful, it could well be the Dragon Storm reemerging.

APOD: 2006 February 12 - Phoebe: Comet Moon of Saturn
Explanation: Was Saturn's moon Phoebe once a comet? Images from the robotic Cassini spacecraft taken two years ago when entering the neighborhood of Saturn indicate that Phoebe may have originated in the outer Solar System. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was once part of the Kuiper belt of icy comets beyond Neptune before being captured by Saturn. Visible in the above image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 30,000 kilometers out from this 200-kilometer diameter moon. Two weeks after taking the above image, Cassini fired its engines to decelerate into orbit around Saturn.

APOD: 2006 January 28 - Saturn in the Hive
Explanation: If you can find Saturn in tonight's sky, then you can also find M44, popularly known as the Beehive star cluster. In fact, with a pair of binoculars most casual skygazers should find it fairly easy to zero in on this celestial scene. Saturn is at opposition - opposite the Sun in Earth's sky - so, the bright planet rises in the east at sunset and is visible throughout the night. Near the stationary part of its wandering path through the heavens, Saturn will obligingly linger for a while in the vicinity of M44 in the relatively faint constellation Cancer. Seen here in a photograph from January 25, Saturn (lower right) is strongly overexposed with the stars of M44 swarming above and to the left. The picture approximately corresponds to the view when looking through a typical pair of binoculars. Saturn is about 64 light-minutes from our fair planet while M44, one of the closest star clusters, is around 600 light-years away.

APOD: 2006 January 3 - Dark Terrain on Saturn s Iapetus
Explanation: Why are vast sections of Iapetus as dark as coal? No one knows for sure. Iapetus, the third largest moon of Saturn, was inspected again as the Saturn-orbiting robot Cassini spacecraft swooped past the enigmatic world again late last year. The dark material covers most of the surface visible in the above image, while the small portion near the top that appears almost white is of a color and reflectance more typical of Saturn's other moons. The unknown material covers about half of the 1,500 kilometer wide moon. The material is so dark that it reflects less than five percent of incident sunlight, yet overlays craters indicating that it was spread after the craters were formed. Iapetus has other unexplained features. The bright part of Iapetus is covered with unexplained long thin streaks. The orbit of Iapetus is also unusual, being tilted to the plane of Saturn's orbit by an unusually high fifteen degrees. A strange ridge about 13 kilometers high crosses much of Iapetus near the equator and is visible near the bottom. Oddly, this ridge is almost exactly parallel with Iapetus' equator. The exact shape of Iapetus remains undetermined, but images indicate that it is quite strange -- something like a walnut. Research into the formation and history of mysterious Iapetus is active and ongoing.

APOD: 2005 December 31 - A Year at Saturn
Explanation: Arriving at Saturn in July of 2004, the Cassini spacecraft has now spent a year and a half exploring the magnificent rings and moons of the distant gas giant. The year 2005 began with Cassini's Huygens probe landing on Saturn's large moon Titan. Cassini's continuing series of close flybys also revealed details and discoveries across the surface of the smog shrouded moon. In fact, with a ringside seat throughout 2005, Cassini's cameras have made spectacular pictures of Titan along with Saturn's other moons and rings almost common place. But often, Saturn itself provided the most dramatic backdrop. In this view, Saturn's moon Dione lies in front of edge-on rings and the gas giant's cloud tops draped with broad ring shadows. Dione is 1,118 kilometers across and lies about 300,000 kilometers from the ring's edge.

APOD: 2005 December 19 - Thin Rings Around Polarized Saturn
Explanation: How thin are the rings of Saturn? Brightness measurements from different angles have shown Saturn's rings to be about one kilometer thick, making them many times thinner, in relative proportion, than a razor blade. This thinness sometimes appears in dramatic fashion during an image taken nearly along the ring plane. The robot Cassini spacecraft now orbiting Saturn has now captured another shot that dramatically highlights the ring's thinness. The above artistic looking image was taken early last month in infrared polarized light. If alone in space, the unlit part of Saturn would be much darker. Reflection of light off of moons like Enceladus (pictured) and the billions of small particles in Saturn's rings, however, gives the giant space orb an unusual glow, an effect highlighted in polarized light.

APOD: 2005 November 23 - Pandora: A Shepherd Moon of Saturn
Explanation: What does Saturn's small moon Pandora look like? To help find out, the robot Cassini spacecraft now orbiting Saturn passed about 50,000 kilometers from the unusual moon in early September. The highest resolution image of Pandora ever taken was then captured and is shown above in representative colors. Features as small as 300 meters can be discerned on 80-kilometer wide Pandora. Craters on Pandora appear to be covered over by some sort of material, providing a more smooth appearance than sponge-like Hyperion, another small moon of Saturn. Curious grooves and ridges also appear to cross the surface of the small moon. Pandora is partly interesting because, along with its companion moon Prometheus, it helps shepherd the particles of Saturn's F ring into a distinct ring.

APOD: 2005 November 2 - Epimetheus and Janus: Interchangeable Moons of Saturn
Explanation: These two moons change places. Epimetheus and Janus, two small moons of Saturn, actually switch positions as they orbit their home planet. The orbital radii of the moons are strangely separated by less than the radii of the moons themselves: about 50 kilometers. One moon orbits Saturn well ahead of the other, at first. As the two moons gravitationally attract, they approach each other and, every few years, actually pass and trade orbits. This strange dance creates speculation that Epimetheus and Janus were once joined and later split from each other. Pictured above, the two moons were photographed rounding their orbits just outside of Saturn's F ring. The above image was taken in early September by the robot Cassini spacecraft, also orbiting Saturn.

APOD: 2005 October 10 - The Swirling Storms of Saturn
Explanation: Storms larger than hurricanes continually dot the upper atmosphere of the planet Saturn. A view of many storms occurring simultaneously was captured in July by the robot Cassini spacecraft now orbiting Saturn. An image of unusually high detail was made possible at that time when Cassini isolated a very specific color of polarized infrared light. The numerous white and dark spots visible above are the swirling storm systems. On Saturn, storms like these typically last for months and have even been seen merging. Bands of clouds that circle the entire planet are also clearly visible. Saturn's complex and majestic ring system is seen both in the foreground and the background. The above image has been digitally shortened along the vertical.

APOD: 2005 June 22 - Saturn's Rings from the Other Side
Explanation: What do Saturn's rings look like from the other side? From Earth, we usually see Saturn's rings from the same side of the ring plane that the Sun illuminates them. Geometrically, in the above picture taken in April by the robot Cassini spacecraft now orbiting Saturn, the Sun is behind the camera but on the other side of the ring plane. Such a vantage point gives a breathtaking views of the most splendid ring system in the Solar System. Strangely, the rings have similarities to a photographic negative of a front view. For example, the dark band in the middle is actually the normally bright B-ring. The ring brightness as recorded from different angles indicates ring thickness and particle density of ring particles. Images like these are also interesting for what they do not show: spokes. The unexpected shadowy regions once recorded by the Voyager missions when they passed Saturn in the early 1980s are not, so far, being seen by Cassini. Extra credit: Can you spot the small moon (Prometheus) among the rings?

APOD: 2005 June 6 - Saturn: Dirty Rings and a Clean Moon
Explanation: Eating surface ice from Enceladus might be healthier than eating ice from Saturn's rings -- it certainly appears cleaner. From their apparent densities and reflectance properties, both the rings of Saturn and its shiniest moon, Enceladus, are thought to be composed predominantly of water ice. For reasons that are not yet understood, however, many of Saturn's ring particles have become partly coated with some sort of relatively dark dust, while the surface of Enceladus appears comparatively bright and clean. The contrast between the two can be seen in the above image taken last month by the robot Cassini spacecraft now in orbit around Saturn. Bright Enceladus shines in the background in contrast to the darker foreground rings. The reason why Enceladus is so bright is currently unknown but might involve bringing fresh water to its surface with water volcanoes.

APOD: 2005 May 23 - A Wavemaker Moon in Saturn's Rings
Explanation: What causes small waves in Saturn's rings? Observations of rings bordering the Keeler gap in Saturn's rings showed unusual waves. Such waves were first noticed last July and are shown above in clear detail. The picture is a digitally foreshortened image mosaic taken earlier this month by the robot Cassini spacecraft now orbiting Saturn. The rings, made of many small particles, were somehow not orbiting Saturn in their usual manner. Close inspection of the image shows the reason - a small moon is orbiting in the Keeler gap. The previously unknown moon is estimated to span about seven kilometers and appears to have the same brightness as nearby ring particles. The gravity of the small moon likely perturbs the orbits of ring particles that come near it, causing them to shimmy back and forth after the moon passes. Since inner particles orbit more quickly than outer particles, only the leading particles of the inner rings and the trailing particles of the outer rings show the wave effect.

APOD: 2005 February 11 - Blue Saturn
Explanation: Serene blue hues highlight this view of Saturn's northern hemisphere from the Cassini spacecraft. The image has been adjusted to approximate the natural blue color of visible sunlight scattered by the gas giant's upper atmosphere. Saturn's famous rings cast the dark shadows stretching across the frame with infamous cratered moon Mimas lurking at the lower left. Orbiting beyond the main inner rings, Mimas itself is 400 kilometers across and lies nearly 200,000 kilometers, over 3 Saturn radii, from the center of the planet. Still, Mimas orbits within Saturn's faint and tenuous outer E ring.

APOD: 2005 February 10 - Red Saturn
Explanation: This strange, false-color image of otherwise familiar planet Saturn shows temperature changes based on thermal infrared emission in the gas giant's atmosphere and rings. Recorded from the Keck I telescope on Mauna Kea, the sharp, ground-based picture of Saturn's southern hemisphere is a mosaic of 35 images. Based on the effects of sunlight during the southern summer season, general warming trends were anticipated. But a surprising result of the infrared image data is the a clear indication of an abruptly warmer polar cap and bright hot spot at Saturn's south pole. The warm south pole and hot spot may be unique in the solar system and a further exploration of the region is planned using instruments on the Cassini spacecraft. So how hot is Saturn's hot spot? The upper tropospheric temperature is a sweltering 91 Kelvin (-296 degrees Fahrenheit) at the pole.

APOD: 2004 November 29 - Saturn's Moon Tethys from Cassini
Explanation: Tethys is one of the larger and closer moons of Saturn. The Cassini spacecraft now orbiting Saturn passed near the frozen moon at the end of October, capturing the most detailed images since the Voyager spacecrafts in the early 1980s. Tethys is composed almost completely of water ice and shows a large impact crater that nearly circles the moon. Because this crater did not disrupt the moon, Tethys is hypothesized to be at least partly liquid in its past. Two smaller moons, Telesto and Calypso, orbit Saturn just ahead of and behind Tethys. Giovanni Cassini discovered Tethys in 1684. The Cassini spacecraft is scheduled for a close fly-by of Tethys in September 2005.

APOD: 2004 November 2 - Storm Alley on Saturn
Explanation: What causes storms on Saturn? To help find out, scientists commanded the robot Cassini spacecraft now orbiting Saturn to inspect a circulating band of clouds nicknamed "Storm Alley." This westwardly moving cloud ring has been unusually active since the beginning of 2004, spawning white swirling storms and dark storms ringed by sprawling white clouds all cascading around the gas giant. The rogue band, as well as other parts of south Saturn, were imaged in stunning detail in a very specific band of infrared light that passes through Saturn's upper haze relatively unblurred. The result was then digitally sharpened, showing more cloud detail but creating fake image artifacts such as a surrounding ring. Speculation on the nature of past Saturn storms included convective motions of small amounts of ammonia and water, seasons, and shadowing effects of the great ring system. Although the above image provides data and clues, the power behind Saturn's storms still remains a mystery.

APOD: 2004 October 18 - Southern Saturn from Cassini
Explanation: What happens to Saturn's pervasive clouds at its South Pole? Visible in the above image of Saturn are bright bands, dark belts and a dark spot right over the South Pole. The above image in infrared light spans over 30,000 kilometers and was taken early last month by the robot Cassini spacecraft currently orbiting Saturn. Saturn's atmosphere is about 75 percent hydrogen, 25 percent helium, and small amounts of heavier compounds including water vapor, methane, and ammonia. The relatively low gravity at Saturn's cloud tops result in a thicker haze layer, which in turn makes atmospheric features blurrier than Jupiter.

APOD: 2004 September 20 - Seeing Through Saturn's C Ring
Explanation: Are Saturn's rings transparent? The Cassini spacecraft that recently entered orbit around Saturn has confirmed that some of Saturn's rings are more transparent than others. Pictured above, Saturn's main A, B, and C rings can be seen, top to bottom, superposed against the gas giant planet. Although the B-ring across the top is opaque, Saturn's cloud tops can be clearly seen through the lower C-ring. The translucent nature of the C-ring likely indicates that it is less densely populated with ring particles than the B-ring. The above image was taken on July 30 while Cassini was over 7 million kilometers from Saturn.

APOD: 2004 July 21 - A Shadow on the Rings of Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. Rather, this picture was taken by the robot Cassini spacecraft that began orbiting Saturn earlier this month. The dark western limb of Saturn looms large on the image right, while complex concentrations of small ring particles reflect sunlight on the image left. Saturn's enigmatic F ring is visible around the outside, showing mysterious knots. The small moon Epimetheus, only about 100 kilometers across, can also been seen on the far left. Cassini is scheduled to drop a probe toward the largest moon Titan in December.

APOD: 2004 June 30 - Phoebe: Comet Moon of Saturn
Explanation: Was Saturn's moon Phoebe once a comet? Images from the robotic Cassini spacecraft taken two weeks ago when entering the neighborhood of Saturn indicate that Phoebe may have originated in the outer Solar System. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was once part of the Kuiper belt of icy comets beyond Neptune before being captured by Saturn. Visible in the above image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 30,000 kilometers out from this 200-kilometer diameter moon. Late today, Cassini will begin to fire its engines to decelerate into orbit around Saturn.

APOD: 2004 May 31 - 24 Million Kilometers to Saturn
Explanation: Next stop: Saturn. The Cassini-Huygens spacecraft is approaching Saturn and will fire its engines to break into orbit around the ringed giant on July 1. The robot spacecraft was launched in 1997 and rounded Jupiter in 2001. As Cassini orbits Saturn over the next four years, it will swoop past many of Saturn's moons for unprecedented close-ups and even drop a probe onto Titan. Pictured above, Cassini imaged Saturn two weeks ago as it closed to only 24 million kilometers out. Visible are complex cloud patterns, thousands of rings, a shadow angle not visible from Earth, and a moon (if you can find it).

APOD: 2004 April 30 - Eyeful of Saturn
Explanation: Now a bright speck of light wandering through Earth's night sky, magnificent planet Saturn lies nearly 1.5 billion kilometers from the Sun. But after an interplanetary voyage of seven years the planet's stunning rings nearly fill the field of the Cassini spacecraft's narrow angle camera in this image recorded on March 27. Tip to tip, the ring system spans about 270,000 kilometers. Named for discoverers, the large, easily visible gap in the rings is known as the Cassini division, while the narrower outer gap is the Encke division. Illuminated from below and to the right, the rings cast a shadow on Saturn's upper hemisphere, interrupted where sunlight streams through the Cassini division and creates a light blue streak. At the left, Saturn also casts a stark shadow across the planet girdling rings. On July 1, the Cassini spacecraft is scheduled to fire its main engine and enter Saturn orbit.

APOD: 2004 March 12 - X-Ray Saturn
Explanation: Above, the ringed planet Saturn shines in x-rays. Otherwise beyond the range of human vision, the eerie x-ray view was created by overlaying a computer generated outline of the gas giant's disk and ring system on a false-color picture of smoothed, reconstructed x-ray data from the orbiting Chandra Observatory. The data represent the first clear detection of Saturn's disk at x-ray energies and held some surprises for researchers. For starters, the x-rays seem concentrated near the planet's equator rather than the poles, in marked contrast to observations of Jupiter, the only other gas giant seen at such high energies. And while Saturn's high energy emission is found to be consistent with the reflection of x-rays from the Sun, the intensity of the reflected x-rays was also found to be unusually strong. Outside the planet's disk, only a faint suggestion of x-rays from Saturn's magnificent ring system is visible at the left.

APOD: 2004 March 1 - Cassini Closes in on Saturn
Explanation: Are they gone? They were not originally predicted to even be there. The mystery revolves around strange shadow-like spokes that appeared on Saturn's large B-ring, the large middle ring in the complex system of particles that orbits Saturn. The spokes were discovered 23 years ago by the passing Voyager spacecraft and attributed to very fine dust of unknown origin. The missing spokes were noted in the above image, taken last month, from the robot Cassini spacecraft now approaching Saturn. Launched in 1997, the distance remaining between Cassini and Saturn is now less than half that between the Earth and the Sun. Cassini is expected to enter orbit around the ringed Jovian giant planet in July and drop a probe onto Titan, Saturn's largest moon.

APOD: 2004 January 17 - Saturn: Lord of the Rings
Explanation: Born in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings, After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. Accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. On December 31, 2003, Saturn made its closest approach to Earth for the next 29 years, a mere 1,200,000,000 kilometers. It will remain a tantalizing target for earthbound telescopes in the coming months.

APOD: 2003 December 10 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has now closed to within a single Earth-Sun separation from the ringed giant. Early last month, Cassini snapped the contrast-enhanced color composite pictured above. Many features of Saturn's rings and cloud-tops now show considerable detail. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

APOD: 2003 September 18 - Saturn by Three
Explanation: These three views of Saturn were recorded by the Hubble Space Telescope on March 7th of this year, as the southern hemisphere of the solar system's most gorgeous planet reached its maximum 27 degree tilt toward Earth. The images used to construct the false-color pictures were made through a combination of filters covering the electromagnetic spectrum from ultraviolet (top), to visible (middle) and infrared (bottom) wavelengths highlighting different features in the Saturnian atmospheric bands and rings. Well known for its bright ring system and large, mysterious moon Titan, gas giant Saturn is also a planet with a dynamic atmosphere and high-speed winds. In fact, in the 1980s, Voyager spacecraft measured equatorial winds of over 1,000 miles per hour. Giant storm systems, comparable in size to planet Earth itself, have been seen erupting in Saturn's cloud tops.

APOD: 2003 August 17 - Natural Saturn On The Cassini Cruise
Explanation: What could you see approaching Saturn aboard an interplanetary cruise ship? Your view would likely resemble this subtly shaded image of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently en route, the Cassini spacecraft. Cassini flew by Jupiter at the turn of the millennium and is scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth.

APOD: 2003 April 5 - The Seasons of Saturn
Explanation: Since Saturn's axis is tilted as it orbits the Sun, Saturn has seasons, like those of planet Earth ... but Saturn's seasons last for over seven years. So what season is it on Saturn now? Orbiting the equator, the tilt of the rings of Saturn provides quite a graphic seasonal display. In fact, this month, Saturn's rings will reach their most "open" angle after appearing nearly edge on in the mid-1990s. The ringed planet is also well placed in evening skies providing a grand view as summer comes to Saturn's southern hemisphere and winter to the north. The Hubble Space Telescope took the above sequence of images about a year apart, starting on the left in 1996 and ending on the right in 2000. Although they look solid, Saturn's Rings are likely less than 50 meters thick and consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2003 February 22 - Infrared Saturn
Explanation: This delightfully detailed false-color image of Saturn was taken in January 1998 by the orbiting Hubble Space Telescope. The picture is a combination of three images from Hubble's NICMOS instrument and shows the lovely ringed planet in reflected infrared sunlight. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere. The bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left. Presently, Saturn shines brightly in evening skies as a pale yellow "star" near the constellation Orion.

APOD: 2002 May 19 - Saturn's Moon Tethys
Explanation: Tethys is one of the larger and closer moons of Saturn. It was visited by both Voyager spacecraft - Voyager 1 in November 1980 and by Voyager 2 in August 1981. Tethys is now known to be composed almost completely of water ice. Tethys shows a large impact crater that nearly circles the planet. That the impact that caused this crater did not disrupt the moon is taken as evidence that Tethys was not completely frozen in its past. Two smaller moons, Telesto and Calypso, orbit Saturn just ahead of and behind Tethys. Giovanni Cassini discovered Tethys in 1684. In 1997, NASA launched a spacecraft named Cassini to Saturn that will arrive in 2004.

APOD: 2002 February 15 - Saturn: Lord of the Rings
Explanation: Born on today's date in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings. After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. But accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. Still mesmerized, some will take advantage of next week's (February 20th) favorable lunar occultation of Saturn to search for evidence of ring material outside the well known boundaries of the ring system. The presence of such a "lost" ring of Saturn was first hinted at in reports dating back to the early 20th century.

APOD: 2001 July 2 - The Seasons of Saturn
Explanation: Soon it will be winter in Saturn's northern hemisphere. Since Saturn is tilted in its orbit around the Sun, it has seasons just like the Earth. When a hemisphere is tilted so that the Sun passes more directly overhead, summer occurs. Half an orbit later -- about 15 (Earth) years for Saturn -- winter occurs. Since the rings of Saturn orbit the equator, they provide a quite graphic seasonal display. The Hubble Space Telescope took the above sequence of images about a year apart, starting on the lower left in 1996. Saturn's rings are less than 50 meters thick and are composed of pebble and boulder sized chunks of dusty water ice.

APOD: 2001 May 25 - Saturn The Giant
Explanation: Forty years ago today (May 25, 1961) U.S. president John Kennedy announced the goal of landing Americans on the Moon by the end of the decade. Kennedy's ambitious speech triggered a nearly unprecedented peacetime technological mobilization and one result was the Saturn V moon rocket. Its development directed by rocket pioneer Wernher Von Braun, the three stage Saturn V stood over 36 stories tall. It had a cluster of five first stage engines fueled by liquid oxygen and kerosene which together were capable of producing 7.5 million pounds of thrust. Giant Saturn V rockets ultimately hurled nine Apollo missions to the Moon and back again with six landing on the lunar surface. The first landing, by Apollo 11, occurred on July 20, 1969 achieving Kennedy's goal. Bathed in light, this Saturn V awaits an April 11, 1970 launch on the third lunar landing mission, Apollo 13.

APOD: 2001 March 7 - Saturn At Night
Explanation: From a spectacular vantage point over 1.4 billion kilometers from the sun, the Voyager 1 spacecraft looked back toward the inner solar system to record this startling view of Saturn's nightside. The picture was taken on November 16, 1980, some four days after the robot spacecraft's closest approach to the gorgeous gas giant. The crescent planet casts a broad shadow across its bright rings while the translucent rings themselves can be seen to cast a shadow on Saturn's cloud tops. Since Earth is closer to the sun than Saturn, only Saturn's dayside is visible to Earth-bound telescopes which could never take a picture like this one. After this successful flyby two decades ago, Voyager 1 has continued outward bound and is presently humanity's most distant spacecraft. The next spacecraft to approach Saturn will be Cassini, on course to arrive in 2004.

APOD: 2000 November 3 - New Moons For Saturn
Explanation: Which planet has the most moons? For now, it's Saturn. Four newly discovered satellites bring the ringed planet's total to twenty-two, just edging out Uranus' twenty-one for the most known moons in the solar system. Of course, the newfound Saturnian satellites are not large and photogenic. The faint S/2000 S 1, the first discovered in the year 2000, is the tiny dot indicated at the lower right of this August 7th image made with the ESO 2.2 meter telescope at La Silla, Chile. (An eye-catching spiral galaxy at the upper left is in the very distant background!) Unlike Saturn's larger moons whose almost circular orbits lie near the planet's equatorial plane, all four newly discovered moons have irregular, skewed orbits drifting far from the planet. With sizes in the 10 to 50 kilometer range, they are are likely captured asteroids. The international team of astronomers involved in the discoveries hopes to get many observations of the tiny satellites allowing accurate orbital computations before Saturn is lost in the solar glare around March 2001. The team has also found several other irregular satellite candidates which are now being followed. Saturn's only previously known irregular satellite is Phoebe, discovered over 100 years ago by W. H. Pickering,

APOD: 2000 March 30 - Saturn-Sized Worlds Discovered
Explanation: The last decade saw the profound discovery of many worlds beyond our solar system, but none analogs of our home planet Earth. Exploiting precise observational techniques, astronomers inferred the presence of well over two dozen extrasolar planets, most nearly as massive as gas giant Jupiter or more, in close orbits around sun-like stars. Less massive planets must certainly exist, and yesterday preeminent planet-finders announced the further detection of two more new worlds -- each a potentially smaller, saturn-sized planet. The parent suns are 79 Ceti (constellation Cetus), at a distance of 117 light-years, and HD46375 (constellation Monoceros), 109 light-years away. With at least 70 percent the mass of Saturn, 79 Ceti's planet orbits on average 32.5 million miles from the star compared to 93 million miles for the Earth-Sun distance. This arresting artist's vision depicts the newly discovered world with rings and moons, known characteristics of giant planets in our solar system. HD46375's planet is at least 80 percent Saturn's mass, orbiting only 3.8 million miles from its parent star. While Saturn's mass is only one third of Jupiter's, it is still about 100 times that of Earth, and dramatic discoveries in the search for smaller planets are still to come.

APOD: 2000 March 4 - Saturn At Night
Explanation: From a spectacular vantage point over 1.4 billion kilometers from the sun, the Voyager 1 spacecraft looked back toward the inner solar system to record this startling view of Saturn's nightside. The picture was taken on November 16, 1980, some four days after the robot spacecraft's closest approach to the gorgeous gas giant. The crescent planet casts a broad shadow across its bright rings while the translucent rings themselves can be seen to cast a shadow on Saturn's cloud tops. Since Earth is closer to the sun than Saturn, only Saturn's dayside is visible to Earth-bound telescopes which could never take a picture like this one. After this successful flyby two decades ago, Voyager 1 has continued outward bound and is presently humanity's most distant spacecraft. The next spacecraft to approach Saturn will be Cassini, on course to arrive in 2004.

APOD: 2000 February 12 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2000 January 29 - Natural Saturn On The Cassini Cruise
Explanation: What could you see approaching Saturn aboard an interplanetary cruise ship? Your view would likely resemble this subtly shaded image of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently enroute - the Cassini spacecraft, launched in 1997 and scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth. But where is Cassini now? Still about 980 million kilometers from Saturn, last Sunday the spacecraft flew by asteroid 2685 Masursky.

APOD: January 30, 1999 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's Rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: November 5, 1998 - Natural Saturn On The Cassini Cruise
Explanation: What you could see approaching Saturn aboard an interplanetary cruise ship would closely resemble this subtly shaded view of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently enroute - the Cassini spacecraft, launched just over a year ago and scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth.

APOD: September 2, 1998 - Saturn from Earth
Explanation: Saturn is the second largest planet in our Solar System. Saturn has been easily visible in the sky since history has been recorded. Galileo used one of the first telescopes in 1610 to discover Saturn's rings, which he first thought were moons. Maxwell showed in 1856 that Saturn's rings couldn't be a single solid, since Saturn's own gravity would break it up. Were Saturn's rings assembled into a single body, it would measure less than 100 kilometers across. The origin of Saturn's rings, and of unusual radial patterns that appear on them called spokes, are still unknown. The above representative-color picture was taken from Earth in infrared light. A robot spacecraft Cassini launched in 1997 will reach Saturn in 2004.

APOD: April 24, 1998 - Infrared Saturn
Explanation: This delightfully detailed false color image of Saturn has been earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January of this year with the Hubble's new NICMOS instrument and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left.

APOD: January 18, 1998 - Saturn, Rings, and Two Moons
Explanation: NASA's robot spacecraft Voyager 2 made this image of Saturn as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots on the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting its elaborate ring system. A robot spacecraft named Cassini was launched last October and is expected to rendezvous with the giant gas planet in 2004.

APOD: October 29, 1997 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's Rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: September 21, 1997 - Looking Down on Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this photo was taken by the Voyager 1 spacecraft as it flew by Saturn in November 1980. The next spacecraft to approach Saturn will be Cassini which is currently scheduled to be launched later this year and reach Saturn in 2004.

APOD: August 29, 1997 - Cassini To Saturn
Explanation: Scheduled for launch in October, the Cassini spacecraft will spend seven years traveling through the Solar System -- its destination, Saturn. On arrival Cassini will begin an ambitious mission of exploration which will include parachuting a probe to the surface of Titan, Saturn's largest moon. This artist's vision offers a dramatic view of Cassini's engine firing during the SOI (Saturn Orbit Insertion) maneuver as it passes above the ring plane. Before the development of the telescope, the gas giant Saturn was the most distant planet known to astronomers. Ten times farther from the Sun it receives only 1 percent of the sunlight that Earth does. Operating in this faint sunlight, the Cassini spacecraft can't use solar arrays so, like other missions to the outer Solar System, it will be powered by radioisotope thermoelectric generators (RTGs).

APOD: May 31, 1997 - Saturn with Moons Tethys and Dione
Explanation: Saturn and two of its larger moons - Tethys and Dione - were photographed by the Voyager 1 spacecraft which flew by the planet in November of 1980. This picture gives an indication of Saturn's extensive ring system, which can be seen casting a shadow on the planet, as does Tethys. Saturn's rings are composed of many chunks of ice ranging in size from a pebble to a car. The rings have several large gaps, the largest of which is clearly visible in the picture and is named the Cassini Division, after its discoverer. Saturn appears brighter than most stars in the sky, and its rings can be discerned with a small telescope. A new spacecraft - Cassini - will visit Saturn and is currently scheduled for launch later in 1997.

APOD: May 24, 1997 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: March 12, 1997 - Saturn in Color
Explanation: Saturn is unusual but photogenic. The second largest planet in our Solar System, behind Jupiter, has been easily identifiable at night since history has been recorded. It was only with the invention of the telescope, however, that any evidence of its majestic ring system became apparent. Saturn itself is composed of mostly hydrogen and helium gas. Saturn's rings are composed of many ice chunks ranging in size from a penny to car. The above picture of Saturn is one of the earliest taken by the Hubble Space Telescope and is a digital reconstruction of three color images. The Cassini mission to Saturn is scheduled to be launched later this year and should reach Saturn in 2004.

APOD: July 17, 1996 - Looking Down on Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this photo was taken by the Voyager 1 spacecraft as it flew by Saturn in November 1980. The next spacecraft to approach Saturn will be Cassini which is currently scheduled to be be launched next year and reach Saturn in 2004.

APOD: July 16, 1996 - A Portrait of Saturn from Titan
Explanation: The above artistic portrait of Saturn depicts how it might look from Titan, Saturn's largest moon. In the foreground sits ESA's Huygens probe, which will be released by NASA's Cassini spacecraft. The Cassini mission to Saturn in currently planned for launch in late 1997. Cassini will reach Saturn in 2004 and will release the Huygens probe later that year. Titan is one of only two moons in the Solar System to have an atmosphere, It has been suggested Titan might have gasoline-like lakes, and may even harbor life.

APOD: April 29, 1996 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: March 18, 1996 - Saturn with Moons Tethys and Dione
Explanation: Saturn and two of its larger moons - Tethys and Dione - were photographed by the Voyager 1 spacecraft which flew by the planet in November of 1980. This picture gives an indication of Saturn's extensive ring system, which can be seen casting a shadow on the planet, as does Tethys. Saturn's rings are composed of many chunks of ice ranging in size from a pebble to a car. The rings have several large gaps, the largest of which is clearly visible in the picture and is named the Cassini Division, after its discoverer. Saturn appears brighter than most stars in the sky, and its rings can be discerned with a small telescope. A new spacecraft - Cassini - will visit Saturn and is currently scheduled for launch in 1997.

APOD: July 6, 1995 - Saturn, Rings, and Two Moons
Explanation: This image of Saturn was made by NASA's robot spacecraft Voyager 2 as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots in the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting its elaborate ring system.

APOD: July 5, 1995 - The Night Side of Saturn
Explanation: This image of Saturn was made in November 1980 by the Voyager 1 spacecraft as it flew past the ringed gas giant planet. From a spectacular vantage point, looking back toward the inner solar system, the robot spacecraft recorded this view of the night side of Saturn casting a sharp shadow across the bright rings. No Earth based telescope could ever take a similar picture. Since Earth is closer to the sun than Saturn, only the day side of the planet is visible from the Earth.