Astronomy Picture of the Day
APOD: 2001 August 26 - Uranus: The Tilted Planet
Explanation: Uranus is the third largest planet in our Solar System after Jupiter and Saturn. Uranus is composed mostly of rock and ices, but with a thick hydrogen and helium atmosphere. The blue hue of Uranus' atmosphere arises from the small amount of methane which preferentially absorbs red light. This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus has many moons and a ring system. Uranus, like Venus, has a rotation axis that is greatly tilted and sometimes points near the Sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are quite similar: Uranus is slightly larger but less massive.
APOD: 1997 November 26 - Uranian Moons, Rings, And Clouds
Explanation: The giant planet Uranus is faint and featureless when viewed in visible light. But this pair of near-infrared mosaics from the Hubble Space Telescope's NICMOS camera reveals moons, rings, and clouds of this distant gas planet. The color coded images highlight different atmospheric layers - blue represents the deepest layers while the highest cloud features have a reddish tinge. Racing around the planet, high, bright clouds are seen to move substantially between the two pictures taken only ninety minutes apart. Ring systems are a common to the solar system's giant planets. Here the main Uranian ring seems to vary in width and is clearly brightest near the top. The eight specks beyond the ring system are small Uranian moons which also show counter-clockwise motion over ninety minutes as traced by the arrows on the right hand image.
APOD: 1996 April 30 - Uranus' Ring System
Explanation: The rings of Uranus are thin, narrow, and dark compared to other planetary ring systems. Brightened artificially by computer, the ring particles reflect as little light as charcoal, although they are really made of ice chucks darkened by rock. This false-color, infrared picture from the Hubble Space Telescope taken in July 1995 shows the rings in conjunction to the planet. The infrared light allows one to see detail in different layers of Uranus' atmosphere, which has been digitally enhanced with false color. Three other planets in our Solar System are known to have rings: Jupiter, Saturn, and Neptune. Four of Uranus' moons are visible outside the ring plane. The rings of Uranus were discovered from ground-based observations in 1977.
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