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7 known moons of Uranus. Discovered on 24 October 1851 by William Lassell, it is named for a sky spirit in Alexander Pope's The Rape of the Lock and Shakespeare's The Tempest. Like its parent planet, Ariel orbits on its side, giving it an extreme seasonal cycle. As of 2011, almost all knowledge of Ariel derives from a single flyby of Uranus performed by the spacecraft Voyager 2 in 1986, which managed to image 35% of the moon's surface. There are no plans at present to return to study the moon in more detail.
After Miranda, Ariel is the second-smallest of Uranus's five round satellites, and the second-closest to its planet. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), Ariel is composed of roughly equal parts ice and rocky material. Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it may be differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface comprising extensive cratered terrain cross-cut by a system of scarps, canyons and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.
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Did you know

... that William Herschel's 40-foot telescope (pictured) was the largest telescope in the world for 50 years?
...that Zeeman-Doppler imaging is a technique used to map the surface magnetic field of stars?
...that Astronomische Nachrichten, founded by H. C. Schumacher in 1821, is the world's oldest existant astronomical journal?
...that the Stingray Nebula, thought to have formed around 1987, is the youngest known planetary nebula?
...that the Mark II radio telescope built in 1964 at Jodrell Bank Observatory, UK was the first ever telescope to be controlled by a digital computer?
...that Comet Shoemaker-Levy 9 passed within Jupiter's Roche limit in 1992, causing it to break up into smaller pieces two years before it collided with the planet?
...that the Kaidun meteorite fell on March 12, 1980 on a Soviet military base in Yemen and may be from Phobos?

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Selected picture

Credit: NASA/JPL-Caltech/T. Megeath (Harvard-Smithsonian CfA).
NGC 7129 is an open cluster star-forming region in a reflection nebula which a circa 2004 survey shows has over 130 very young stars each less than 1 million years old. This image of the open cluster was taken by Spitzer Space Telescope.
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Astronomy News
Wikinews Space portal

June 17: Study says young stars 'shoot water bullets'
January 11: Moon water possibly originated from comets, data shows
October 15: Brazilian astronomers propose new model of our galaxy
October 1: New planet found in 'Habitable Zone'
September 19: Jupiter at its brightest in 47 years
August 25: Astronomers discover large exo-solar system
August 22: Scientists say the moon is slowly shrinking
August 20: Scientists conclude that the universe may expand forever
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Wikibooks

These books may be in various stages of development. See also the related Science and Mathematics bookshelves.
Astronomy
GAT: A Glossary of Astronomical Terms
Introduction to Astrophysics
General relativity
Observing the Sky from 30°S
Observing the Sky from 40°N
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Solar System
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Astronomical events
1 May, 00:06 Venus at max brightness
6 May, 05:35 Full moon
6 May, 05:40 Moon at perigee
7 May, 08:00 Eta Aquariids peak
13 May, 15:23 Jupiter at Conjunction
19 May, 18:28 Moon at apogee
20 May, 23:53 New moon and annular solar eclipse
27 May, 13:18 Mercury at superior Conjunction