| Umbriel is a moon of Uranus discovered on 1851-10-24 by William Lassell. Umbriel was discovered at the same time as Ariel. Naming of Umbriel. Umbriel Moon of Uranus. 
Moon Umbriel. | | Discovery |
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| Discovered by | William Lassell |
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| Discovered in | October 24, 1851 |
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| Orbital characteristics |
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| Semi-major axis | 266,000 km |
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| eccentricity | 0.0039 |
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| Periastron (Periuranion) | 265,000 km |
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| Apastron (Apuranion) | 267,000 km |
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| Orbital circumference | 1,671,000 km |
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| Orbital period | 4.144 d |
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| Mean orbital speed | 4.668 km/s |
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| Minimum orbital speed | 4.650 km/s |
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| Maximum orbital speed | 4.686 km/s |
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| inclination | 0.205º (to Uranus' equator) |
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| Is a Satellite of | Uranus |
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| Physical characteristics |
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| Mean diameter | 1169.4 km (0.092 Earths) |
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| Surface area | 4,296,000 km2 (0.008 Earths) |
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| Volume | 837,300,000 km3 (0.0008 Earths) |
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| mass | 1.2±0.1×1021 kg (0.0002 Earths) |
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| Mean density | 1.40±0.16 g/cm3 |
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| Surface gravity | 0.23 m/s2 (0.023 g) |
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| escape velocity | 0.52 km/s |
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| Rotation period | synchronous |
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| axial tilt | zero |
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| Rotation velocity | 36.94 km/h (at the equator) |
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| Albedo | 0.21 |
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| Surface temp. | |
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| Atmospheric pressure | 0 kPa |
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The name "Umbriel" and the names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell. Lassell had earlier endorsed Herschel's 1847 naming scheme for the seven then-known satellites of Saturn and had named his newly-discovered eighth satellite Hyperion in accordance with Herschel's naming scheme in 1848. Umbriel is the 'dusky melancholy sprite' in Alexander Pope's The Rape of the Lock, and the name suggests the Latin umbra, shadow. Umbriel is also designated Uranus II. Physical characteristics of Umbriel. So far the only close-up images of Umbriel are from the Voyager 2 probe, which made observations of the moon during its Uranus flyby in January, 1986. During the flyby the southern hemisphere of the moon was pointed towards the Sun so only it was studied. Umbriel's surface is the darkest of the Uranian moons, and it is also the least geologically active. It is mostly composed of water ice, with the balance made up of silicate rock, and probably other ices such as carbon dioxide and/or methane. Umbriel's most prominent feature is Wunda, a large ring of bright material near Umbriel's equator (see picture; the viewpoint is nearly polar). Wunda is presumably some kind of crater, but its exact nature is mysterious. Nearby, seen along the equator, is the crater Skynd, which lacks a bright rim but possesses a bright central peak.
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