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Umbriel is a moon of the planet Uranus.

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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.
Discovered by William Lassell
Discovered in October 24, 1851
Orbital characteristics
Semi-major axis 266,000 km
eccentricity 0.0039
Periastron (Periuranion) 265,000 km
Apastron (Apuranion) 267,000 km
Orbital circumference 1,671,000 km
Orbital period 4.144 d
Mean orbital speed 4.668 km/s
Minimum orbital speed 4.650 km/s
Maximum orbital speed 4.686 km/s
inclination 0.205º (to Uranus' equator)
Is a Satellite of Uranus
Physical characteristics
Mean diameter 1169.4 km (0.092 Earths)
Surface area 4,296,000 km2 (0.008 Earths)
Volume 837,300,000 km3 (0.0008 Earths)
mass 1.20.11021 kg (0.0002 Earths)
Mean density 1.400.16 g/cm3
Surface gravity 0.23 m/s2 (0.023 g)
escape velocity 0.52 km/s
Rotation period synchronous
axial tilt zero
Rotation velocity 36.94 km/h (at the equator)
Albedo 0.21
Surface temp.
 ? K ~61 K  ? K
Atmospheric pressure 0 kPa

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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