Umbriel (astronomy)

Umbriel (astronomy), large moon of the planet Uranus. Umbriel orbits Uranus at a distance of about 266,000 km (about 160,000 mi), completing one orbit in just over four Earth days. Umbriel’s orbit is circular and parallels Uranus’s equator. Because the planet’s equator is tilted relative to its orbit around the sun, Umbriel’s orbit is tilted nearly perpendicular to the orbits of most other worlds in the solar system.

Umbriel is spherical and measures about 1170 km (about 702 miles) in diameter. It is about one-third as wide as Earth’s moon. Measurements of Umbriel’s density indicate that it is about half ice and about half rock.

Umbriel has the darkest surface of any moon of Uranus—it reflects only about half as much light as the neighboring moon Ariel. Umbriel’s surface is apparently uniformly old and heavily cratered, with few distinctive features. In photos taken by the United States Voyager 2 probe in 1986, only one feature stands out. This is a bright, ring-shaped feature that is barely visible at one edge of Umbriel in the Voyager 2 pictures. The feature is named Wunda and is more than 140 km (more than 84 mi) across. Wunda is probably a large crater with a bright ice floor, dark rim, and dark central peak. Much remains unknown about Umbriel because Voyager 2 took only a few pictures near one of Umbriel’s poles.

Umbriel was discovered in 1851 by English astronomer William Lassell, who named his find for a character in English poet Alexander Pope’s poem The Rape of the Lock. Individual features on Umbriel are named for evil spirits from folktales and myths around the world.