Definition of Umbriel
: the third largest satellite of Uranus with a diameter of approximately 730 miles (1170 km) Earlier this year, Buratti and her co-workers reported other evidence suggesting that Umbriel might be a Uranian oddity. Analyzing photographs taken through various color filters by the Voyager 2 spacecraft, they found that the ratio of green light to violet light varies widely over Umbriel's surface. — Science News, 16 Nov. 1991
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