Umbriel is one of the moons of Uranus, the seventh planet from the Sun. It is the second-largest of Uranus's moons, measuring about 1,169 kilometers (726 miles) in diameter. Umbriel is known for its dark surface, which is covered in a mix of ice and rock, and it has a heavily cratered landscape, indicating that it has been geologically inactive for a long time. Discovered in 1851 by the astronomer William Lassell, Umbriel is named after a character from Alexander Pope's poem "The Rape of the Lock." It orbits Uranus at a distance of approximately 266,000 kilometers (165,000 miles) and takes about 4.1 days to complete one orbit.