Umbriel is the third largest and the darkest moon of Uranus. It is 16th moon in order from the planet. Umbriel is named after the "dusky melancholy sprite" in Alexander Pope's poem "The Rape of the Lock" and is also known as Uranus II. It was discovered by William Lassell on 24 October 1851, at the same time as Ariel.

 

Umbriel's size and density are about the same as those of Ariel, while its surface is old with little variation in the type of terrain from one region to another. A puzzling bright ring in Umbriel's surface, initially nicknamed the "fluorescent cheerio" and later formally named Wunda, may be the floor of a crater but its exact nature is unknown.

discovery	1851, by William Lassell
orbit semi-major axis	266,300 km (165,510 mi)
diameter	1,169 km (727 mi)
mean density	1.40 g/cm3
escape velocity	0.540 km/s (1944 km/h, 1208 mph)
orbital period	4.144 days (4 d 3 h 27 min)
axial period	4.144 days
orbital eccentricity	0.0039
orbital inclination	0.36°
visual albedo	0.21