William Lassell discovers Ariel and Umbriel, moons of Uranus

Background. William Lassell was an English merchant and astronomer who built an observatory at his house 'Starfield' in West Derby, a suburb of Liverpool. He was remembered for his improvements to the reflecting telescope and his discoveries of four planetary satellites, including Triton, the largest moon of Neptune, which he found in 1846 using his self-built instrument.

Discovery. On 24 October 1851, Lassell discovered Ariel and Umbriel, two moons of Uranus, at the same time. Umbriel is the third-largest moon of Uranus. William Herschel, who had discovered Titania and Oberon in 1787, had claimed to have observed four additional moons, but those observations were never confirmed and the four objects are now thought to be spurious.

Naming. The names of the four known Uranian satellites, including Ariel and Umbriel, were suggested by John Herschel in 1852 at Lassell's request. The names are derived from characters created by William Shakespeare or Alexander Pope. It remains uncertain whether Herschel devised the names or if Lassell proposed them and sought Herschel's permission.