Amateur Astronomer William Lassell and the Discovery of Uranus Moons Ariel and Umbriel

The discovery of Ariel and Umbriel, two prominent moons of Uranus, took place 175 years ago on October 24, 1851. These satellites were discovered by the English amateur astronomer William Lassell.

The Path to Astronomy

Born in Bolton, England on June 18, 1799, Lassell moved to Toxteth, Liverpool in 1815 and apprenticed in a merchant's office until 1822. He started a beer business on Milton Street in Liverpool by November 1824, which brought him wealth and leisure time. This financial success allowed him to pursue astronomy and build his own observatory called 'Starfield' on West Derby Road by the summer of 1839. He built a 9-inch telescope himself and later visited Irish astronomer William Parsons, 3rd Earl Rosse, who designed the Leviathan of Parsonstown.

The Grand Amateurs

Lassell partnered with two other prominent amateur astronomers: William Rutter Dawes, an English astronomer with an observatory in Ormskirk, and James Nasmyth, a Scottish engineer who invented the steam hammer. Together, they were known as the 'Grand Amateurs' of astronomy. Using Lassell's workshops at Starfield, the trio built a 24-inch diameter telescope with a 20-foot-long tube and a mirror polished by a machine of Lassell's own design. This telescope achieved equatorial mounting, which allowed them to track and observe specific objects for longer periods.

Telescope Discoveries

Over five years, this telescope helped Lassell discover four major solar system satellites. On October 10, 1846, he discovered Triton, the largest moon of Neptune, just 17 days after Neptune itself was found. In 1848, he discovered Hyperion, a moon of Saturn, sharing this find with the American father-son duo William Cranch Bond and George Phillips Bond. Finally, on October 24, 1851, he discovered Ariel and Umbriel, adding them to the previously known Uranus moons Titania and Oberon found in 1787.

Later Years and Honours

Lassell built a final Newtonian Reflecting Telescope that was 48 inches wide and 37 feet long, and travelled between Liverpool and Malta for better viewing conditions. His scientific contributions brought him many honours, including the Royal Astronomical Society Gold Medal in 1849, election as a fellow of the Royal Society in 1849, the Royal Medal in 1858, and serving as the president of the Royal Astronomical Society from 1870 to 1872. He passed away in 1880.

Context

William Lassell belongs to a historical era when wealthy amateurs played a massive role in scientific progress before astronomy became heavily institutionalized. Uranus currently has 29 officially recognized moons, with Ariel and Umbriel remaining among its most prominent inner satellites. Telescopes with equatorial mounts revolutionized early astronomy by letting observers compensate for Earth's rotation to keep stars and planets in steady focus.