Uranus

13 March 1781, between ten and eleven at night

Between 10 and 11 p.m. on 13 March 1781, William Herschel was examining small stars near H Geminorum when one object struck him as visibly larger than the others. He was working from the garden of his home in Bath, England, with a reflecting telescope he had built himself.

The instrument had a 6.2-inch aperture, and it showed him a resolved disc rather than an ordinary point of light. The object was in Gemini, not Taurus — Uranus had been catalogued in Taurus by earlier observers who took it for a star.

Herschel did not conclude that night that he had found a planet. His initial suspicion was that the object was a comet.

Four nights later, the wrong answer

On 17 March the object had changed position, and Herschel treated that as confirmation of a comet. The motion only established that it was not a fixed star; the comet reading was his own inference, and it was wrong.

He reported a supposed comet to the Royal Society. Nevil Maskelyne, the Astronomer Royal, helped circulate and evaluate the observation among other astronomers, and Anders Johan Lexell's calculation of a nearly circular orbit helped establish that the object was a planet rather than a comet.

Georgium Sidus, and how long it lasted

Herschel later proposed the name Georgium Sidus in honour of George III, and subsequently received royal patronage worth £200 a year. Johann Elert Bode proposed Uranus in 1782 to preserve the mythological naming pattern.

Bode's name did not take over at once. Britain's Nautical Almanac retained the Georgian name until 1850.