Other Discoveries

describe the discoveries of Ceres, Uranus, Neptune and Pluto and the techniques involved

Ceres

Ceres was the first asteroid ever discovered. A Sicilian monk, Giuseppe Piazzi first saw it in January 1801. At first he thought it was a faint fixed star but he noticed that it moved position, as planets do, over the next few days. He took careful measurements of its position over a period of 41 days but then fell ill.

Ceres was lost due to the glare of the Sun but its position was predicted thanks to new mathematical techniques and it was found again. Over the next few years several similar objects were discovered in roughly the same region and the term asteroid was introduced.

Uranus

Uranus had been observed many times by early astronomers but because of its dimness and slow orbit it was always assumed to be a star. It was first recognised as a planet by William Herschel in 1781 with the aid of his telescope.

He noticed that over a period of time its position changed in the sky. At first he announced that he had discovered a comet but, because it has no tail and a very circular orbit, it was soon realised to be a planet.

Neptune

The existence of Neptune was predicted before its discovery because of irregularities in the orbit of Uranus. It was realised that another large body with significant gravity was producing these irregularities. It was eventually observed in 1846 by Heinrich D'arrest in a position predicted by mathematician Urbain Le Verrier.

Pluto

The presence of Pluto was also predicted from irregularities in the orbit of Uranus in 1906 by American Percival Lowell. Despite an extensive search no planet was found until in 1930 Clyde Tombaugh, after a year of searching, found the elusive object.

He used a device called a blink comparator. Two photographs of a section of the sky are taken several days apart then these photographs are observed quickly, one after the other. Any object that has moved would appear to blink.

Once Pluto's mass had been calculated it was realised that it could not have caused the irregularities in the orbit of Uranus that led to the search for it.