Nevil Maskelyne. Portrait of the English astronomer Nevil Maskelyne (1732-1811), from 1765, the fifth Astronomer Royal. In 1756 he joined the Royal Society who, in 1761, sent him to the South Atlantic island of St Helena to observe a transit of Venus. Such transits were used to find the Earth's distance from the Sun and the size of the Solar System. During the voyage he tried to accurately determine his longitude from the Moon's position. In 1764 he sailed to Barbados to test the ability of John Harrison's marine chronometer (naval clock) ^IH4^i to find longitude. Maskelyne later carried out an experiment to determine the Earth's density. From a painting by J. Downman.