Halley, Edmond

Halley, Edmond (1656–1742)English scientist. From 1676 he spent two years observing the southern sky from St Helena, and in 1678 published a catalogue of 341 stars, the first southern star catalogue compiled from telescopic observations. He was the first to suggest that observations of transits of Venus could be used to measure the Sun's distance, which was eventually done by N.Maskelyne long after his death. In 1683 Halley commenced a long series of lunar studies, discovering the Moon's secular acceleration in 1693. In 1684, having deduced the inverse-square law, he visited I.Newton and persuaded him to write the Principia. In 1705 Halley published the Synopsis of Cometary Astronomy in which he concluded that the comet he had observed in 1682 was the same as those of 1531 and 1607. He predicted it would return in 1758, which it did and was named Halley's Comet. In 1718 he concluded that the brightest stars had changed position since the time of Ptolemy's Almagest, thus discovering proper motion. As the second Astronomer Royal (from 1720) he initiated a series of lunar and solar observations spanning 18 years—a complete saros. In 1721 he raised the problem of what has come to be called Olbers' paradox.