Second Astronomer Royal, in office 1721-1742. The son of wealthy parents, Halley is the most celebrated of all the distinguished scientists who have held the office of Astronomer Royal. He was only 16 years of age on entering The Queen's College, Oxford, in 1673 and was soon making serious astronomical observations. In 1675 he suggested an expedition to the island of St. Helena in the South Atlantic to observe the southern stars, and under the auspices he undertook this task in 1676-1677, publishing his observations in his Catalogue of Southern Stars of 1697. Whilst on St. Helena, he observed the transit of the planet Mercury across the Sun's disc in 1677. The success of the latter observations convinced him that observations from widely spaced stations of the next transit of Venus, not due until 1761, would yield an accurate value of the solar parallax, and he appealed to the next generation of astronomers to undertake such observations. Later in life, he similarly appealed to younger astronomers when he realized that the comet he had observed in 1682 was identical to that seen in 1607 and 1531, which he thus predicted would return to the Earth's skies late in 1758. Halley was instrumental in persuading Isaac Newton to publish his fundamental work in natural philosophy, Philosphiae Naturalis Principia Mathematica, and when the Royal Society proved to be too short of funds, also paid for the printing of the first edition of 1687, perhaps the most important single publication in the history of science. Less happily, Halley was also at the center of the dispute with John Flamsteed over the publication of the Historia Coelestis of 1712 [see notes to classmark RGO 1]. Halley was appointed the commander of a ship in the Royal Navy and in three voyages between 1698 and 1701 made the variation of the magnetic declination in the Atlantic Ocean, from which he produced an isogonal world map. Another study in historical astronomical observations lead Halley to realize that the heavens were not immutable and that stars could be possessed of their own "proper motion." Given the hostility which existed between Halley and Flamsteed, it is ironic that Halley should be appointed as Flamsteed's successor at Greenwich. He spent the remaining years of his life there, not producing the wealth of inspired philosophical ideas which characterized his earlier life, but essentially undertaking the observation of the Moon over a complete sidereal cycle of the nodes of its orbit, a period of over 18 years, an ambitious goal for a man who was 64 years of age on taking up his post at Greenwich.