Astronomer, Scientist. He was the second English Astronomer Royal, a position he held from 1720 until his death, and is best remembered for discovering the comet that bears his name. The son of a prosperous soap maker he studied at Saint Paul's School in London, England and attended The Queen's College at Oxford, graduating in 1676. He then visited the island of Saint Helena in the south Atlantic Ocean, setting up an observatory to catalog the stars of the southern hemisphere. During this time he observed a transit of the planet Mercury and determined that a similar transit of Venus could be used to measure the absolute size of the Solar System. Returning to England in 1678, he was asked by the Royal Society to travel to Danzig (now Gdansk, in present-day Poland), to settle a dispute between German astronomer Johannes Hevelius and English physicist Robert Hooke over the accuracy of Hevelius' astronomical observations, who did not use a telescope when taking astronomical measurements, and was able to satisfactorily verify the quality of Hevellus' observations. In 1679 he published the first part of his results from his observations at Saint Helena, the "Catalogus Stellarum Australium," which included the details of 341 southern stars. He then received his Master of Arts Degree at Oxford University and was elected as a Fellow of the Royal Society. In 1686 he published the second part of his Saint Helena expedition, a paper and charts on trade winds and monsoons, in which he identified solar heating as the cause of atmospheric motions as well as establishing the relationship between barometric pressure and the height above sea level. He also had an interest in the problems of gravity, particularly the proof of Kepler's laws on planetary motion. In 1664 he traveled to Cambridge, England to discuss it with Sir Isaac Newton, only to learn that he had solved the problem but had not published his findings. Halley convinced him to write the "Phiolsophae Naturalis Principia Mathematica" and it was published in 1687 at his expense. In 1691 he built a diving bell which he and five others demonstrated by lowering it into the Thames River at a depth of 60 feet, where they remained for over an hour and a half. Later that same year, he introduced a rudimentary working model of a magnetic compass to the Royal Society that contained a liquid-filled housing to help stabilize the swing and wobble of the magnetized needle. In 1692 he suggested the idea of a hollow earth, consisting of a 500 mile thick outer shell with two inner concentric shells and an innermost core, speculating that atmospheres separated each shell and that each shell had its own magnetic poles that rotated at a different speed, in an effort to try and explain anomalous compass readings. The following year, he published an article on life annuities that featured an analysis of mortality rates, based on statistics provided by the German professor and clergyman Caspar Neumann. By 1706 he had learned Arabic and had completed the translation of Books V through VII of Apollonius' "Conics" from copies at the Leiden and Bodleian Library at Oxford University. He also completed a new translation of Book I through IV of "Conics" from the original Greek, and published all these along with his own reconstruction of Book VIII in the first complete Latin edition in 1710. In 1698 he was given command of the English ship Paramour, to conduct investigations in the South Atlantic Ocean into laws governing the variation of the compass, but was forced to abandon the mission and return to England due to insubordination of the ship's crew. He received a temporary commission as a Captain in the British Royal Navy and sailed again on the Paramour from September 1699 until September 1700, publishing is results in "General Chart of the Variation of the Compass." In November 1703 he was appointed Savilian Professor of Geometry at Oxford University, eventually receiving an honorary doctor of laws degree. In 1705 he published his "Synopsis Astronomia Cometicae" which stated his belief that the comet sightings of 1456, 1531, 1607, and 1682 were of the same comet and predicted it would return in 1758 (now known as Halley's Comet, in his honor). In 1716, following the method described by Scottish mathematician James Gregory in his "Optica Promota," he suggested a high-precision measurement of the distance between the Earth and the Sun by timing the transit of the planet Venus. In 1718 he discovered the proper motion of the "fixed" stars by comparing his own astrometry measurements with those found in Ptolemy's "Almagest." Following the death of John Flamsteed in 1719, he was appointed in 1720 as successor to the position of Astronomer Royal. That same year, he participated with English antiquarian William Stukeley in the first attempt to scientifically date Stonehenge, based on the assumption it had been laid out using a magnetic compass. Arriving at three potential dates (920 AD, 220 AD, and 460 BC) they chose the earliest one. Although they were mistaken by several thousand years, the concept that scientific methods could be used to date ancient monuments was revolutionary in its day. He died at the age of 85. The Halley Craters on the Moon and the planet Mars are named in his honor.