Edmond (or Edmund) Halley (; 8 November [O.S. 29 October] 1656 – 25 January 1742 [O.S. 14 January 1741]) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.
From an observatory he constructed on Saint Helena in 1676–77, Halley catalogued the southern celestial hemisphere and recorded a transit of Mercury across the Sun. He realised that a similar transit of Venus could be used to determine the distances between Earth, Venus, and the Sun. Upon his return to England, he was made a fellow of the Royal Society, and with the help of King Charles II, was granted a master's degree from Oxford.
Halley encouraged and helped fund the publication of Isaac Newton's influential Philosophiæ Naturalis Principia Mathematica (1687). From observations Halley made in September 1682, he used Newton's law of universal gravitation to compute the periodicity of Halley's Comet in his 1705 Synopsis of the Astronomy of Comets. It was named after him upon its predicted return in 1758, which he did not live to see.
Beginning in 1698, Halley made sailing expeditions and made observations on the conditions of terrestrial magnetism. In 1718, he discovered the proper motion of the "fixed stars".
Halley was born in Haggerston in Middlesex. His father, Edmond Halley Sr., came from a Derbyshire family and was a wealthy soap-maker in London. Halley showed an early interest in mathematics. He studied at St Paul's School, where he developed an interest in astronomy and was elected captain of the school in 1671. The following year, his mother, Anne (probably née Robinson) died. In July 1673, Halley entered The Queen's College, Oxford. He took a twenty-four-foot (7.3 m) long telescope with him, apparently paid for by his father. While still an undergraduate, Halley published papers on the Solar System and sunspots. In March 1675, he wrote to John Flamsteed, the first Astronomer Royal, telling him that the leading published tables on the positions of Jupiter and Saturn were erroneous, as were some of Tycho Brahe's star positions. His association with Flamsteed soon led to Halley's first publication and to his expedition to Saint Helena.
In 1676, Flamsteed helped Halley publish his first paper, titled "A Direct and Geometrical Method of Finding the Aphelia, Eccentricities, and Proportions of the Primary Planets, Without Supposing Equality in Angular Motion", on planetary orbits, in Philosophical Transactions of the Royal Society. Influenced by Flamsteed's project to compile a catalogue of stars of the northern celestial hemisphere, Halley proposed to do the same for the southern sky. He left Oxford to undertake the project, chosing the South Atlantic island of Saint Helena, from which he could observe southern stars as well as some northern stars for comparison. King Charles II supported the expedition.
Halley sailed to Saint Helena in late 1676 and established an observatory with a large sextant with telescopic sights. Over the following year, he made observations for what became the first telescopic catalogue of the southern sky, and observed a transit of Mercury across the Sun. Focusing the Mercury transit, Halley recognised that observations of planetary solar parallax, especially during a transit of Venus, could be used to determine trigonometrically the distances between Earth, Venus and the Sun.
Halley returned to England in May 1678, and used his data to produce a map of the southern stars. Oxford would not allow Halley to return because he had violated his residency requirements when he left for Saint Helena. He appealed to Charles II, who signed a letter requesting that Halley be unconditionally awarded his Master of Arts degree, which the college granted on 3 December 1678. Just a few days before, Halley had been elected as a fellow of the Royal Society, at the age of 22. In 1679, he published Catalogus Stellarum Australium ('A catalogue of the stars of the South'), which includes his map and descriptions of 341 stars. Robert Hooke presented the catalogue to the Royal Society. In mid-1679, Halley went to Danzig (Gdańsk) on behalf of the society to help resolve a dispute: because astronomer Johannes Hevelius' observing instruments were not equipped with telescopic sights, Flamsteed and Hooke had questioned the accuracy of his observations; Halley stayed with Hevelius and checked his observations, finding that they were quite precise.
By 1681, Giovanni Domenico Cassini had told Halley of his theory that comets were objects in orbit. In September 1682, Halley carried out a series of observations of what became known as Halley's Comet; his name became associated with it because of his work on its orbit and predicting its return in 1758 (which he did not live to see). In early 1686, Halley was elected to the Royal Society's new position of secretary, requiring him to give up his fellowship and manage correspondence and meetings, as well as edit the Philosophical Transactions. Also in 1686, Halley published the second part of the results from his Helenian expedition, being a paper and chart on trade winds and monsoons. The symbols he used to represent trailing winds still exist in most modern day weather chart representations. In this article he identified solar heating as the cause of atmospheric motions. He also established the relationship between barometric pressure and height above sea level. His charts were an important contribution to the emerging field of information visualisation.
Halley spent most of his time on lunar observations, but was also interested in the problems of gravity. One problem that attracted his attention was the proof of Kepler's laws of planetary motion. In August 1684, he went to Cambridge to discuss this with Isaac Newton, much as John Flamsteed had done four years earlier, only to find that Newton had solved the problem, at the instigation of Flamsteed with regard to the orbit of Kirch's Comet, without publishing the solution. Halley asked to see the calculations and was told by Newton that he could not find them, but promised to redo them and send them on later, which he eventually did, in a short treatise titled On the motion of bodies in an orbit. Halley recognised the importance of the work and returned to Cambridge to arrange its publication with Newton, who instead went on to expand it into his Philosophiæ Naturalis Principia Mathematica published at Halley's expense in 1687. Halley's first calculations with comets were thereby for the orbit of Kirch's Comet, based on Flamsteed's observations in 1680–1681. Although he was to accurately calculate the orbit of the comet of 1682, he was inaccurate in his calculations of the orbit of Kirch's Comet. They indicated a periodicity of 575 years, thus appearing in the years 531 and 1106, and presumably heralding the death of Julius Caesar in a like fashion in 45 BC. It is now known to have an orbital period of circa 10,000 years.
In 1691, Halley built a diving bell, a device in which the atmosphere was replenished by way of weighted barrels of air sent down from the surface. In a demonstration, Halley and five companions dived to 60 feet (18 m) in the River Thames, and remained there for over an hour and a half. Halley's bell was of little use for practical salvage work, as it was very heavy, but he made improvements to it over time, later extending his underwater exposure time to over 4 hours. Halley suffered one of the earliest recorded cases of middle ear barotrauma. That same year, at a meeting of the Royal Society, Halley introduced a rudimentary working model of a magnetic compass using a liquid-filled housing to damp the swing and wobble of the magnetised needle.
In 1691, Halley sought the post of Savilian Professor of Astronomy at Oxford. While a candidate for the position, Halley faced the animosity of the Astronomer Royal, John Flamsteed, and the Anglican Church questioned his religious views, largely on the grounds that he had doubted the Earth's age as given in the Bible. After Flamsteed wrote to Newton to rally support against Halley, Newton wrote back in hopes of reconciliation, but was unsuccessful. Halley's candidacy was opposed by both the archbishop of Canterbury, John Tillotson, and Bishop Stillingfleet, and the post went instead to David Gregory, who had Newton's support.