Edmond Halley

Edmond (or Edmund) Halley FRS (/ˈhæli/; 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. As a child, Halley was very interested in mathematics. He studied at St Paul's School, where he developed his initial interest in astronomy, and was elected captain of the school in 1671. The following year, Halley's mother, Anne (probably née Robinson) died. In July 1673, he began studying at The Queen's College, Oxford. Halley 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 Astronomer Royal (England's first), 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.

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", about 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, dropping out of school to do so. He chose the south Atlantic island of Saint Helena (west of Africa), from which he would be able to observe not only the southern stars, but also some of the northern stars with which to cross-reference them. King Charles II supported his endeavour. Halley sailed to the island in late 1676, then set up an observatory with a large sextant with telescopic sights. Over a year, he made observations with which he would produce the first telescopic catalogue of the southern sky, and observed a transit of Mercury across the Sun. Focusing on this latter observation, Halley realised that observing the solar parallax of a planet—more ideally using the transit of Venus, which would not occur within his lifetime—could be used to trigonometrically determine 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.