The English astronomer Edmund Halley (1656-1742) studied the orbital movements of the moon and of comets and discovered the proper motion of the fixed stars.

The son of a prosperous London soap-boiler, Edmund Halley was born on November 8, 1656, in Haggerston near London. He attended St. Paul's School, where he excelled in classics and mathematics and early developed an interest in astronomy. At the age of 16, when he entered Queen's College, Oxford, he was already an accomplished astronomical observer. He continued his observations at Oxford and, before he was 20, had sent to the Royal Society an explanation of an improved means of calculating planetary orbits.

Recognizing the need for more accurate star charts, Halley, while still an undergraduate, proposed a plan for surveying the stars of the southern hemisphere as a supplement to the surveys then being made of the northern hemisphere by John Flamsteed and Johannes Hevelius. He left Oxford without a degree and in 1676 journeyed to the island of St. Helena in the South Atlantic. St. Helena's frequent cloud cover made it poorly suited for astronomical observations, although in 18 months on the island Halley managed to determine the position of approximately 350 stars. In addition, he made one of the first complete observations of a transit of Mercury; it occurred to him that similar transits might be used to accurately calculate the sun's distance from the earth. He returned to England in 1678, published his results, and was dubbed by Flamsteed "the Southern Tycho," a reference to the famous Danish astronomer Tycho Brahe.

Publication of Newton's "Principia"

Upon his return Halley received, by royal mandate, his Oxford degree and, at the age of 22, was elected a fellow of the Royal Society. After two years of traveling on the Continent, he returned to London, where he married and, in 1682, began a lengthy series of lunar observations. Designed to last for 18 years, these observations were to correct tables of the moon's position in an effort to solve the problem of accurately determining longitude. Such a lengthy project was not, however, well suited to Halley's temperament, and he was soon diverted to other concerns.

Intensely interested in the problem of gravitation, Halley had obtained by 1684 an inverse-square relationship, but since he was unable to deduce from it the planetary motions, in August that year he traveled to Cambridge to seek the assistance of Newton. What would be the orbit of a planetary body subjected to such a force? An ellipse, Newton replied. He had earlier proved that this was so and shortly thereafter sent Halley a copy of his demonstration. Realizing the significance of what Newton had done, Halley, utilizing great skill and tact, persuaded the reluctant Newton to develop and publish his ideas on celestial mechanics. Newton's Principia was published in 1687. Halley read the manuscript, corrected the proofs, and paid the publication costs out of his own pocket. A lasting friendship ensued, and in 1696, through Newton's influence, Halley was appointed deputy comptroller of the Mint at Chester.

Astronomical and Physical Observations

Halley maintained a lifelong interest in the declination of the magnetic compass, and he published two significant papers (1683 and 1692) discussing the causes of this variation and its change with time. Between 1698 and 1702 he undertook a series of government-sponsored expeditions to make extensive measurements of terrestrial magnetism in the South Atlantic and to study in detail the tides and coast of the English Channel. He correlated the data from his South Atlantic voyages with other measurements he had been collecting and in 1702 published for the first time a map showing lines of equal declination. Of great navigational value, these lines (known today as isogonics) were for years called "Halleyan lines."

Halley's calculation of the periodic nature of comets was perhaps his most significant contribution to astronomy. In his Synopsis of the Astronomy of Comets (1705) he collected and analyzed all known observations of comets and computed the parabolic orbits of 24 comets dating from 1337 to 1698. The orbital elements of three (1531, 1607, and 1682) were so similar as to suggest that they were in fact the successive returns of a single body whose orbit was an enormous elongated ellipse, rather than a parabola, and whose period of revolution was approximately 76 years. Halley successfully predicted the return of this comet in 1758 and suggested that other comets might also have elliptical orbits. Halley's comet, as it is known today, returned on schedule in 1835, 1910, and 1986.

Before Halley's discovery of the "proper motion" of fixed stars, it was believed that they (unlike the planets) never moved in relation to each other. In 1718, however, Halley pointed out that three of the brightest stars (Sirius, Procyon, and Arcturus) had apparently changed their relative positions markedly since having been observed by the Greeks. In fact, Sirius appeared to have moved perceptibly since observed by Tycho Brahe only a century and a half earlier. After carefully comparing the positions of other stars and establishing that this apparent movement could not be accounted for by any motion of the earth, Halley concluded that the three had actually shifted their relative positions and suggested that, if observed over sufficiently long periods, this proper motion might also be detected in other stars as well.

Halley's knowledge and interests were extensive. He pursued such varied topics as the magnetic origin of the aurora borealis, the design and construction of diving bells, and the establishment of quantitatively accurate mortality tables. He continued his astronomical observations until a few months before his death on January 14, 1742.

Further Reading

• Selections from Halley's correspondence and unpublished papers, together with two 18th-century biographical memoirs, are in Eugene Fairfield MacPike, ed., Correspondence and Papers of Edmund Halley (1932). The best biography of Halley is Angus Armitage, Edmund Halley (1966). Also useful is Colin A. Ronan, Edmund Halley: Genius in Eclipse (1969). For Halley's relations with contemporary astronomers see Eugene Fairfield MacPike, Hevelius, Flamsteed and Halley (1937).
• Standing on the Shoulders of Giants: A Longer View of Newton and Halley, Berkeley: University of California Press, 1990.