Sir Edmond Halley

1656-1742

English Astronomer and Physicist

Edmond Halley is best known for predicting the return of the comet that today bears his name and for the instrumental role he played in the publication of Isaac Newton's Principia.

Halley was born at Haggerton, near London, in late 1656 to a wealthy landowner and soapmaker. He attended St. Paul's School before matriculating at Queen's College, Oxford (1673). An experienced astronomical observer even before entering Oxford, Halley began assisting the Astronomer Royal John Flamsteed (1646-1719).

Leaving Oxford, Halley traveled to the island of St. Helena, where he established the southern hemisphere's first observatory and proceeded with the first telescopic mapping of the southern skies. Upon returning to England he published his Catalogus Stellarum Australium (1678) which gained him election to the Royal Society at age 22.

Also while at St. Helena he observed the transit of Mercury and realized that simultaneous measurements of a transit by astronomers at locations across Earth's surface could be used to determine the astronomical unit—distance from Earth to the Sun. He later concluded that the 1761 Venus transits would be more suitable (1679) and devised an appropriate observational method (1691,1694,1716). Measurements were made with a simplified version of Halley's method and a distance of 95 million miles was computed, which compares well with the present value of 92.9 million.

Good friends with Isaac Newton (1642-1727), Halley encouraged him to publish his work on gravitation and motion through the auspices of the Royal Society (1684). However, due to the Society's financial difficulties, Halley assumed full financial responsibility. The Principia finally appeared in 1687.

In 1695 Halley undertook his now-famous study of comets. Compiling records of previous comets, he made calculations of their orbits. This led him to believe that the comets of 1531, 1607, and 1682 were the same object. In 1705 Halley predicted the object would reappear in December 1758. Confirmation of this prediction, 16 years after his death, was one of the three main supports for Newton's gravitational theory during the Enlightenment. The object is now referred to as Halley's comet in his honor. Halley was also the first to suggest that nebulae are clouds of interstellar gas (1715).

Halley's next great achievement was based on his study of Ptolemy's (c. 100-170) writings. His close scrutiny of Ptolemy's star catalog revealed discrepancies between positions measured in Ptolemy's day and those taken 1,500 years later. Allowing for observational errors and precession, Halley was left to conclude that stars have individual or proper motions. He detected such motions in 1718.

In 1720 Halley succeeded Flamsteed as Astronomer Royal. Correctly believing that accurate lunar measurements would improve methods for determining longitude at sea, he initiated observations of the Moon though its Saros cycle of 18 years—the period after which Sun, Moon, and Earth return to their same relative positions. Halley completed the measurements but died before their publication in 1749.

Halley's contributions outside astronomy include the first map of Earth's winds (1686), a mathematical relationship between height and pressure (1686), and research on tidal phenomena (1684-1701). His most significant geophysical contribution, though, was on terrestrial magnetism. Compass data indicated Earth's magnetic field was slowly drifting westward, which seemed impossible if Earth's interior were solid as was then generally believed. Halley proposed a core-fluid-crust model to explain the phenomenon (1692). According to the model, Earth's magnetic field is produced by a solid iron core. Earth's outer shell or crust is separated from the core by an effluvium-filled region. The westward drift of the magnetic field was the result of the core rotating eastward slightly slower than the crust. Halley further suggested that escaping effluvium, governed by Earth's magnetic field, produced the aurora borealis.

STEPHEN D. NORTON