Edmund Halley (1656-1742) first studied at St Paul’s School (est.1509), London and then went to Queen’s College (1341), Oxford. While still a student Halley published a little book on Kepler’s laws. In 1676 he left Oxford without finishing his degree and in November sailed for St Helena to make the first catalogue of the stars of the southern hemisphere.
He returned to England in 1678 and published a star chart. Despite not having graduated from Oxford he was now regarded as a leading astronomer and was elected to the Royal Society on 30 November. The following year he published Catalogus Stellarum Australium (‘Catalogue of the Southern Stars’), which included details of 341 southern stars.
It was generally thought that a comet came into the Solar System, swerved past the Sun and then continued straight on out of the Solar System. In his Principia Newton proposed that unless the velocity of a departing comet was sufficiently large then it would be unable to escape the magnetic pull of the Sun and would therefore one day be drawn back into the Solar System but with its path modified by the gravitational pull of the planets. In 1705 Halley, using historical astronomy records, published Synopsis Astronomiae Cometicae (‘Summary of Comet Astronomy’). In this he stated his belief that the comet sightings of 1456, 1531, 1607 and 1682 were related to the same comet and predicted it would return in 1758. In his prediction Halley allowed for the pull of Jupiter on the comet as it approached its passage around the Sun but not for the effect as it left the Solar System. In June 1757, Alexis-Claude Clairaut (1713-65) predicted that the comet would swing round the Sun in 1758/9. It was first seen on Christmas day 1758 by Johann Palitzsch (1723-88), an amateur astronomer living in Dresden.
The first professional to observe the comet was Charles Joseph Messier (1730-1817), who went on to discover some fifteen comets. During his searches he noted various objects (nebulae and star clusters) that could be mistaken for comets, starting with the Crab Nebula in 1758. Later observers extended Messier’s list and astronomers still refer to these Messier objects by their Messier, or M, numbers.
In 1716 Halley suggested a measurement of the distance between Earth and the Sun by timing the transit of Venus. In doing so he was following the method described by James Gregory (1638-75) in his Optica Promota (‘The Advance of Optics’) in 1663. In 1761, sixty-two observing stations monitored the transit of Venus, and a similar number observed the transit of 1769. When the data had been analysed the average value achieved was 95 million miles (≈153 million km).
In 1718 Halley discovered that three of the brightest stars had changed their positions since Ptolemy. He suspected that the discrepancies between the ancient charts and those of his own time were too large to be attributed to errors in measurement. His suspicions were confirmed when he checked the position of Sirius when Tycho had observed it. The shift had been so gradual that it had only become apparent over a span of several generations. This change of position of a star relative to our star (the Sun) is called its proper motion. In 1721 Halley raised the problem of what has come to be called Olbers’ paradox, in honour of Heinrich William Matthias Olbers (1758-1840) who in 1826 published his discussion of the apparent contradiction between the simple observation that the night sky is dark and the theoretical expectation that an infinite, static Universe, filled with more or less uniformly with stars and galaxies, should be as bright as the surface of a star. Edgar Allan Poe (1809-49), a keen amateur scientist, gave the first correct explanation of this paradox in a lecture published in 1848 in which he wrote that the dark gaps between the stars could be comprehended only by supposing the distance to the invisible background to be so immense that light from it has not yet been able to reach us.
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