Kepler’s laws explained how the planets moved, but not why. He believed that the Sun somehow pushed the planets around; otherwise they would come to rest. René Descartes (1596-1650), on the other hand, taught that a planet would continue to move in a straight line at uniform speed unless subjected to an outside influence.
Descartes’ universe, however, was completely filled with matter. Planets were carried around in a solar vortex, jostled on all sides by matter whose motions made it invisible and whose pressure bent planetary movements, which would otherwise be in straight lines, into nearly circular orbits. There were other large vortexes, each having a sun (a star) at its centre. The Sun was therefore a typical star.
Isaac Newton (1643-1727) was born prematurely on 4 January 1643 at Woolsthorpe, near Grantham in Lincolnshire. When he was three years old his mother remarried and moved away and he was brought up at Woolsthorpe by his maternal grandmother. In 1653 his mother moved back to Woolsthorpe after the death of her second husband. Shortly after this, Newton began attending the King’s School (1528) in Grantham.
In 1661 he went to Trinity College (est.1546), Cambridge. Despite evidence that his progress had not been particularly good he received his bachelor’s degree in 1665. In the summer of 1665 the college was closed during the Great Plague (1665-6) and he returned to Woolsthorpe. There, in a period of less than two years, he developed his theories on calculus, optics and the law of gravitation. He returned to Cambridge in April 1667 and was elected to a minor fellowship at Trinity College. In the next year he took his Master of Arts degree and from 1669 to 1695 he was Lucasian Professor of mathe-matics. In 1672 he became a fellow of the Royal Society.
In January 1684, three members of the Royal Society, Christopher Wren (1632-1723), Robert Hooke (1635-1703) and Edmond Halley (1656-1742) argued as to whether the elliptical orbits of the planets could result from a gravitational force towards the Sun proportional to the inverse square of the distance. The problem, as all three admitted, was to find the mathematical means of proving it.
Hooke had clashed with Newton over the nature of light and in November 1679 had begun to exchange letters with him on the question of planetary motion. Although Newton quickly broke off the correspondence, Hooke’s letters had provided a link between central attraction and the force falling off with the square of the distance. Hooke now claimed he had the proof but was going to keep it a secret for the time being.
In August Halley went to Cambridge and put the problem to Newton. Without hesitation Newton said he had solved it four years earlier. He said he could not find the proof among his papers but promised to redo his calculations and send them on to Halley in London. In November a copy of De Motu Corporum in Gyrum (‘On the Motion of Bodies in an Orbit’) was delivered to Halley. In December, Halley rose to address his fellow members of the Royal Society. He gave an account of his visit with Newton and of De Motu. Halley was urged to push Newton to publish this work as soon as possible.
Newton, however, was already improving and expanding on De Motu. On 5 July 1687 Philosophiae Naturalis Principia Mathematica (‘Mathematical Principles of Natural Philosophy’) now known as the Principia, was published. In it Newton states his laws of motion (the foundation of classical mechanics), his law of universal gravitation, and his derivations of Kepler’s laws for the motion of the planets.
After publishing the Principia, Newton became involved in public affairs. In 1689 he was elected to represent Cambridge in Parliament and in 1696 he was appointed Master of the Mint. He argued with Hooke about who deserved the credit for discovering the connection between elliptical orbits and the inverse square law until Hooke died in 1703. Newton died on 31 March 1727 and was buried in Westminster Abbey.
The last decades of the eighteenth century and the early decades of the nineteenth were dominated by Pierre Simon de Laplace (1749-1827) who established celestial mechanics as a mathematical discipline by applying Newton’s theory of gravitation to the study of orbits, perturbations and stability in the Solar System. Laplace’s researches culminated in the publication of his greatest work, the five-volume Mécanique Céleste (‘Celestial Mechanics’) in 1799 to 1825. In 1796 Laplace published his Exposition du Système du Monde (‘Exposition of a System of the World’), a popular work aimed at describing the nebular hypothesis, which proposes that the Solar System formed from the gravitationally-induced contraction of an initially large, diffuse and slowly-rotating cloud of interstellar gas. He was apparently unaware that Immanuel Kant (1724-1804) had proposed a similar cosmology in 1755.
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