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Development of the Solar System

planets, laplace, nebula, rotation, sun, hypothesis and motions

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Laplace's Nebular Hypothesis, Not long after Galileo's invention of the telescope in 1610, and its application to the heavens by his immediate successors, it was found that the Sun and Mars were rotating on their axes, that Jupiter had equatorial belts and a system of four satellites whirling about that great planet, and that the planet Saturn was surrounded by the system of rings. The rotation of the earth on its axis had been taught by Aristarchus of Samos (who flourished about 270 i.e.), and was fully established by the great work of Coper nicus, published in 1543. Similar rotatory mo tion of the other planets was now proved by the telescopic discoveries of Galileo. So many motions of rotation and of revolution naturally increased the complexity of the system of the world; and when. philosophers began to cast about for the cause of these things. Ken* suspected that the rotation of the sun had some influence on the orbital motions of the planets. In fact, he thought that subtle spirits emanating from the surface of the sun bad iinpressed themselves on the motions of the planets, so that they were carried around tangentially in the same direction. This is the earhest hint of the theory afterward developed by Kant and Laplace under the name of °nebular hy pothesis? The term nebula designates patches of luminous matter occurring in the sky and differing from the stars in that they present large surfaces of varying brilliancy instead of small definite points of light.

We shall not go into the details of the early history of the nebular hypothesis, beyond re -marking that in various forms it was imper fectly outlined by Emanuel Swedenborg (1688 1772) more satisfactorily treated by Thomas Wridit (1711-86), of Durham, England; Ins .manuel Kant (1724-1804), of Konigsberg,. Prus sia; and finally put on a much more scientific basis by the telescopic explorations of Sir Wil liam Herschel (1738-1822), and the mathe matical genius and dynamical judgment of the great French astronomer Laplace Swedenborg's Cosmogony was the earliest, 1734. Wright's work dates from 1750; Kant's

from 1755, and he acknowledges his indebted ness to Wright; Herschel's from 1791, after years of mature observations on nebula; while Laplace's theory of 1796 was the outcome of a quarter of a century of profound study of the •mechanics of the solar system.

On account of the great mathematical pres tige of the illustrious of the Mecan Ique Celeste it was natural that Laplace's 'theory should be the, one most generally ac cepte• by men of science. According to this view the matter now constituting the sun and planets was originally diffused into an oblate planetary nebula. Sir William Herschel had recently observed and catalogued great num bers of planetary nebula throughout the side real universe. To account for the motions of the planets close to one fundamental plane and in nearly circular orbits, Laplace conceived that the primordial nebula had been endowed -with a rapid motion of rotation; and when thus rendered very oblate by the effects of centrifu gal force, had detached or thrown off succes 'sive rings of vapor from the equatorial por tion of the mass, which was supposed to be at high temperature and kept in equilibrium under the pressure and attraction of its parts. These rings of vapor were imagined to have condensed into the planets, which therefore re volve in nearly circular orbits. The satellites were explained in the same way, by the con densation of rings of vapor imagined to have been thrown off from the several planets, as they cooled and contracted and accelerated cor respondingly their velocity of axial rotation.

In very brief outline this is the celebrated nebular hypothesis of Laplace, which has ex erted so great an influence on philosophic thought for more than 100 years. In some form or other it enters into all the books on astronomy, as giving the best available explana tion of the origin of the solar system; but it will be shown below that the "hypothesis of de tachmentD postulated by Laplace is entirely er roneous, and will have to be supplanted by a very different theory.

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