a history of science-2-第15章
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re。 For our present purpose it must suffice to point out the three important theories； which； as culled from among a score or so of incorrect ones； Kepler was able to demonstrate to his own satisfaction and to that of subsequent observers。 Stated in a few words； these theories； which have come to bear the name of Kepler's Laws； are the following： 1。 That the planetary orbits are not circular； but elliptical； the sun occupying one focus of the ellipses。 2。 That the speed of planetary motion varies in different parts of the orbit in such a way that an imaginary line drawn from the sun to the planetthat is to say； the radius vector of the planet's orbitalways sweeps the same area in a given time。
These two laws Kepler published as early as 1609。 Many years more of patient investigation were required before he found out the secret of the relation between planetary distances and times of revolution which his third law expresses。 In 1618； however； he was able to formulate this relation also； as follows： 3。 The squares of the distance of the various planets from the sun are proportional to the cubes of their periods of revolution about the sun。
All these laws； it will be observed； take for granted the fact that the sun is the centre of the planetary orbits。 It must be understood； too； that the earth is constantly regarded； in accordance with the Copernican system； as being itself a member of the planetary system； subject to precisely the same laws as the other planets。 Long familiarity has made these wonderful laws of Kepler seem such a matter of course that it is difficult now to appreciate them at their full value。 Yet； as has been already pointed out； it was the knowledge of these marvellously simple relations between the planetary orbits that laid the foundation for the Newtonian law of universal gravitation。 Contemporary judgment could not； of course； anticipate this culmination of a later generation。 What it could understand was that the first law of Kepler attacked one of the most time…honored of metaphysical conceptionsnamely； the Aristotelian idea that the circle is the perfect figure； and hence that the planetary orbits must be circular。 Not even Copernicus had doubted the validity of this assumption。 That Kepler dared dispute so firmly fixed a belief； and one that seemingly had so sound a philosophical basis； evidenced the iconoclastic nature of his genius。 That he did not rest content until he had demonstrated the validity of his revolutionary assumption shows how truly this great theorizer made his hypotheses subservient to the most rigid inductions。
GALILEO GALILEI While Kepler was solving these riddles of planetary motion； there was an even more famous man in Italy whose championship of the Copernican doctrine was destined to give the greatest possible publicity to the new ideas。 This was Galileo Galilei； one of the most extraordinary scientific observers of any age。 Galileo was born at Pisa； on the 18th of February （old style）； 1564。 The day of his birth is doubly memorable； since on the same day the greatest Italian of the preceding epoch； Michael Angelo； breathed his last。 Persons fond of symbolism have found in the coincidence a forecast of the transit from the artistic to the scientific epoch of the later Renaissance。 Galileo came of an impoverished noble family。 He was educated for the profession of medicine； but did not progress far before his natural proclivities directed him towards the physical sciences。 Meeting with opposition in Pisa； he early accepted a call to the chair of natural philosophy in the University of Padua； and later in life he made his home at Florence。 The mechanical and physical discoveries of Galileo will claim our attention in another chapter。 Our present concern is with his contribution to the Copernican theory。 Galileo himself records in a letter to Kepler that he became a convert to this theory at an early day。 He was not enabled； however； to make any marked contribution to the subject； beyond the influence of his general teachings； until about the year 1610。 The brilliant contributions which he made were due largely to a single discoverynamely； that of the telescope。 Hitherto the astronomical observations had been made with the unaided eye。 Glass lenses had been known since the thirteenth century； but； until now； no one had thought of their possible use as aids to distant vision。 The question of priority of discovery has never been settled。 It is admitted； however； that the chief honors belong to the opticians of the Netherlands。 As early as the year 1590 the Dutch optician Zacharias Jensen placed a concave and a convex lens respectively at the ends of a tube about eighteen inches long； and used this instrument for the purpose of magnifying small objectsproducing； in short； a crude microscope。 Some years later； Johannes Lippershey； of whom not much is known except that he died in 1619； experimented with a somewhat similar combination of lenses； and made the startling observation that the weather…vane on a distant church…steeple seemed to be brought much nearer when viewed through the lens。 The combination of lenses he employed is that still used in the construction of opera…glasses； the Germans still call such a combination a Dutch telescope。 Doubtless a large number of experimenters took the matter up and the fame of the new instrument spread rapidly abroad。 Galileo； down in Italy； heard rumors of this remarkable contrivance； through the use of which it was said 〃distant objects might be seen as clearly as those near at hand。〃 He at once set to work to construct for himself a similar instrument； and his efforts were so far successful that at first he 〃saw objects three times as near and nine times enlarged。〃 Continuing his efforts； he presently so improved his glass that objects were enlarged almost a thousand times and made to appear thirty times nearer than when seen with the naked eye。 Naturally enough； Galileo turned this fascinating instrument towards the skies； and he was almost immediately rewarded by several startling discoveries。 At the very outset； his magnifying…glass brought to view a vast number of stars that are invisible to the naked eye； and enabled the observer to reach the conclusion that the hazy light of the Milky Way is merely due to the aggregation of a vast number of tiny stars。 Turning his telescope towards the moon； Galileo found that body rough and earth…like in contour； its surface covered with mountains； whose height could be approximately measured through study of their shadows。 This was disquieting； because the current Aristotelian doctrine supposed the moon； in common with the planets； to be a perfectly spherical； smooth body。 The metaphysical idea of a perfect universe was sure to be disturbed by this seemingly rough workmanship of the moon。 Thus far； however； there was nothing in the observations of Galileo to bear directly upon the Copernican theory； but when an inspection was made of the planets the case was quite different。 With the aid of his telescope； Galileo saw that Venus； for example； passes through phases precisely similar to those of the moon； due； of course； to the same cause。 Here； then； was demonstrative evidence that the planets are dark bodies reflecting the light of the sun； and an explanation was given of the fact； hitherto urged in opposition to the Copernican theory； that the inferior planets do not seem many times brighter when nearer the earth than when in the most distant parts of their orbits； the explanation being； of course； that when the planets are between the earth and the sun only a small portion of their illumined surfaces is visible from the earth。 On inspecting the planet Jupiter； a still more striking revelation was made； as four tiny stars were observed to occupy an equatorial position near that planet； and were seen； when watched night after night； to be circling about the planet； precisely as the moon circles about the earth。 Here； obviously； was a miniature solar systema tangible object…lesson in the Copernican theory。 In honor of the ruling Florentine house of the period； Galileo named these moons of Jupiter； Medicean stars。 Turning attention to the sun itself； Galileo observed on the surface of that luminary a
