476 POPULAR SCIENCE MONTHLY. 



interesting yet fanciful speculation. It remained for a subsequent age 

 to furnish proof of the truth of the Copernican system which could 

 not be gainsaid or resisted. 



At the beginning of the seventeenth century, the dicta of Aristotle, 

 in regard to matters of science as well as philosophy, were still accepted, 

 as they had been for many centuries preceding, as of infallible author- 

 ity. In regard to the subject of our inquiry, he taught that bodies at 

 the surface of the earth fell or tended to fall toward the center of the 

 earth, not in virtue of any attraction of the earth, but in virtue of the 

 fact that the center of the earth was the center of the material universe 

 — that if the earth itself should be moved out of its place and then left 

 free to move, it would return to its place by the same law of nature 

 which controlled all terrestrial bodies. He taught moreover that celes- 

 tial bodies were different in kind from bodies terrestrial — that whilst 

 the latter were imperfect, corruptible and changeable, the former were 

 perfect, (and therefore, according to his fancy, perfectly spherical in 

 form) incorruptible, unchangeable and self-luminous. Being different 

 in kind, he held that they were subject to entirely different physical 

 laws; that whereas the motion of terrestrial bodies when free to move 

 was rectilinear, by a necessity of their nature, the motion of celestial 

 bodies was circular by a like necessity of their nature. His language 

 on this point is worth quoting as an illustration of the contrast be- 

 tween the ancient and modern method of philosophizing in regard to 

 natural phenomena. He says: "All simple motion must be rectilinear 

 or circular; either to a center or from a center, each of which is recti- 

 linear, or about a center. It is natural for two of the elements — 

 earth and water — which are heavy, to tend to a center; two — air and 

 fire — which are light, to tend from a center. As the motion of all 

 terrestrial elements is therefore rectilinear, it seems reasonable that 

 celestial bodies, which are of a different nature, should have only 

 the other simple motion possible, namely, circular motion." 



The year 1609 marks a new era in the history of astronomy. In 

 this year two events occurred, independent, yot alike memorable as con- 

 tributing to the overthrow of the theory in r>_gard to the structure of 

 the material universe which had previously prevailed and establishing 

 the doctrine of Copernicus upon an immovable foundation. The inven- 

 tion of the telescope by Galileo, and the immediate discovery by means 

 of it of the inequalities of the moon's surface, the phases of Venus, the 

 satellites of Jupiter and the rings of Saturn, at once annihilated the 

 fancies of Aristotle as to the perfectly spherical form of the planets, 

 their self-luminosity, and their difference in kind from bodies terres- 

 trial. The other memorable event referred to was the publication of 

 Kepler's great work on 'The Motions of Mars,' in which, with much 

 that was fanciful, two of the three laws of planetary motion were for 



