292 POPULAR SCIENCE MONTHLY. 



Greek astronomers had conceived the heliocentric motions of the 

 planets, but this true theory was set aside by the ingenious Ptolemy, 

 who assumed the earth as the center of motion, and explained the 

 apparent motions of the planets by epicycles so well that his theory 

 became the one adopted in the schools of Europe during fourteen cen- 

 turies. The Ptolemaic theory flattered the egotism of men by making 

 the earth the center of motion, and it corresponded well with old 

 legends and myths, so that it became inwoven with the literature, art 

 and religion of those times. Dante's construction of Hell, Purgatory 

 and Paradise is derived from the Ptolemaic theory of the universe. His 

 ponderous arrangement of ten divisions of Paradise, with ten Purga- 

 tories and ten Hells, is said by some critics to furnish convenient places 

 for Dante to put away his friends and his enemies, but it is all derived 

 from the prevailing astronomy. Similar notions will be found in 

 Milton, but modified by the ideas of Copernicus, which Milton had 

 learned in Italy. The Copernican theory won its way slowly, but surely, 

 because it is the system of nature, and all discoveries in theory and prac- 

 tical astronomy helped to show its truth. Kepler's discoveries in 

 astronomy, Galileo's discovery of the laws of motion and Newton's dis- 

 covery of the law of gravitation, put the Copernican theory on a solid 

 foundation. Yet it was many years before the new theories were fully 

 accepted. Dr. Johnson thought persecution a good thing, since it weeds 

 out false men and false theories. The Copernican and Newtonian 

 theories have stood the test of observation and criticism, and they now 

 form the adopted system of astronomy. 



The laws of motion, together with the law of gravitation, enable the 

 astronomer to form the equations of motion for the bodies of our solar 

 system ; it remains to solve these equations, to correct the orbits, and to 

 form tables of the Sun, Moon and the planets. This work was begun more 

 than a century ago, and it has been repeated for the principal planets 

 several times, so that now we have good tables of these bodies. In the 

 case of the principal planets the labor of determining their orbits was 

 facilitated by the approximate orbits banded down to us by the ancient 

 astronomers; and also by the peculiar conditions of these orbits. For 

 the most part the orbits are nearly circular ; the planets move nearly in 

 the same plane, and their motions are in the same directions. These 

 are the conditions Laplace used as the foundation of the nebular 

 hypothesis. With approximate values of the periods and motions, and 

 under the other favoring conditions, it was not difficult to form tables 

 of the planets. However the general problem of determining an orbit 

 from three observations, which furnish the necessary and sufficient data, 

 was not solved until about a century ago. The orbits of comets were 

 first calculated with some precision. Attention was called to these 

 bodies by their threatening aspects, and by the terror they inspired 



