SCIENCE. 



[N. S. Vol. XVII. No. 418. 



These results indicate that more than 

 two thousand years ago there existed re- 

 corded observations of astronomy. Hip- 

 parehus appears to have been one of those 

 clear-headed men who deduce results from 

 observations with good judgment. There 

 was a time when those ancient Greek as- 

 tronomers 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 mo- 

 tions of the planets by epicycles so well 

 that his theory became the one adopted 

 in the schools of Europe during fourteen 

 centuries. 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 be- 

 came inwoven with the literature, art and 

 religion of those times. Dante's construc- 

 tion 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 pre- 

 vailing 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 practical astronomy helped 

 to show its truth. Kepler's discoveries in 

 astronomy, Galileo's discovery of the laws 

 of motion and Newton's discovery of the 

 law of gravitation, put the Copernican 

 theory on a solid foundation. Yet it 

 was many years before the new theories 

 wer.e 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 re- 

 peated 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 handed down to us by the ancient 

 astronomers; and also by the peculiar con- 

 ditions 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 direction. 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 favor- 

 ing 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 vpith 

 some precision. Attention was called to 

 these bodies by their threatening aspects, 

 and by the terror they inspired among 

 people. It was therefore a happy duty 

 of the astronomers to show that the comets 

 also move in orbits around the Sun, and 

 are subject to the same laws as the planets. 

 This work was easier because the comets 

 move nearly in parabolas, which are the 

 simplest of the conic sections. Still the 

 general problem of finding the six ele- 



