232 W. Hardness — Magnitude of the Solar System. 



your attention to the present condition of our knowledge 

 respecting the magnitude of the solar system, but in so doing 

 it will be necessary to introduce some considerations derived 

 from laboratory experiments upon the luminiferous ether, 

 others derived from experiments upon ponderable matter, and 

 still others relating both to the surface phenomena and to the 

 internal structure of the earth, and thus we shall deal largely 

 with the border land where astronomy, physics and geology 

 merge into each other. 



The relative distances of the various bodies which compose 

 the solar system can be determined to a considerable degree of 

 approximation with very crude instruments as soon as the true 

 plan of the system becomes known, and that plan was taught 

 by Pythagoras more than five hundred years before Christ. 

 It must have been known to the Egyptians and Chaldeans still 

 earlier, if Pythagoras really acquired his knowledge of astron- 

 omy from them as is affirmed by some of the ancient writers, 

 but on that point there is no certainty. In public Pythagoras 

 seemingly accepted the current belief of his time, which made 

 the earth the center of the universe, but to his own chosen dis- 

 ciples he communicated the true doctrine that the sun occupies 

 the center of the solar system, and that the earth is only one of 

 the planets revolving around it. Like all the world's greatest 

 sages, he seems to have taught only orally. A century elapsed 

 before his doctrines were reduced to writing by Philolaus of 

 Crotqna, and it was still later before they were taught in pub- 

 lic for the first time by Hicetas, or as he is sometimes called 

 Nicetas, of Syracuse. Then the familiar cry of impiety was 

 raised, and the Pythagorean system was eventually suppressed 

 by that now called the Ptolemaic which held the field until it 

 was overthrown by Copernicus almost two thousand years 

 later. Pliny tells us that Pythagoras believed the distances to 

 the sun and moon to be respectively 252,000 and 12,600 stadia, 

 or taking the stadium at 625 feet, 29,837 and 1492 English 

 miles ; but there is no record of the method by which these 

 numbers were ascertained. 



After the relative distances of the various planets are known, 

 it only remains to determine the scale of the system, for which 

 purpose the distance between any two planets suffices. We 

 know little about the early history of the subject, but it is clear 

 that the primitive astronomers must have found the quantities 

 to be measured too small for detection with their instruments, 

 and even in modern times the problem has proved to be an 

 extremely difficult one. Aristarcus of Samos who flourished 

 about 270 B. C. seems to have been the first to attack it in a 

 scientific manner. Stated in modern language, his reasoning 

 was that when the moon is exactly half full, the earth and sun 



