THE SULAR SYSTEM. 191 



to adopt the latter course, and I shall invite your attention to the 

 present condition of our knowledge respecting the magnitude of 

 thesolar 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 ponder- 

 able matter, and still others relating both to the surface pheno- 

 mena 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 astronomy 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 centre of 

 the universe ; but to his own chosen disciples he communicated 

 the true doctrine that the sun occupies the centre 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 Crotona, and it was still later 

 before they were taught in public for the first time by Hicetas, or, 

 as he is sometimes called, Nicetas, of Syracuse. Then the fami- 

 liar 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 1,492 

 .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 



