HISTORY OF SCIENCE. 



porters, and the contest really lay between the Ptolemaic system and the 

 Aristotelean science on the one hand, and the Copernican system and 

 the mechanics of Galileo on the other. The grounds on which Tycho 

 refused to accept the annual motion of the earth have been, for the 

 most part, referred to in our notice of some earlier astronomers. The 

 strongest was the absence of an annual parallax of the fixed stars. 

 That is, the stars remain unchanged in their positions, whereas if the 

 earth revolve about the sun so Tycho urged she will at intervals 

 of six months be in places distant from each other by the whole 

 diameter of that orbit, and consequently some change in the relative 

 position of the fixed stars would be apparent, unless the angle which 

 would be formed between two lines drawn from the extremities of the 

 orbit's diameter to the nearest fixed star should include an inappre- 

 ciable angle between them. But as the diameter of the earth's orbit 



is of so great a length, Tycho thought 

 this would be impossible. We need 

 hardly tell the reader that we have now 

 incontestable proof of the earth's mo- 

 tion, and that we know, however as- 

 tonishing the fact may appear, that the 

 earth's orbit is a mere point compared 

 with its distance from the fixed stars. 



The vast series of accurate observa- 

 tions made by Tycho became a great 

 storehouse of facts from which another 

 illustrious astronomer of a different 

 genius obtained the means of deducing 

 the laws of the planetary motions. 

 When Tycho, in the fifty-fourth year of 

 his age, was carrying on his observa- 

 tions at Prague, he was assisted by this 

 eminent man, of whose own labours we 

 must give a short account. 



This was JOHN KEPLER (1571 1630), who at Tycho's death was 

 appointed his successor as principal mathematician to the Emperor. 

 But although the salary attached to the appointment was nominally 

 liberal, it was in reality inadequate to provide Kepler with a livelihood, 

 since the payments were constantly in arrears, owing to the exhaustion 

 of the imperial treasury by the expenses of a war. He resorted to the 

 practice of casting nativities as a means of adding to his resources. 



In the year 1609 Kepler published his great work entitled "The 

 New Astronomy, or Commentaries on the Motions of Mars." This 

 remarkable book forms the connecting-link between the astronomical 

 discoveries of Copernicus and those of Newton, and upon it chiefly 

 rests the scientific reputation of its author. The inquiries by which 

 he was led to his discoveries had been entered upon when Kepler 



FIG. 36. KEPLER. 



