S96 LECTURE XLVIII. 



made in China, some very accurate observations, which are valuable for the 

 precision Avith which they ascertain the obliquity of the ecliptic: their date is 

 about 1278. 



It was not long after the time of Ulugh Beigh, that Copernicus laid the 

 foundation of the more accurate theories which modern improvements have 

 introduced into astronomy. Dissatisfied with the complicated hypotheses of 

 the Ptolemaean system, he examined the works of the ancients, inquest of 

 more probable opinions. He found from Cicero that Nicetas and other 

 Pythagoreans had maintained, that the sun is placed in the centre of the 

 system, and that the earth moves round him in common with the other 

 planets. He applied this idea to the numerous observations which the dili- 

 gence of astronomers had accumulated, and he had the satisfaction to find 

 them all in perfect conformity with this theory. He quickly discarded the 

 Ptolemaean epicycles, imagined in order to explain the alternations of the 

 direct and retrograde motions of the planets; in these remarkable phenomena, 

 Copernicus saw nothing but the consequences necessarily produced by the 

 combination of the motions of the earth and planets round the sun; and from 

 a minute examination of these circumstances he calculated the relative dis- 

 tances of the planets from the sun, which till then had remained unknown. 

 In this system, every thing had the marks of that beautiful simplicity which 

 pervades all the works of nature, and which, when once understood, carries with 

 itself sufficient evidence of its truth. Copernicus was born at Thorn, in Polish 

 Prussia, in the year 1475; he studied in Italy; he taught mathematics at 

 Rome, and afterwards settled on a canonicate at Frauenberg, where, in 

 56 years of retirement and meditation, he completed his work on.the celes- 

 tial revolutions, which was scarcely published when he died. 



About this time, William the Fourth, Landgrave of Hesse Cassel, not only 

 enriched astronomy by his own observations, but also exerted his influence 

 with Frederic, King of Denmark, to obtain his patronage for the celebrated 

 Tycho Brahe. Frederic agreed to give him the little island Huen, at the 

 entrance of the Baltic, where Tycho built his observatory of Uraniburg, 

 and, in a period of 21 years, made a prodigious collection of accurate obser- 

 vations. After the death of his patron, his progress was impeded, and he 

 sought an establishment at Prague, under the emperor Rudolph. Here he 



