NEW DISCOVERIES. 



469 



left a valuable collection of observations on record. In this volume, called 



the " Almagest," he reviewed the 

 state of the science, and gave a 

 catalogue of stars, as well as a 

 description of the heavens. He 

 discovered the lunar evection. 



After his time astronomy, 

 though it was not neglected, ap- 

 peared to droop, and it is at a 

 comparatively late period that we 

 again open the records viz., in 

 1543, the year in which Coper- 

 nicus died. This philosopher, 

 who was born in 1473, promul- 

 gated the true theory of the solar 

 system. He placed the sun in 

 the centre of the planets, and by 

 \ s . 494-Ptokmaic System. this he explained their motion 



around the sun, though they appeared to be carried round the earth. The 

 book in which he explained his theory, " De Revolutionibus Orbium Celes- 

 tium," was not finished till a day or two before he died. 



The justly celebrated Tycho Brahe was the most important of the 

 successors of Copernicus, but he opposed the Copernican theory, while other 

 able philosophers agreed with it. Brahe was a Dane; he died in 1601. 

 He adopted the theory that the sun and moon revolved around the earth, 

 while the (other) planets moved around the sun. This theory did not gain 

 much credence, but he, again, though he could not defeat Copernicus, and 

 though he was wrong in his assumption, made many important investi- 

 gations. After him came Kepler, whose observations upon the planet Mars 

 cleared away many complications, and he laid down three laws, which are as 

 follows : 



1. Every planet describes an elliptic orbit about the sun, which 

 occupies one focus of each such ellipse. 



2. If a line be drawn from the sun, con- 

 tinually, to any planet, this line will sweep over 

 equal areas in equal times. 



3. The squares of the periodic times of the 

 planets are proportional to the cubes of their mean 

 distances from the sun. 



Kepler also remarked that gravity was a 

 power existing between all bodies, and reasoned 

 upon the tides being caused by the attraction of 

 the moon for the waters. 



It was about this time viz., the beginning of 

 the seventeenth century that the telescope was invented, and logarithms 



Fig. 495. Copernican System. 



