HOW HE STUDIED MARS. 301 



however, been left to depend entirely on his own resources, 

 he might, perhaps, have never completed his task. A for- 

 tunate circumstance brought him an unexpected ally. Tycho, 

 having taken refuge in Bohemia, sent for the young astrono- 

 mer (Kepler then was but twenty-nine years old), to assist 

 him in the composition of the " Rudolphine Tables." * 



"This," says Kepler, "was a providential interposition. 

 I repaired to Bohemia early in the year 1600, in the hope of 

 learning the correction of the eccentricities of the planets. 

 Perceiving that Tycho made use of a mixed system (which 

 made Mercury and Venus revolve around the sun, and all 

 these planets, with their companions, around the earth), I 

 asked his permission to follow out my own ideas. It was the 

 will of Providence again, that we should occupy ourselves 

 with Mars. My whole attention, therefore, was directed to 

 this planet : and it is through the movements of Mars we 

 must obtain our insight into the secrets of astronomy, or 

 remain ignorant of them for ever (ex Martis motibus omnino 

 necesse est nos in cognitionem astronomies arcanorum venire aut ea 

 perpetuo nestire)" f 



Why this preference given to Mars? In the first place, 

 because, among all the planets then known, it was Mars which, 

 in its movement round the sun, departed most from the circle ; 

 next, its orbit approaches nearest to the earth's ; the earth is 



* Certain astronomical calculations, so called because begun under the 

 patronage of Rudolf II., Emperor of Germany (1576-1612). 



t Kepler, " Astronomia Nova, seu de Motibus Stellae Martis," p. 53 

 (ed. 1609). 



