444 HISTORY OF FORMAL ASTRONOMY. 



with reference to the sun as well as to the earth; 

 but this was not possible, for though the geocentric 

 position, or the direction in which the planet was 

 seen, could be observed, its distance from the earth 

 was not known. Hence, when Kepler attempted to 

 determine the orbit of a planet, he combined the 

 observed geocentric places with successive modifi- 

 cations of the theory of epicycles, till at last he was 

 led, by one step after another, to change the epicy- 

 olical into the elliptical theory. We may observe, 

 moreover, that at every step he endeavoured to 

 support his new suppositions by what he called, in 

 his fanciful phraseology, " sending into the field a 

 reserve of new physical reasonings on the rout and 

 dispersion of the veterans (s) :" that is, by connect- 

 ing his astronomical hypotheses with new imagina- 

 tions, when the old ones became untenable. We 

 find, indeed, that this is the spirit in which the 

 pursuit of knowledge is generally carried on with 

 success: those men arrive at truth who eagerly 

 endeavour to connect remote points of their know- 

 ledge, not those who stop cautiously at each point 

 till something compels them to go beyond it. 



Kepler joined Tycho Brahe at Prague in 1600, 

 and found him and Longomontanus busily employed 

 in correcting the theory of Mars ; and he also then 

 entered upon that train of researches which he 

 published in 1609 in his extraordinary work On the 

 Motions of Mars. In this work, as in others, he 

 gives an account, not only of his success, but of his 



