ON THE HISTORY OF ASTRONOMY. 5^7 



died soon after, at the age of 55. Struck with the objections made to the 

 system of Copernicus, principally such as were deduced from a misinterpre- 

 tation of the scriptures, he imagined a new theory, which, although mechani- 

 cally absurd, is still astronomically correct; for he supposed the earth to 

 remain at rest in the centre, the stars to revolve round it, together with the 

 sun and all the planets, in a sidereal day, and the sun to have, besides, an 

 annual motion, carrying with him the planets in their orbits. Here the 

 apparent or relative motions are precisely the same as in the Copernican 

 sj'stem; the argument that Tycho Brahe drew from the scriptures in favour 

 of his theory was, therefore, every way injudicious; for it is not to be 

 imagined that any thing but relative motion or rest could be intended in the 

 scriptures, when the sun is said to move, or to standstill. But in the Copernican 

 system, there was an evident regularity in the periods of all the planets, that 

 of the earth being longer than that of Venus, and shorter than that of Mars, 

 which were the neighbouring planets on each side; and when Tycho imagined 

 the sun to move round tlie earth, this analogy was entirely lost. Tycho 

 Brahe was the discoverer of the variation and of the annual equation of the 

 moon, the one being an irregularity in its velocity, dependent on its position 

 with respect to the sun, the other a change in the magnitude of all the per- 

 turbations produced by the sun, dependent on his distance from the earth. 

 (Plate XXXVIII. Fig. 529.) 



Kepler was the pupil and assistant of Tycho, whose observations were the 

 basis of his important discoveries: he succeeded him in his appointments at 

 Prague, and enjoyed the title of Imperial Mathematician. Adopting the 

 Copernican system, which was then becoming popular, he proceeded to 

 examine the distances of the celestial bodies from each other at various time?}; 

 and after many fruitless attempts to reconcile the places of the planets with 

 the supposition of revolutions in eccentric circles, at last discovered that 

 their orbits are ellipses, and demonstrated, chiefly from his observations on 

 the planet Mars, that the revolving radius, or the line drawn from the sun to 

 the planet, always describes equal areas in equal times. By comparing the 

 periods and the mean distances of the different planets with each other, he 

 found, after 17 years calculation, tliat the squares of the times of revolution 

 are always proportional to the cubes of the mean distances from the sun. 



VOL. I. 4 E 



