ON THE SOLAR SYSTEM. 505 



he found that a right line, joining the sun and any planet, describes always 

 equal areas in ecjual ^imes. 1 he ohsi-rvations, on which Ktpler founded these 

 important laws, were made pnntipally on the phmet Mars. He determined 

 by calculation, upon the supposition which was then generally adopted, of a 

 motion in an eccentric circle, what must be nearly the situati;)n of the 

 planet, with respect to the sun, that is, its heliocentric place, and observing 

 its geocentric place, with respect to the earth, he was thus able to construct 

 a triangle representing the situation of the three bodies; repeating this 

 operation in various parts of the orbit, he discovered its form; and having 

 done this, the velocity of the motion in different parts of the orbit was easily 

 determined from the apparent change of place in a given time. (Plate 

 XXXII. Fig. 471.) 



The same as'' ronomer also ascertained, that the squares of the times of re- 

 volution of the different planets are in proportion to the cubes of their mean 

 distances from the sun. For example, if oneplann were four times as distant 

 as aiother, it wjuld revolve in a period eight times as.long, since the cube of 

 4 is equal to the square of 8; thus Mars is nearly four times as remote from 

 the sun as Mercury, and the Georgian planet four times as remote as Jupifer, 

 and their periods are nearly eight times as long respectively. 



It is probable that all the planets have a rotatory motion from west to east, 

 cither perfectly or very nearly equable. This motion has been observed in 

 Venus, the Earth, Mars, Jupirer, and Saturn; and from some phenomena 

 of the satellites of the Georgian planet, Mr. Laplace thinks that it may also 

 be assumed as nearly certain that this planet has also a rotatory motion. 

 The figure of the planets is spheroidical; they are more or less flattened 

 at the poles, as they revolve more or less rapidly on their axes. These 

 axes retain, with a very slight deviation, a situation always parallel, in 

 every part of the orbits. 



But, in the course of time, the gradual change of the position of the axis 

 produces a sensible effect. In the case of the earth, this effect is denomi- 

 nated the precession of the equinoxes. The equinoctial points are the in- 

 tersections of the apparent ecliptic, or the path of the sun in the heavens, 



VOL. I 3 T 



