73 



THK WOTU.I). 



the lengths C O and S O, or C D, and S D, and all are equal to the 



length of the major axis A B. By placing two pins, one at eacli 

 focus of the ellipse, and tying a thread around them of such length 

 as will give the requisite major axis, a true ellipse may be described, 

 by stretching the string and moving a pencil around in the angle. 

 In the preceding diagram, we may suppose S E C, S O C, S I) 

 C, to be three positions of the string, the pencil being placed in 

 the angles E, O, and D. Such is the peculiar property of the 

 ellipse, and in such an orbit the earth is moving around the sun. 

 Let S be the position of the sun, and A the position of the earth, 

 at the time when nearest the sun, and when, consequently, the 

 sun's diameter appears the largest. This point in the orbit, is 

 called the perihelion point, from two Greek words, which mean 

 near or about the sun. The point B is called the aphelion point, 

 or point away from the sun ; when the earth is in this position, 

 the sun's diameter appears the. smallest. The line B A, is called 

 the line of the apsides, La. the line without deviation, or change 

 in length, for we shall show, presently, that whatever changes the 

 earth's orbit may undergo, tlys line will remain unaltered. In 

 the preceding chapter, we observed that the sun's motion was 

 not uniform in the heavens, or did not correspond with the indi- 

 cations of a well regulated clock. It will not be difficult to under- 

 stand, that since it is the attraction of the sun which causes the 

 motion of the earth', it will, while approaching the sun, have its 

 motion continually accelerated, or quickened, until it sweeps 

 around the perihelion point A, with its greatest velocity, itfi motion 



