418 ASTRONOMY. 



at midnight, with a declination as far from the 

 equator on one side as the sun's is on the other 

 side ; and it is evident the sun must be in that 

 part of the heavens which is diametrically opposite 

 to this point. By either of these methods you 

 may obtain a series of points in the heavens through 

 which the sun passes, forming a circle called the 

 ecliptic. This circle has its name from thence, that 

 all the eclipses of the sun and moon are performed 

 either actually in, or very near the circumference 

 of that circle. 



To conceive this combined annual and diurnal 

 motion of the sun, suppose a globe to represent the 

 celestial sphere. Place an insect upon it at an 

 equal distance from both the poles, and turn the 

 globe upon its axis : the insect will turn with the 

 globe, and describe the equator. But while the 

 globe is moving in one direction, the insect may be 

 moving very slowly in another, and will thereby 

 imitate the annual motion of the sun, which ad- 

 vances by degrees towards the east, though it is 

 carried round every day by the diurnal motion to- 

 wards the west. 



The ecliptic, or annual path of the sun, differs 

 in situation from the equator ; for the sun rises 

 above the equator in summer, and does not rise so 

 high in winter. The points of the ecliptic, where 

 the sun is situated when he is most distant from 

 the equator, are called solstitial points ; and the 

 distance between the equator and the ecliptic at 

 the solstitial points, is called the obliquity of the 

 ecliptic; this is found to be about 23J°. AB 

 (Fig. 1.) represents the ecliptic inclined 23J° to 

 the equator. 



The equinoctial colure is the great circle which 

 passes at right angles to the equator, through those 



