ASTRONOMY. 5 



7. A circle, the plane of which passes through 

 the centre of the earth, and is perpendicular to 

 the axis above defined, is called the Equator. 

 The line in which this plane cuts the surface of 

 the earth is called the Equinoctial. 



a. The circles described by the stars in their diurnal 

 motion, are all parallel to the equator. 



8. If any plane pass through the poles of the 

 celestial sphere, the circle in which it cuts the 

 surface of that sphere is called a Meridian Cir- 

 cle ; and the section of the same plane with the 

 surface of the earth, is called the Meridian of 

 any of the points through which it passes. 



a. The planes of all the Meridians cut the plane of 

 the Equator at right angles. 



9. If at any place on the earth's surface, the 

 direction of gravity, (that is, the direction in 

 which bodies fall to the ground, or the line in 

 which the plummet hangs), be produced up- 

 ward, the point in which it cuts the celestial 

 sphere is called the Zenith of the place ; and if 

 the same line be produced downward, the point 

 in which it cuts the celestial sphere is called the 

 Nadir. If a plane perpendicular to this line 

 pass through the place itself, the circle in which 

 it cuts the heavens is called the Horizon. 



^ The 



