140 PRACTICAL ASTRONOMY 



to the hour circle passing through the star. AS is the declina- 

 tion of the star; that is, its angular distance from the equator. 

 The declination is considered positive when the star is north 

 and negative when south of the equator. The complement 

 angle of the declination, SP, is called the polar distance of 

 the star. 



The zenith of a point on the earth's surface is the point in 

 which the line passing through the center of the earth and the 

 given point intersects the celestial sphere above the given 

 point. The horizon is the plane passing through the given 

 point and perpendicular to this line. In the illustration, Z is 

 the zenith, and NVM is the celestial horizon. 



The celestial meridian of a given point is a great circle passing 

 through the zenith of the point and the poles. The celestial 

 meridian cuts the horizon in two points N and M, called, 

 respectively, the north point and the south point. 



A vertical circle is one that passes through the zenith and is 

 perpendicular to the horizon. 



The prime vertical is the vertical circle at right angles to the 

 meridian; it intersects the horizon in two points V and V, 

 called the west and the east point, respectively. 



The altitude of a heavenly body is its angular distance from the 

 horizon, measured along the vertical circle passing through the 

 body. The zenith distance is the angular distance of the star 

 from the zenith, measured along the same circle. The zenith 

 distance is the complement of the altitude. In the illustration, 

 DS and SZ are, respectively, the altitude and zenith distance 

 of 5. 



The azimuth of a star is the angle in the plane of the horizon 

 intercepted by the planes of the meridian and the vertical 

 circle passing through the star. It is measured from the north 

 point toward the east or from the south point toward the 

 west. NMD is the azimuth of S, measured from the north 

 toward the east, and M D is the azimuth of 5 when measured 

 from the south toward the west. 



The hour angle of a star is the arc intercepted on the equator 

 between the meridian and the foot of the hour circle passing 

 through the star. It is measured from the meridian toward 

 the west. In the illustration, QA is the hour angle of S. 



