CHAPTERS ON THE STARS. 381 



pole. Then as meridians are drawn from pole to pole on the earth, 

 cutting the equator at different points, so imaginary meridians are 

 conceived as drawn from pole to pole on the celestial sphere. Cor- 

 responding to parallels of latitude on the earth we have parallels of 

 declination on the celestial sphere. These are parallel to the equator, 

 and become smaller and smaller as we approach either pole. The 

 correspondence of the terrestrial and celestial circles is this: 



To latitude on the earth's surface corresponds declination in the 

 heavens. 



To longitude on the earth corresponds right ascension in the 

 heavens. 



A little study of these facts will show that the zenith of any point 

 on the earth's surface is always in a declination equal to the latitude 

 of the place. For example, for an observer in Philadelphia, in 40° 

 latitude, the parallel of 40° north declination will always pass through 

 his zenith, and a star of that declination will, in the course of its diurnal 

 motion, also pass through his zenith. 



So also to an observer on the equator the celestial sphere always 

 spans the visible celestial hemisphere through the east and west points. 



In the case of the right ascension, the relation between the terres- 

 trial and celestial spheres is not constant, because of the diurnal motion, 

 which keeps the terrestrial meridians in constant revolution relative to 

 the celestial meridians. Allowing for this motion, however, the system 

 is the same. As we have on the earth's surface a prime meridian 

 passing from pole to pole through the Greenwich Observatory, so in 

 the heavens a prime meridian passes from one celestial pole to the 

 other through the vernal equinox. Then to define the right ascension 

 of any star we imagine a great circle passing from pole to pole through 

 the star, as we imagine one to pass from pole to pole through a city on 

 the earth of which we wish to designate the longitude. The actual 

 angle which this meridian makes with the prime meridian is the right 

 ascension of the star as it is the longitude of the place on the earth's 

 surface. 



There is, however, a difference in the unit of angular measurement 

 commonly used for right ascensions in the heavens and longitude on 

 the earth. In astronomical practice, right ascension is very generally 

 expressed by hours, twenty-four of which make a complete circle, cor- 

 responding to the apparent revolution of the celestial sphere in twenty- 

 four hours. The reason of this is that astronomers determine right 

 ascension by the time shown by a clock so regulated as to read hrs., 

 min., sec. when the vernal equinox crosses the meridian. The hour 

 hand of this clock makes a revolution through twenty-four hours during 

 the time that the earth makes one revolution on its axis, and thus re- 

 turns to hrs., min., sec. when the vernal equinox again crosses the 



