410 ASTROXOMY. 



below the horizon are seen only to cross it once, 

 because when they cross it a second time they are 

 invisible. 



Three great circles are thus established in the 

 heavens ; the horizon, the equator, and the meri- 

 dian. The first determines the rising and setting 

 of the heavenly bodies, and also the altitude of any 

 of them, at any time of their course. For this 

 purpose you must suppose another great circle to 

 pass through the star and the zenith ; it will, con- 

 sequently be perpendicular to the horizon. This 

 is called a vertical circle, and upon this circle we 

 reckon the number of degrees which the star is 

 distant from the horizon. The quadrant is an in- 

 strument for measuring the number of degrees of 

 altitude which any body has. 



The three great circles mentioned above form 

 the basis of all observations upon the heavenly 

 bodies, and to them all their situations must be 

 referred. It is necessary, therefore, to deter- 

 mine the relative situations of these circles. If 

 the polar star had been accurately at the pole of 

 the heavens, nothing more would be necessary, in 

 order to obtain the altitude of the pole, but to 

 take the altitude of this star ; but this star is 

 situated 2° distant from the pole : 2° must, there- 

 fore, be added to this altitude to find that of the 

 pole. 



The elevation of the pole being discovered, it is 

 easy to find that of the equator. Thus, in the dia- 

 gram (Fig. 1.), H M O, or the visible part of the 

 heavens, contains 180°; but it is 90° from the pole 

 P, to E the equator. If you take away P E from 

 the semi-circle HMO, there remains 90° for the 

 other two arcs \ or in other words, the elevation of 

 the pole and the equator, are together equal to 90°; 



