88 OUTLINES OF NATURAL PHILOSOPHY. 



The manner of referring any s point to the ecliptic, of 

 which the position is known with respect to the 

 equator, or of finding its longitude and latitude 

 from its right ascension and declination, has been 



already explained, 30. 



y F 9 y 



If the point is in the ecliptic ; if, for example, it is 

 the sun itself of which the longitude is required, 

 from knowing the right ascension or declination ; 

 we have 



tan. #1 

 tan. long. = CQg Qbl ^ c > or sin long. = 



/Ki> 80 - The character JR denotes 

 sin Ubl. .be. J 



the Right Ascension. 



8.9. The point in which the equator and ec 

 jUptic intersect, is not iinmoveable, but appears, 

 in respect of the fixed stars, to recede towards 

 the west, at the rate of 50"^ nearly per annum, 

 or about 1 in 72 years. 



This motion is called the Precession of the Equinoxes ; 

 and by means of it, the equinoctial points describe 

 an entire circle in 25867 years, which istheannus 

 magnus of the ancients. In consequence of this, 

 the longitude of the stars continually increases, at 

 the rate of a degree in 72 years nearly. When the 

 zodiac was first delineated by the ancient astrono- 

 mers, the middle of the constellation Aries was at 

 the vernal equinox, from which it is now distant 



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