SEASONS, LONGITUDE, ETC. 21 



from 12 hours, and for east longitude add to 12 hours (i. e., 

 so many hours past 12), and the result will be the corre- 

 sponding time. 



EXAMPLE. Required the time at longitude 50 31/ west, 

 corresponding to noon at Greenwich ? 



SOLUTION. 50 31' x 4=3 hours 22 min. 4 sec. 12=8 

 h. 37 min. 56 sec. A.M. Ans. 



NOTE. Time is both apparent and mean. The sun is 

 on the meridian at 12 o'clock on four days only in the 

 year. It is sometimes as much as 16J minutes before or 

 after 12 when its shadow strikes the noon mark on the sun- 

 dial. This is occasioned by the irregular motion of the 

 earth on its axis and the inclination of its poles. This is 

 called apparent time. Mean time is determined by the 

 equation of these irregularities for every day in the year, 

 and is noted in all good almanacs. ' The latter is the true 

 or correct time. The foregoing rule is applicable to 

 either. 



When you buy an almanac, buy one that expresses on 

 each calendar page the mean time when the sun reaches 

 the meridian, or the shadow the noon-mark on the dial, 

 and set your time-piece fast or slow as indicated in the 

 almanac. 



To ascertain the length of the day and night. 

 At any time in the year, add 12 hours to the time of the 

 sun's setting and from the sum subtract the time of rising 



