LATITUDE AND LONGITUDE. 93 



that is VO -f. WO = 2 PO 

 orPO = VO-f WO 



2 



which is half the sum of the two altitudes VO and WO 

 equal PO = the latitude. 



But although a variety of methods may be proposed 

 to determine the latitude, the most simple is that before 

 observed ; in finding the meridian altitude of the stars, 

 or their meridian distance from the zenith, that is to say, 

 their distance from the zenith or point immediately 

 over our heads when on the meridian, whenever this is 

 practicable we should not employ any other method. 



To ascertain the latitude by those means we must 

 know the declination or the distance of the sun or star 

 from the equator either north or south, to facilitate this 

 calculation, tables have been constructed by which we 

 are enabled to ascertain the declination of the sun, moon 

 and a variety of stars, at any given time. 



This understood, we may observe the following rule 

 in order to ascertain the latitude by a meridian distance 

 from the zenith. 



If the distance from the zenith to the star is of the 

 same denomination as the declination, (that is, if they 

 are both north or south) take their difference and we 

 shall have the latitude ; if, on the contrary, they are of 

 different denominations, add them together, and the 

 sum will give the latitude. 



Should a star be under or below the elevated pole, 

 we add the declination and zenith distance, and take 

 the supplement of the sum, which gives the latitude. 



In order more clearly to understand the reason of 

 this, we may employ the annexed figure, where HLON 

 represents the rational horizon, PEAT the meridian, 

 L the place, ET the equator extending to the heavens, 

 P the pole, GZ the zenith distance of the sun, RZ the 

 zenith distance of a star ; and we may further suppose 

 that in passing over the meridian, a star may happen to 

 be between O and E, or between E and Z, or between 

 Z and P, or lastly between P and H. 



