THE CONQUEST OF THE ZONES 



Kes near the horizon. Everyone is familiar with the 

 oval appearance of the rising or setting sun, which is 

 due to refraction. With the sun at the meridian, the 

 refractive error is comparatively slight; and when a 

 star is observed at the zenith the refractive error dis- 

 appears altogether. 



By parallax, as here employed, is meant the error 

 due to the difference in the apparent position of the sun 

 as viewed by an observer at any point of the earth's 

 surface from what the apparent position would be if 

 viewed from the line of the center of the earth, from 

 which theoretical point the observations are supposed 

 to be made. In the case of bodies so distant as the sun, 

 this angle is an exceedingly minute one, and in the case 

 of the fixed stars it disappears altogether. The sun's 

 parallax is very material indeed from the standpoint of 

 delicate astronomical observations, but it may be ig- 

 nored altogether by the practical navigator in all ordi- 

 nary observations. There is one other correction that 

 he must make, however, in case of sun observations; 

 he must add, namely, the amount of semi-diameter 

 of the sun to his observed measurement, as all calcula- 

 tions recorded in the Nautical Almanac refer to the 

 center of the sun's disk. 



PERFECTING THE CHRONOMETER 



The observation of the sun's height, with the various 

 corrections just suggested, suffices by itself to define 

 the latitude of the observer. Something more is re- 

 quired, however, before he can know his longitude. 



[23] 



