152 



PRACTICAL ASTRONOMY 



about 15 min. before apparent noon and repeat them every 

 minute or two. At first the altitude will be increasing; then, 

 it will be decreasing. The maximum altitude obtained will 

 be the apparent meridian altitude. To this the corrections 

 that follow must be applied, giving the true altitude. The 

 true altitude is then subtracted from 90, and the remainder is 

 the zenith distance. The latitude is then equal to the algebraic 

 sum of the zenith distance and the declination of the sun at the 

 instant of apparent noon. 



Corrections for Altitude. The observed altitude of a heav- 

 enly body must be corrected: (1) for index error, (2) refrac- 

 tion, (3) parallax, and (4) semi-diameter. 



1. The index error is a purely instrumental error and is 

 explained under the heading Hydrographic Surveying. 



2. Refraction is the change of direction of the rays of light 

 when they pass from one medium into another of different 

 density. Its amount for different altitudes is given in the 

 accompanying table. It is subtractive. When the altitude 

 is less than about 8 to 10, the refraction becomes so uncer- 

 tain that the measurement is of no value for accurate work. 



3. Parallax is the difference in direction of a heavenly body 

 as actually observed and the direction it would have if seen 

 from the earth's center. This correction is necessary when 



SUN'S PARALLAX IN ALTITUDE TO BE APPLIED TO 

 ALL MEASURED ALTITUDES OF THE SUN 



(Additive to observed altitude) 



