156 



PRACTICAL ASTRONOMY 



in diagonal corners of the field of view and taking the mean 

 of the observations, the corrections (both horizontal and ver- 

 tical) due to the semi-diameter of the sun are eliminated. To 

 simplify the notes, 180 should be added to (or subtracted 

 from) the horizontal plate reading when the instrument is 

 inverted. 



EXAMPLE. The following measurements were taken in the 

 manner just described. The four means of the circle readings 

 were formed in the field. The declination of the sun was 

 -9 30' 5", and the approximate latitude +39 57'. Find 

 the azimuth of the reference mark. 



SOLUTION. 



Mean of the four vertical circle readings . 



Refraction 



Parallax. . . 



18 37' 11" 

 -2 48 



+8 



True altitude of center 18 34' 31" 



Zenith distance = 90 true altitude 71 25' 29* 



To find the azimuth of the sun : z = 7 1 25' 29" ; <t> = 39 57' 0" ; 



930 / 5"; } ( a +<f>-fS) = 5056'12 w ; * (z+<t>- &) = 60 



26' 17*. Substituting these values in the formula for the 

 azimuth of the sun, 



