PRACTICAL ASTRONOMY 



155 



must also be applied to the reading of the horizontal circle; 

 this may be found by dividing the correction for altitude by 

 the cosine of the sun's altitude. This correction is to be added 

 to the reading of the horizontal circle if the hair is placed 

 tangent to the left edge of the sun, and subtracted from the 

 reading of the horizontal circle if the hair is placed tangent 

 to the right edge of the sun. 



In making observations of the sun for azimuth, the errors 

 of adjustment, the index error, and the correction for semi- 

 diameter may be eliminated by the following method, which 

 assumes that the vertical circle of the transit is complete. 



The instrument is set up with the horizontal plate reading 

 when sighting at the azimuth mark. For forenoon work, the 

 sun should be so sighted that it occupies position 1, Fig. 1, 



/Iffernoon 



FlG. 1 



FIG. 2 



with reference to the cross-hairs. The time, vertical angle, 

 and horizontal angle are noted. Then the upper plate is 

 loosened, the instrument turned 180 in azimuth, the tele- 

 scope inverted, and the sun sighted again, as in position 2, 

 Fig. 1. In position 1, the sun is moving toward both hairs; in 

 position 2, the telescope should be set approximately as shown 

 by the dotted circle, so that the sun will clear both hairs at the 

 same instant. For afternoon work, the positions shown in 

 Fig. 2 should be used. The observations are taken in pairs; if 

 the second observation of a pair cannot be obtained promptly 

 after the first one (owing to a passing cloud, or some other 

 ciuse), the first must be ignored and considered as useless. 



It should be noted that the reversal ef the transit between 

 the observations eliminates the index error of the vertical 

 circle, the error of level in the horizontal axis of the telescope, 

 and the error of collimation of the telescope. By sighting 



