PRACTICAL ASTRONOMY 147 



This means that upper culmination will occur when i4 h 25.7 a 

 has elapsed since local noon Sept. 6; that is, at 2 h 25.7 m A.M., 

 Sept. 7. 



DETERMINATION BY OBSERVING POLARIS AT 

 ELONGATION 



When a star is at its extreme westerly or easterly position, 

 it is said to be at western or eastern elongation. This position 

 with reference to the meridian of the place is determined by the 

 angle that a vertical plane passing through the star and the point 

 of observation is making with the meridian. This angle is called 

 the azimuth of the star, and its values for Polaris, for the years 

 1913 to 1922 and latitudes 5 to 74, are given in the accom- 

 panying table. 



Polaris is at eastern elongation about 5 h 55 m before it reaches 

 its upper culmination; and at western elongation, 5 h 55 m after 

 upper culmination. The times of elongation can, therefore, 

 be readily determined from those of culmination taken from 

 the table. 



EXAMPLE. Find the time of western elongation of Polaris 

 on Mar. 1,1914. 



SOLUTION. On referring to the table, it is found that the 

 upper culmination is at 2 h 52. 5 m , local astronomical time, or 

 2 h 52. 5 m , P. M., local civil time. Polaris is at western elongation 

 5 h 55 m later or at 8 h 47. 5 m p. M. local civil time. 



Making the Observation and Marking the Meridian. Deter- 

 mine the approximate time of elongation as just explained. 

 About 20 min. before that time, set the transit over a point 

 properly marked, and level it carefully. Set the vernier at zero. 

 Direct the telescope to the star, and, with both clamps set, 

 follow the star by means of the lower tangent screw. If the 

 star is approaching eastern elongation, it will be moving to the 

 right; if western, to the left. About the time of elongation, it 

 will be noticed that the star ceases to move horizontally, and 

 that its image appears to follow the vertical cross-hair of the 

 instrument. The star has then reached its elongation and the 

 observation is completed. Take the azimuth from the table. 

 Depress the telescope, and turn it through an angle equal to the 

 azimuth, to the west or to the east, according as the star was 



