144 



PRACTICAL ASTRONOMY 



Pole 



DETERMINATION OF MERIDIAN 



DETERMINATION BY OBSERVING POLARIS AT 

 CULMINATION 



The position of Polaris, or the north star, can easily be ascer- 

 tained by means of the group of stars called the Dipper, or the 

 Great Bear. As shown in the accompanying illustration, a 

 straight line joining the stars, 

 and j8, called the pointers, nearly 

 intersects Polaris. There are two 

 times during the day when the star 

 crosses the meridian. It is then 

 said to be at its upper or lower 

 culmination, as the star is then 

 occupying either the highest or the 

 lowest position with reference to the 

 horizon. When the star is in either 

 one of these positions, the vertical 

 plane passing through it and the ob- 

 server's station is the meridian of 

 the place, and its intersection with 

 the horizon is therefore a true north- 

 and-south line. 



Field Work. Select a date on 

 which Polaris is at either lower or 

 upper culmination during the night 

 (preferably during the early part of 

 the evening). Determine, by means of the accompanying 

 table, the exact time of culmination, being careful to reduce 

 the tabular values to standard civil time. It is safer, 

 order to avoid confusion, for the observer to set 

 'watch to show local time. About 15m. before the time 

 culmination, set the transit in such a position that an unot 

 structed view toward the north may be obtained for a distai 

 of between 300 and 500 ft. Drive a stake, and mark by a 1 

 the exact point occupied by the instrument. About 

 before the time of culmination, direct the telescope to the 

 star, holding a lamp in front and a little toward one side of the 



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Dipper or Great JBar 



