INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 19 



Apparent right ascension of Polaris this 



date (A. E. and N. A., apparent place H. m. s. H. m. s. H. m, s. 

 ofstars) 1 31 35 1 31 35 1 31 35 



Hour angle of Polaris before upper cul- 

 mination 10 15 25 10 02 25 9 47 25 



H. m. n. m- H. m. 



Same in decimals of minutes 10 15.42 10 02.42 9 47.42 



Azimuth of Polaris at this hour angle 



and latitude (A. E. and N. A., table 



IV) 39.58 44.23 50.29 



Same reduced to degrees .66 .74 .84 



Observed azimuth of Polaris 237. 80° 237. 89° 237. 98° 



True north on base plate 237. 14° 237. 15° 237. 14° 



44. Accepting 237.14° as the direction of true north when the the- 

 odolite is set with 189.64° on the reference point, the bearing, or hori- 

 zontal angle between the reference point and true north, will be the 

 difference between 237.14° and 189.64°, or 47.50°. Now, then, with 

 the zero of base-plate setting on north, the azimuth bearing of the 

 reference point is 360°, minus 47.50°, or 312.50°. In preparation for 

 an observation the theodolite setting on this point would be 132.50.° 



45. Third method. — The south point may be determined by pointing 

 the theodoUte at the sun at true solar noon. To do this it will be nec- 

 essary to protect the object glass with a piece of smoked or colored 

 glass. In order to determine the time the sun crosses the meridian, 

 the observer must ascertain the exact difference between the standard 

 time in use at his station and the true local time. To this difference 

 must then be added or subtracted, as the case may require, the 

 so-caUed equation of time, which is the number of minutes before 

 or after local noon at which the sun passes the meridian. The equation 

 of time (Mean— App.) for Washington may be found in the Nautical 

 Almanac or in Circular D, Instrument Division. When the equation 

 of time is + , the sun is slower than the clock and the specified number 

 of minutes must be added to the true local noon time to give the time 

 at which the sun passes the meridian, and, similarly when the sign 

 is — , the number of minutes must be subtracted from local noon. 



For example, suppose the south point is to be located at some 

 station on October 3, and where the local time is 24 minutes faster 

 than standard time in use — 



Difference between standard and local time is —24^00" (use + 

 when standard meridian is east of station and — when west) . 



Equation of time, October 3 = — ll'OO" 



Total correction = —35' 00" 



Therefore the sun will be exactly on the meridian at 12 o'clock minus 

 35 minutes = 11.25 a. m., and if the theodolite is pointed at the sun 

 at this time it will be pointing due south. Of course the north point 

 is found by reversing the telescope of the instrument. 



Note that the equation of time as given from Greenwich is App. — 

 Mean. In this case the signs must be changed before it is applied. 



46. Orientation points, base lines, etc. — When the true north point 

 has been determined, the bearing or azimuth of reference point from 

 the north point with at least two or three others at different distances 

 from the observation point should be determined. These points with 

 their bearings from north will constitute the orientation points of 



