18 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



But, if the theodolite is set up with zero of base plate on south (wiiich 

 is Weather Bureau practice), then the azimuth bearing of the church 

 spire will be 180° more, or 207.75°. 



42. Second method. — Polaris, in its apparent counterclockwise revo- 

 lution about the pole, takes 23 hours 56.1 minutes of our regular 24- 

 hour day, thus culminating or crossing the meridian twice in 24 hours, 

 and nearly 4 minutes earUer each day. From this we see that t'ne 

 position of Polaris east or west of the meridian for any specified time 

 will vary from day to day. Knowing the correct local mean time and 

 the time of upper culmination, the hour angle of Polaris (or the angle 

 at the pole between the north-south line and the hour circle passing 

 through Polaris) may be found. From the hour angle of Polaris, 

 with the aid of the American Ephemeris and Nautical Almanac, the 

 true azimuth of Polaris may be easily computed. The observations 

 are made on Polaris at any convenient time after it becomes visible. 



43. The theodohte is carefully set and leveled over the exact point 

 of observation as in the preceding method, the cross hairs are likewise 

 illuminated, and the watch compared with the correct local mean 

 time. After the base plate is locked, the telescope is sighted upon 

 some well-defined point as a reference mark, and the azimuth reading 

 carefully noted and recorded. The telescope is then trained upon 

 Polaris, and at the instant that the intersection of cross hairs is brought 

 centrally over Polaris, the exact watch time to seconds is first noted, 

 followed by the reading on the same azimuth vernier from which 

 the azimuth reading of the reference point was. made. All angles will 

 be read to the nearest hundredth of a degree. A series of three or more 

 observations, 10 to 15 minutes apart, should be taken as a check on 

 the first and the computation as a whole. The final result of each 

 computation should be no more than 0.02 or 0.03 of a degree from the 

 mean result. 



Example 2: On July 2, 1919, m lat.42°27' N., long. 76°29' W., or 5 h. 

 06 m. earlier than Greenwich, a series of three observations was made 

 at 8 h. 42 m. 00 s., 8 h. 55 m. 00 s., and 9 h. 10 m. 00 s., seventy-fifth 

 meridian time. The base-plate reading of the right azimuth vernier, 

 when sighted upon a definite point on the left of north, was 189.64°. 

 The azimuth readings from the same vernier when sighted upon 

 Polaris during the observations were 237.80°, 237.89°, and 237.98°, 

 respectively. 



Date, July 2, 1919. Position, lat. 42°27' N., long. 76°29' W.— 

 5 h. 06 m. earlier than Greenwich. 



H. m. s. H. m. s. H. m. s. 



Time of observation (St. 75th) 8 42 00 8 55 00 9 10 00 



Earlier than seventy-fifth meridian 06 00 06 00 06 00 



Local mean time 8 36 00 8 49 00 9 04 00 



Reduction to sidereal time (A. E. and 



N. A., table III) +01 25 +01 27 +01 29 



Sidereal time mean noon Greenwich, 



or right ascension of mean sun this 



date (A. E. and N. A., for Greenwich 



mean noon) 6 37 54 6 37 54 6 37 54 



Correction for long., 5 h. 06 m. 00 s., 



(A. E. and N. A., table III) +00 50 +00 50 +00 50 



Local sidereal time 15 16 09 15 29 11 15 44 13 



