OBSERVING THE SUN AT NOON. 21 



OBTAINING A TRUE MERIDIAN BY OBSERVING THE 

 SUN AT APPARENT NOON. 



In addition to the instructions given on pages 16 to 

 19, there is a method of obtaining a true meridian 

 by observing the sun with a sight compass at the exact 

 time it is due south. The time of this southing is called 

 apparent noon and changes from day to day. It is not 

 the same as local mean noon, nor standard time noon. 

 It is best to set your watch for local mean time, since 

 you can then observe a southing at the time given in 

 Table 2. If your watch is set for standard time, it will 

 be necessary to set it ahead or back by adding or sub- 

 tracting a correction, according as the longitude of 

 your station is either east or west of one of the standard 

 meridians. These are: 



Local mean time at 



Longitude 75==Eastern standard time. 

 Longitude 90=Central standard time. 

 Longitude 105=Mountain standard time. 

 Longitude 120=Pacific standard time. 



The correction for a degree of longitude is 4 minutes 

 of time; the correction for a minute of longitude is 4 

 seconds of time. To illustrate: The local mean time 

 in longitude 108 will evidently be 12 minutes behind 

 Mountain standard time, or 48 minutes ahead of 

 Pacific standard time. The local mean time in longi- 

 tude 114 35' will be 21 minutes and 40 seconds ahead 

 of Pacific standard time. The method is: 



Pacific standard time is for longitude 120 00' 



Local mean time is required for longitude 114 35' 



The difference in longitude is 5 25' 



Then 5 25' 



Multiplied by 4 4 



Gives 20 m. 100 s., or 21 m. 40 s. 



