48 HANDBOOK OF MAGNETIC COMPASS 



70. Curve of magnetic azimuths. — When swinging ship for pur- 

 poses of compass correction it is necessary for the operator to be able 

 to determine the sun's azimuth, either true or magnetic, at any moment 

 without the delay which would result if each azimuth had to be ob- 

 tained from the tables as required. Therefore, a prepared table or 

 curve of azimuths for the required length of time, plotted against 

 L. A. T. or watch time, is very useful and will facilitate the procedure. 

 For such a table or curve, the Eq. T. and the sun's declination may be 

 used as of the middle instant of the pre-selected period without ap- 

 preciable error, as discussed previously under article 65. If interpo- 

 lation is necessary the combined correction for ^, d^ and L, as deter- 

 mined for the middle instant, may be used on the base values of Z 

 throughout the period provided the period does not border too close 

 to noon L. A. T. It is simpler, of course, to make such a table or curve 

 of azimuths for even units of L. A. T., thus eliminating the t correction. 



Extreme care must he exercised mhen using the sun between 1100 

 and 1300 L. A . T., si7ice the azimuth changes very raj)idly at that time 

 and the sun is generally at a high altitude. 



