46 HANDBOOK OF MAGNETIC COMPASS 



Inasmuch as the various azimuth tables tabulate the solution of the 

 astronomical triangle, in terms of t^ d^ and Z, these arguments are gen- 

 erally used to enter the tables. The meridian angle, t^ is expressed 

 either asL. A, T. or as L. H. A., as previously discussed. The declina- 

 tion ^, of the sun, with its sign, is obtained from the Ncmtioal Almanac 

 for the appropriate G. C. T. and date. The latitude, Z, may be 

 obtained from a chart of the locality. 



To obtain the sun's true azimuth, Zn, from the azimuth tables the 

 following basic procedure is presented : 



(1) Compute the meridian angle, t, 



(2) Enter the proper part of the table, according to whether 

 the latitude and declination are of the same or different names, 

 and select the page headed with the value of the latitude, Z. 



(3) Select the declination column headed with the value of 

 declination, d. In this column select the value of Z tabulated 

 against the desired value of meridian angle, t. 



(4) Convert Z to Zn according to rules previously given. 



67. Use of United States Navy Hydrographic Office Publication 



H. O. 71. — In the H. O. 71 tables the meridian angle, t^ is expressed as 

 apparent time a. m. and p. m. for each 10 minutes, and the latitude and 

 declination are given for each whole degree. Apparent time, as tabu- 

 lated, can be converted to L. A. T., as defined, by using the a. m. values 

 directly and by adding IS** to the p. m, values. This table consists of a 

 single volume covering declinations from 0° to 23° and is, therefore, 

 convenient for use with the sun. 



Interpolation in H. O. 71 is necessary when the values of Z and d are 

 not in whole degree and the L. A. T. is not an even ten-minute value. 

 To interpolate, list the necessary arguments, t^ d^ and Z ; ^ to the nearest 

 t«nth of a minute, and d and Z to the nearest tenth of a degree. Select 

 the value of Z for the next lower degree of latitude and declination and 

 the next lower 10 minutes of the tabulated value of t. This value of 

 Z is the hose. 



Then keeping Z and d^ as before, select the value of Z for the next 

 higher tabulated value of t. The difference between this Z and the 

 base Z is caused by a ten minute change in L. A. T. Multiply this 

 difference by the difference in the number of minutes, expressed to 

 the nearest tenth of a minute, between the given value of L. A. T. 

 and the L. A. T. used as a base, and divide by ten. This result is the 

 time correction. 



Next, using the values of t and Z as used for the base, select the 

 value of d which is 1° greater than d used for the base Z. The differ- 

 ence between this value of Z and the base Z is the difference for 1° 

 change of declination. Multiply this difference by the difference in 



