44 HANDBOOK OF MAGNETIC COMPASS 



61. Meridian angle, t. — The meridian angle^ t, of the sun is the 



angle at the pole measured from the meridian of the observer to the 

 hour circle of the sun eastward or westward from 0'' to 12^ (or from 

 0° to 180°). Thus t is an expression in hours, minutes, and seconds 

 (or in degrees, minutes, and seconds) denoting the apparent sun's 

 position east or west of the local meridian. 



62. Hour angle (H. A.). — The hour angle {H. A.) of the sun is 

 the angle at the pole measured from a given meridian to the horn- 

 circle of the sun, positively westward from 0'' to 24^ (or from 0° to 

 360°). H. A., when referred to the local meridian, is called the local 

 hour angle {L. H. A.) and is, therefore, another method for ex- 

 pressing meridian angle, t. Thus, t for the sun west of the observer's 

 meridian is equal to the L. H. A. of the sun, while t for the sun east 

 of the meridian is equal to 24'' minus L. H. A. 



63. Local apparent time (L. A. T.). — Local apparent time 

 (Z. A. T.) is another method for expressing meridian angle, z5, or 

 hour angle, H. A. Since H. A. is measured from the upper branch 

 of the meridian (noon) , and L. A. T. commences when the sun crosses 

 the lower branch of the meridian (midnight), L. A. T. and L. H. A. 

 will differ by 12 hours. Thus 1200 L. A. T. is 0° of L. H. A. 



To find L. A. T., first convert zone time to its equivalent Green- 

 'wich civil time {Q. G. T.). Zone time is standard time in principle, 

 but differs from standard time in that it is kept and expressed on 

 the O'' to 24*" basis, the suffixes a. m. and p. m. not being used. Thus 

 8 : 15 a. m. standard time is 0815 zone time, and 3 : 15 p. m. standard 

 time is 1515 zone time. The earth is considered as divided into 24 

 time zones, each 15° of longitude in width, the middle meridians of 

 the zones being 15° or 1 hour of time apart. The time kept throughout 

 any zone is the civil time of its middle meridian. Thus, zone time 

 differs from G. C. T. by 1 or more whole hours. (See exception in 

 next paragraph.) When the zone is west of Greenwich the zone de- 

 scription, expressed in time, is added to the zone time to obtain G. 

 C. T.; conversely, when the zone is east of Greenwich the zone de- 

 scription, expressed in time, is subtracted from zone time to obtain 

 G. C. T. (See table in Nautical Almanac for conversion of arc to 

 time. 



There are certain excepted areas and, countries where the legal time 

 differs from the zone time. In such places the zone description is 

 the exact amount in hours, minutes, and seconds that must be applied 

 to the legal time to get G. C. T. (See U. S. Navy Hydrographic 

 Office chart H. O. 5192.) 



Having obained G. C. T., select from the Nautical Almanac the 

 Eq. T. corresponding to this G. C. T. and date, and apply it to the 

 G. C. T. in accordance with the sign given. The result will be Green- 



