designated as a  standard time meridian. Each time meridian runs through the center of its time zone, which means that the zone extends for 7°30' on each side of the  meridian.  In  each  zone,  the  time  is  the  same throughout  the  zone. There  is  a  1  hour  difference  in  time  between  a particular  zone  and  the  adjacent  zone.  When determining time in different zones, it is helpful to remember this phrase:  time is later as you move eastward. So, if it is 1200 in your zone, it is 1300 in the next zone to the east and 1100 in the next zone to the w e s t. ZONE   TIME   AND   GREENWICH   MEAN TIME.— The time listed in most of the computational tables used in celestial observations is Greenwich  mean time (GMT)—  meaning the zone time in the Greenwich standard time zone. You must know how to convert the zone time at which you made a particular observation to Greenwich mean time. The procedure is as follows. Each of the time zones has a number that is called the zone description  (ZD). The Greenwich zone is numbered 0. The others are numbered from 1 through 12, east or west of Greenwich. To determine the ZD for any point on the earth, you divide the longitude by 15. If the remainder is greater than  7030', the quotient plus 1 is the ZD. Suppose, for example, that the longitude at the point of your observation is            . Divide this by 15 and you get 9, with a remainder of 7041'. Since the remainder is greater than  7030', the ZD is 9 + 1, or 10. Zones east of Greenwich are minus and zones west of Greenwich are plus. To convert the zone time of an observation  to  the  corresponding  Greenwich  meantime, you apply the ZD  according to its sign  to the zone time. For example, suppose the longitude at your point of observation   is   75°15'37"E   and  the  zone  time  is 16"23m14s.  Divide the longitude by 15 and you get 5, with less than 7°30' left over. ‘The longitude is east; therefore, the ZD is -5; and the GMT of the observation Suppose  now  that  the  longitude  of  the  point  of observation is                and the zone time of the observation is 10h15m08’.  Divide the longitude by 15 and you get 4, with more than  7°30" left over. The ZD is therefore  +5;  and  the  GMT  of  the  observation  is 10h15m08’  + 5h, or 15’’15m08S. ZONE TIME AND DATE.— It may be the case that the date at Greenwich and the date at the point of observation are not the same at the time of observation. Suppose   that   on   1   May   you   are   in   longitude and the zone time of your observation is 16’’24”1ls.   The  ZD  is  +12.  GMT  of  the  observation  is therefore                                  or  28h24mlls. However, 28h24ml1’  on 1 May means 04h24ml1s  on 2 May, and you would refer to the tables for that GMT and date. Suppose now that on 1 May you are in longitude and the zone time of the observation is but 02h15m27s on 1 May can be considered as 26h15m27s  on  30  April.  Therefore,  GMT  for  the observation  was  25h15m27s – 3h, or 23h15m27s, on 30 April. Importance  of  Exact  Time The importance of recording the  exact time  at which an observation is made may be illustrated as follows.  Suppose  a  ship’s  navigator  makes  an  error  of only 1 minute in his time. This could produce an error of as much as 15 miles in the location of his computed and plotted line of position. A 1-minute time error produces  a  15-minute  error  in  longitude  regardless  of the latitude; and on the equator, a minute of longitude equals a nautical mile. You must time the observation to the nearest second, and for this purpose, you must have an accurate watch. It is best that you have an accurate ordinary watch plus a stopwatch. You should set the ordinary watch to exact time shortly before the time of observation. Correct standard time can be obtained from a clock known to be closely  regulated,  or  preferably  from  time  signals broadcast by the U.S. Naval Observatory. Remember, too, that in localities under daylight savings time, the time is 1 hour faster than standard time. ELEMENTS OF FIELD ASTRONOMY Although the earth is not actually a true sphere, it is presumed to be such for the purpose of astronomy. Astronomic  determinations  are  based  on  the relationships   that   exist   among   sets   of   spherical coordinates: the  terrestrial system stated in latitude and longitude; the celestial system of right ascension and declination, or its subsidiary system of hour angle and declination;  and  the  horizon system  in  terms  of  altitude and azimuth. Terrestrial System of Coordinates The terrestrial system of coordinates refers to the location of points on the terrestrial sphere (the earth). In the terrestrial system, the fundamental reference lines (fig. 15-1) are the axis of the earth’s rotation and the earth’s equator. The ends of the axis of rotation are known as the poles, designated as the North and South is 15-2