designated as a standard time meridian. Each timemeridian runs through the center of its time zone, whichmeans that the zone extends for 7°30' on each side ofthe meridian. In each zone, the time is the samethroughout the zone.There is a 1 hour difference in time between aparticular zone and the adjacent zone. Whendetermining time in different zones, it is helpful toremember this phrase: time is later as you moveeastward. So, if it is 1200 in your zone, it is 1300 in thenext zone to the east and 1100 in the next zone to thew e s t.ZONE TIME AND GREENWICH MEANTIME.— The time listed in most of the computationaltables used in celestial observations is Greenwich meantime (GMT)— meaning the zone time in the Greenwichstandard time zone. You must know how to convert thezone time at which you made a particular observation toGreenwich mean time. The procedure is as follows.Each of the time zones has a number that is calledthe zone description (ZD). The Greenwich zone isnumbered 0. The others are numbered from 1 through12, east or west of Greenwich. To determine the ZD forany point on the earth, you divide the longitude by 15.If the remainder is greater than 7030', the quotient plus1 is the ZD. Suppose, for example, that the longitude atthe point of your observation is . Divide thisby 15 and you get 9, with a remainder of 7041'. Sincethe remainder is greater than 7030', the ZD is 9 + 1, or10.Zones east of Greenwich are minus and zones westof Greenwich are plus. To convert the zone time of anobservation 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 ofobservation is 75°15'37"E and the zone time is16"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 observationSuppose now that the longitude of the point ofobservation isand the zone time of theobservation is 10h15m08’. Divide the longitude by 15 andyou get 4, with more than 7°30" left over. The ZD istherefore +5; and the GMT of the observation is10h15m08’ + 5h,or 15’’15m08S.ZONE TIME AND DATE.— It may be the casethat the date at Greenwich and the date at the point ofobservation are not the same at the time of observation.Suppose that on 1 May you are in longitudeand the zone time of your observation is16’’24”1ls. The ZD is +12. GMT of the observation istherefore or 28h24mlls.However,28h24ml1’ on 1 May means 04h24ml1s on 2 May, and youwould refer to the tables for that GMT and date.Suppose now that on 1 May you are in longitudeand the zone time of the observation isbut 02h15m27s on 1 May can be consideredas 26h15m27s on 30 April. Therefore, GMT for theobservation was 25h15m27s – 3h, or 23h15m27s, on 30April.Importance of Exact TimeThe importance of recording the exact time atwhich an observation is made may be illustrated asfollows. Suppose a ship’s navigator makes an error ofonly 1 minute in his time. This could produce an errorof as much as 15 miles in the location of his computedand plotted line of position. A 1-minute time errorproduces a 15-minute error in longitude regardless ofthe latitude; and on the equator, a minute of longitudeequals 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 plusa stopwatch. You should set the ordinary watch to exacttime shortly before the time of observation. Correctstandard time can be obtained from a clock known to beclosely regulated, or preferably from time signalsbroadcast by the U.S. Naval Observatory.Remember, too, that in localities under daylightsavings time, the time is 1 hour faster than standard time.ELEMENTS OF FIELD ASTRONOMYAlthough the earth is not actually a true sphere, it ispresumed to be such for the purpose of astronomy.Astronomic determinations are based on therelationships that exist among sets of sphericalcoordinates: the terrestrial system stated in latitude andlongitude; the celestial system of right ascension anddeclination, or its subsidiary system of hour angle anddeclination; and the horizon system in terms of altitudeand azimuth.Terrestrial System of CoordinatesThe terrestrial system of coordinates refers to thelocation of points on the terrestrial sphere (the earth). Inthe terrestrial system, the fundamental reference lines(fig. 15-1) are the axis of the earth’s rotation and theearth’s equator. The ends of the axis of rotation areknown as the poles, designated as the North and Southis15-2