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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 42042'W. 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 7515'37"E and the zone time is 16h23m14s. Divide the longitude by 15 and you get 5, with less than 730' 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 730" left over. The ZD is therefore +5; and the GMT of the observation is 10h15m08 + 5h, or 15h15m08s.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 17615'22"W and the zone time of your observation is 1624"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 ships 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. |
 
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