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local time at 60o east longitude is later by the respective amounts.
Figure 1.14. Local Time Differences at Different Longitudes.
1.15. Standard Time Zone. The world is divided into 24 zones, each zone being 15o of longitude wide.
Each zone uses the LMT of its central meridian. (A few areas of the world are further divided and use
half hour increments from GMT. Some notable examples include India, Bangladesh, Newfoundland, and
parts of Australia and Thailand.) Since the Greenwich meridian is the central meridian for one of the
zones and each zone is 15o or 1 hour wide, the time in each zone differs from GMT by an integral
number of hours. The zones are designated by numbers from 0 to 12 and -12, each indicating the number
of hours that must be added or subtracted to local zone time (LZT) to obtain GMT. Since the time is
earlier in the zones west of Greenwich, the numbers of these zones are plus; in those zones east of
Greenwich, the numbers are minus (Figure 1.15). Ground forces frequently refer to the zones by letters
of the alphabet, and air forces use one of these letters (Z) for GMT. The zone boundaries have been
modified to conform with geographical boundaries for greater convenience. For example, in case a zone
boundary passed through a city, it would be impractical to use the time of one zone in one part of the city
and the time of the adjacent zone in the other part. In some countries, which overlap two or three zones,
one time is used throughout.
1.16. Date Changes at Midnight. If you travelled west from Greenwich around the world and set your
watch back an hour for each time zone, you would have set your watch back a total of 24 hours on
arriving back at Greenwich and the date would be 1 day behind. Conversely, traveling eastward, the
watch would have been advanced a total of 24 hours, gaining a day.
32 AFPAM11-216 1 MARCH 2001
Figure 1.15. Standard Time Zones.
1.16.1. To keep straight, you must add a day somewhere if going around the world to the west and to
lose a day if going around to the east. The 180o meridian is the international dateline where a day is
gained or lost. The date line follows the meridian except where it detours to avoid eastern Siberia, the
western Aleutian Islands, and several groups of islands in the South Pacific.
Figure 1.16. Zone Date Changes.
AFPAM11-216 1 MARCH 2001 33
1.16.2. The local civil date changes at 2400 or midnight. Thus, the date changes as the mean sun transits
the lower branch of the meridian. Consider the situation in another way. The hour circle of the mean sun
is divided in half at the poles. On the half away from the sun (the lower branch), it is always midnight
LMT. As the lower branch moves westward (Figure 1.16), it pushes the old date before it and drags the
new date after it. As the lower branch approaches the 180o meridian, the area of the old date decreases
and the area of the new date increases. When the lower branch reaches the date line; that is, when the
mean sun transits the Greenwich meridian, the old date is crowded out and the new date for that instant
prevails in the world. Then, as the lower branch passes the date line, a newer date begins east of the
lower branch and the process starts all over again.
1.16.3. The zone date changes at midnight zone time (ZT) or when the lower branch of the mean sun
transits the central meridian of the zone.
1.17. Time Conversion. Sometimes you must convert LMT time to GMT, or GMT to LMT. The Air
Almanac contains a table for conversion of arc to time at a rate of 15o of arc per hour of time (Figure
1.17). This conversion is only good for LMT to GMT, or GMT to LMT. ZT is influenced by daylight
savings time and geographical boundaries. For example, to convert GMT to LMT at 126o-36'W:
126o00 = 8 h 24 min 00s
36' = 02 min 24s
126o36' = 8 h 26 min 24s
To derive LMT from GMT, subtract the time in the Western Hemisphere and add it in the Eastern
Hemisphere. Do the opposite to convert LMT to GMT.
1.18. Sidereal Time. Solar time is measured with reference to the true sun or the mean sun. Time may
also be measured relative to a fixed point in space. Time measured with reference to the first point of
Aries (ϒ), which is considered stationary although it moves slightly, is sidereal or star time. The first
point of Aries is defined as where the sun crosses the equator northbound on the first day of spring.
1.18.1. The sidereal day begins when the first point of Aries transits the upper branch of the observer's
meridian. Local sidereal time (LST) is the number of hours that the first point of Aries has moved
westward from the observers meridian. Expressed in degrees, it equals the local hour angle (LHA) of
Aries (Figure 1.18). LST at Greenwich is Greenwich sidereal time (GST) which is equivalent to the
　
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