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Figure 1-1.—Time zone chart of the world.
Each time zone is different by 1 hour from each of the
zones next to it. See figure 1-1.
The initial time zone is called zero and extends
7 1/2 degrees either side of the zero meridian. The time
of this zone is known as Greenwich mean time (GMT),
often referred to as Zulu time. Each zone, in turn, is
represented by the number that indicates the difference
between the local zone time and GMT.
ZERO DEGREES LINE
Zones lying in longitudes east of zone zero are
numbered from 1 to 12. They are designated minus
because the zone number must be subtracted from local
time to obtain GMT. Zones lying in longitudes west of
zone zero also are numbered from 1 to 12 but are
designated plus, because the zone number must be
added to local zone time to obtain GMT.
Besides its zone number, each zone is assigned a
letter. Zones A through M (J is omitted) are minus
zones; zones N through Y are plus zones. The number
of a zone, preceded by a plus or minus sign, is the zone
description.
A S I A
PRIME MERIDIAN
The 12th zone is divided by the 180th meridian, the
minus half lying in east longitude and the plus half in
west longitude. This meridian is the international date
line, where each worldwide day begins and ends. If a
ship crosses going to the west, the date is advanced one
day. If a ship crosses the line going to the east, the date
becomes one day earlier.
GREENWICH MEANTIME
Greenwich mean time (GMT) was adopted so that
time may be uniform throughout the military services.
This uniformity eliminates any doubt about which time
is used. The designating letter for GMT is Z. (In
lettering or printing, a horizontal bar through the riser of
the capital letter Z helps prevent its being mistaken for
the numeral 2.)
In the 24-hour system, the approved method of
representing time is with the hours and minutes
expressed as a four-digit group. The first two numbers
of the group denote the hour, and the second two, the
minutes. Thus, 6:30 a.m. becomes 0630; noon is 1200;
6:30 p.m. is 1830. Midnight is expressed as 0000 or
2400, and 1 minute past midnight becomes 0001. The
time designation 1327Z shows that it is 27 minutes past
1-8
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