Time Zones

TIME ZONES

Time is measured in terms of the rotation of the earth. Since the earth rotates 360 in a 24-

hour period, we divide 360 by 24 to yield 15of rotation in one hour. This divides the earth

into 24 time zones; each 15 of longitude in width, making the time between each zone

differ by one hour. Each time zone is centered on a meridian that is a multiple of 15. The

time within each zone is called Local Mean Time (LMT).

Figure 4.2-11 Time Zones

Each time zone has been given both alphabetic and numeric designators. The alphabetic

designator for the time zone centered on the zero-degree meridian (the prime meridian) is

"Z" (Zulu). The time within the Zulu time zone is called Greenwich Mean Time (GMT).

Greenwich Mean Time is used as the reference for each of the remaining zones. The zone

description (ZD), numeric designator for any zone, indicates the difference in hours from

local time to GMT.

In air travel, where great distances can be covered in a short time, it is inconvenient to

keep track of time zones being crossed. To avoid confusion, Greenwich Mean Time is the

standard used for aviation since GMT is the same all over the world at any particular

instant in time. For example, weather briefs and flight plans are filed using GMT. Therefore,

you must be able to convert any local time to GMT and GMT to local time.

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