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GLOBAL DIVISION AND DESIGNATORS

To compute time differences, you need to understand the international GMT system. In this system, the surface of the earth is divided into 24 zones, each extending through 15° of longitude, with the initial zone lying between longitudes 71/2' east and 71/2' west of the prime meridian. (Longitude is the name given to the imaginary lines that run lengthwise, north and south, between the North and South Poles. They have east and west designators.) The time system is named after Greenwich, England, because the zero meridian passes directly through that town. Each zone represents a different time in the 24-hour-day cycle, with a 1-hour variation between each time zone. To further aid in zone referencing, each time zone has a numerical, a literal (letter) and, to aid in the mathematical computation, a "+" or a "-" designator.

Numerical Designators

The zero meridian (prime meridian) is the imaginary line running down the center of the initial time zone; thus, this time zone is designated "0" (zero) in the numbering system. The remaining zones are numbered consecu­tively, 1 through 12, both east and west of 71/2° longitude, through 180° longitude. The longitudes of 180° east and 180° west are the same imaginary line. This meridian is the International Date Line.

Let's pause to consider what appears to be a contradiction. We stated that the earth is divided into 24 time zones; however, we have accounted for 25 zones (12 east of zone 0, 12 west of zone 0, and zone 0 itself, a total of 25 zones). This contradiction will be resolved later in the discussion of the International Date Line and the requirement to have a point at which we shift from one day to another. For now, let's agree there are only 24 time zones.

Literal (Letter) Designators

In addition to all zones having an assigned number, each zone also has a letter designator.

The initial time zone, again because of its division by the zero meridian, is designated zone "Z" or ZULU. (Use the phonetic alpha­bet to pronounce the letters of the time zones.)

With 25 designators, we use every letter of the English alphabet except "J." See Like the numbering system, the letters begin with the ZULU (0) time zone and progress to the east and west, consecutively. The zones to the east of ZULU are lettered "A" through "M" (ALFA through MIKE) and the zones to the west of ZULU are lettered "N" through "Y" (NOVEMBER through YANKEE). Re­member, beginning at ZULU and reading from left to right, we have zones ALFA through MIKE (eastern hemisphere). Returning to ZULU and reading from right to left, we find zones NOVEMBER through YANKEE (west­em hemisphere). Don't forget to omit "J" in the eastern hemisphere.

Designators "+" and "-"

Each zone has a designation of either "+" or "-" in addition to the numerical and literal designators. In time-conversion computations, you will see the reason for these designators.

Learning the "+" and "-" designation system is easy. All zones of the western hemisphere have the designation "+." All zones of the eastern hemisphere have the designation see figure 1-1.

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