The standard for time measurement is the apparent movement of the sun from east to west across the sky. The sun moves westward about 15 degrees of longitude each hour, or 360 degrees in 24 hours. Based on this, each time zone is 15 degrees of longitude in width. The same time is kept at all locations within a time zone. Each time zone is given a number from zero to plus twelve, if the zone is in west longitude, or from zero to minus twelve its in east longitude. These numbers are called zone descriptions (ZD).
The central meridian of each time zone is a multiple of 15. For example, 60 degrees west is the central meridian of the + 4 time zone: 60 ÷ 15 = 4. Each time zone then extends 7.5 degrees east and west of the central meridian. The + 4 time zone is from 52.5° W to 67.5° W of longitude.
The upper branch of the Greenwich meridian is the central meridian of the zero time zone, which extends from 7.5 degrees east to 7.5 degrees west longitude. The lower branch of the Greenwich meridian, or 180th meridian, is the central meridian of the + / - 12 time zone. Longitudes from 172.5° west to 180° E / W are in the +12 time zone, and longitudes from 172.5° east to 180° E / W are in the -12 zone. The International Date Line follows the 180th meridian for most of its length. The date line zig-zags in places to keep all islands of a group on the same date. The date on the west side of the date line in ZD -12 (Long. 172.5° E - 180°) is one day later than in ZD + 12 (172.5° W - 180°).
When a ship crosses the date line heading west it becomes the same zone time 24 hours or one day later. When heading east over the date line it becomes one day earlier when the ship enters the + 12 time zone. This seems confusing now but it will become clear once the rules are understood. Determination of the time zone of a particular position is found by dividing its longitude by 15.
Example: Find the zone description of a ship at longitude 141° 30.0' East.
1. Convert the longitude to degrees and tenths of degrees by converting the minutes to the nearest 1/10 of a degree. 141°30.0' = 141.5 degrees.
2. Divide 141.5 by 15 = 9.43, now the zone description is - 9. (It is minus because it is East Longitude).
Note: If the longitude divided by 15 above had been more than 9.5 the zone description would have been rounded to -10. In the example above the central meridian of the time zone is - 9 x 15 = 135 East Longitude.
You should try and memorize the central meridians of the time zones: 0, 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, and 180 east and west. Remember, the central meridians are always even multiples of 15. The time zone boundaries for a given locality can be found by remembering the limits of each zone extend 7.5 degrees of longitude east and west of the standard or central meridians.
A variation to the normal practice of keeping zone time is daylight savings time. When daylight time is in effect the entire time zone keeps the time normally kept by the time zone adjacent to the east. They set their clocks ahead one hour. Knowing the longitude and zone time at one location allows you to determine the zone time at a second location provided you know the longitude of the location.
Example: When it is 1635 zone time on 1 July 1981, for a ship at 43° 22.0' West Longitude, what is the zone time and date at Greenwich? What is the zone time and date for a ship at 108° 42.0' East Longitude?
There are two rules you should know:
Rule 1: When converting from local zone time (ZT) to Greenwich mean time (GMT), the zone description of the local time zone is added or subtracted from the local zone time as indicated by its sign (+ or -).
Rule 2: When converting from Greenwich mean time (GMT) to zone time (ZT) the sign of the local time zone description is reversed before applying it to GMT.
Example: Step 1. Find the zone description of the first ship. The first ship is in the +3 time zone (43° 22.0' or 43.4° divided by 15 = 2.89, which would be rounded up to 3. Since the longitude is west the sign of ZD is a plus.)
Step 2. To convert from zone time (ZT) at the first ship to Greenwich mean time, the zone description at the first ship is applied to the zone time. 1635 ZT +3 = 1935 GMT 1 July 1981.
Step 3. Find the zone description of the second ship. Longitude 108° 42.0' E (108.7°) divided by 15 = 7.25 which is rounded to 7. Since the longitude is east, zone description is -7. To find local zone time you follow Rule 2.
GMT was determined to be 1935, now you have to reverse the sign of zone description 7, which becomes +7, and apply it to GMT. 1935 GMT +7 = 0235 ZT, on 2 July 1981 at the second ship. Knowing the rules for conversion of zone time to GMT and the reverse is very important in solving most of these types of problems found in the celestial portion of the USCG license exams.
