It would be impossible for a ship at sea to keep to the time of its longitude because (unless it is travelling due north or south) the longitude will be constantly changing. For this reason, the sea areas of the Earth are divided into time zones which are 15O (or 1 hour) apart. The central meridian of each zone is an exact number of hours distant from the Greenwich meridian so that zone time differs from GMT by multiples of 1 hour. The time kept in each zone is the time of its central meridian and is plus or minus GMT depending on the zone’s position east or west of Greenwich.
Time Zone System. The Earth is divided into 24 zones of 15O of longitude each, with the centre of the system being the Greenwich meridian. Therefore, the centre zone (zone 0) lies between 7.5O W. and 7.5O E. The zones lying to the West of Greenwich are numbered from +1 to +12 and those to the East from -1 to 12. The 12th zone is divided by the meridian 180O which is known as the International Date Line. The two halves of the 12th zone are marked + or – depending on which side of the date line they lie.
The following table shows the centre meridian of each of the 24 time zones. It should be remembered that each zone is 15O wide and extends 7.5O either side of its centre meridian. Thus, since the centre meridian of zone +4 is 60OW, it’s limits are 52.5OW to 67.OW.
To Convert Zone Time to GMT. The time kept in zone 0 is GMT. To convert any other zone time to GMT, simply apply the sign and number of the zone to the zone time. For example, if it is 1800 in zone +5, GMT will be 2300.
The Greenwich Date (G.D.) is obtained by converting the zone time and date to GMT
Example 1: ZT 0230(-8), 15 Apr
GD = ZT – 8 = 1830, 14 Apr.
Example 2: GD 0515, 25 Dec.
ZT(+10) = GD – 10 = 1915, 24 Dec
Exercise Questions
Question 1. A ship’s longitude is 35O,36’.5W. If GMT is 3h 35m 12s what is zone time?
Question 2. On 5 July, a ship’s longitude is 145O, 15’.33E. If zone time is 3h 20m 30s, what is the Greenwich Date?
Question 3. A ship’s longitude is 158O, 2’.38E. If the GD is 2330 3 July, what is ZT?
Question 4. At GD = 12h 01m, 30th. April, what is the zone time for a ship in position Latitude 0O, longitude 179O,59’.9E ?
Question 5. At GD = 11h 59m , 30 April, what is the zone time for a ship in position Latitude 0O, longitude 179o,59’ W ?
Question 6. Approximately how far apart in geographical miles are the two ships in questions 4 & 5 at GD 12h 00m 30 April?
(Note. The international geographical mile (g.m.) is a unit of length determined by 1 minute of arc along the Earth’s equator and is defined as 1855.32m.).
Click here for answers to exercise questions.
A fuller explanation of this topic is given in the book ‘Astro Navigation Demystified’.
Useful Link: The Sun as a time keeper.
Astro Navigation Demystified 
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