| ZONE TIMES | The division of the world by meridians of longitude into zones and sectors where the same time is kept, particularly in ships at sea. The sun crosses each meridian of longitude at its local noon; four minutes later it reaches the next degree of longitude and crosses it also at its local noon. Without some agreed system of time, all clocks in the world would therefore show different times according to the longitude of the place where they are. The world therefore has been divided into zones of 15 degrees of longitude within which all clocks keep the same time. The sun takes one hour to cross 15 degrees of longitude, and so adjacent zones differ from each other by one hour. Zone times are measured east and west of the longitude of Greenwich (0°), and are designated as plus or minus as to whether the zone difference must be added or subtracted to the local time to indicate the time at Greenwich. Zones east of Greenwich are therefore minus, and zones west are plus. The longitude of 180°, where Zones +12 and -12 meet, is known as the International Date Line. Zone times were introduced in 1918. |