Dividing the world into 24 zones with their own times
The world is divided into time zones corresponding to standard meridians 15 degrees—or 1 hour of longitude—apart, with the Greenwich meridian in London, England, understood to be zero.Each time zone extends 7½ degrees east and west of its meridian. However, in many parts of the world, the borders are bent to include geographical regions with common interests, such as scattered island groups belonging to the same country.The zones extend for 180 degrees east of Greenwich, and 180 degrees west, meeting at the international date line. If you cross the date line in your boat heading west, advance the date by one day; if you’re heading east, subtract one day as soon as you’ve crossed the line.When you’re at sea, you should adjust the ship’s time to correspond with the zones, so that ship’s noon is reasonably close to the time when the sun reaches its zenith on your meridian. As noted, the changeover comes every 15 degrees. If you’re sailing from England to the East Coast of the United States, for example, you would start off in zone 0, or Greenwich time, also known as Zulu. You would enter zone +1 7½ degrees west of the Greenwich
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The Practical Encyclopedia of Boating. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.
