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Longitude and Time
The position of a ship on the earth's surface is described in terms of its latitude and longitude. The equator provides a natural reference circle around the earth which can be called zero latitude, and the latitudes north and south of this zero can be obtained by observations of the pole star and other heavenly bodies.
Longitude, that is measurements east or west around the earth, have no natural zero line and by international consent a line round the earth, through the poles and Greenwich Observatory in England, has been accepted as zero since 1884. Before this date there were thirteen different zeros from which the navigators fixed their longitude and took their time.
A ship having a bearing of 50 degrees north, 30 degrees west, indicates that it is in the Atlantic Ocean, about midway between Ireland and the North American coastline. To locate such a position by day or night requires a knowledge of time and since Greenwich is the zero of longitude it is also accepted as the standard reference of time.
Greenwich Mean Time is used as navigational time for both shipping and aircraft over a large part of the earth's surface. The earth revolves 36° degrees in twenty-four hours, that is 15 degrees every hour. Greenwich Mean Time is set so that the sun is at its highest point in the sky (its zenith) at noon. If a ship observes that from its position at sea the sun is at its zenith at 10 o'clock GMT, two hours before noon, this indicates that it is 15 x 2 = 30 degrees west of Greenwich, or longitude 30 degrees west.
Up to the beginning of the 16th century, sundials, candles, sandglasses and water-clocks were for most people the only available means of keeping time, and these devices could not be used on board ships at sea. Navigators still relied on complicated observations of astronomical events such as eclipses and star positions for their estimates of time. In fact, on long voyages they had to carry with them trained astronomers as part of the ship's crew.