A navigator can know the latitude of his ship by observing the location of stars, where the sun rises in the morning and sets in the evening, and what time of year it is.With this information he knows where his ship is in relation to the north or south pole and the equator.

　　Still, there is one more important piece of information necessary for safely sailing the oceans.For many centuries, scientists, astronomers and inventors searched for a way to tell longitude(经度)．The lines of longitude go the other way from latitude lines.They stretch from the north pole to the south pole, and back again in great circles of the same size.All of the lines of longitude meet at the top and bottom of the world.

　　To learn longitude at any place requires knowledge about time.A navigator needs to know what time it is on his ship and also the time at another place of known longitude—at the very same moment.

　　The Earth takes twenty­four hours to complete one full turn or revolution of 360 degrees.One hour marks one twenty­fourth of a turn, or fifteen degrees.So each hour's time difference between the ship and the starting point marks a ship's progress of fifteen degrees of longitude to the east or west.Those fifteen degrees of longitude mark a distance traveled.

　　At the equator, where the Earth is widest, fifteen degrees stretch about one thousand six hundred kilometers.North or south of that line, however, the distance value of each degree decreases.One degree of longitude equals four minutes of time all around the world.But in measuring distance, one degree shrinks from about one hundred and nine kilometers at the equator to nothing at the north and south poles.

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