In her book, “Longitude,” writer Dava Sobel tells the story about longitude and how the problem of knowing it was solved.
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STEVE EMBER: For centuries, the great scientists of the world struggled to develop a way to learn longitude. 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 twenty-four hours to complete one full turn or revolution of three-hundred-sixty degrees. One hour marks one twenty-fourth 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 stretches 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 nine kilometers at the equator to nothing at the north and south poles.
BOB DOUGHTY: For many centuries, navigators hoped they could find longitude by observing the movement of stars at night. During the day, the sun provided information about the time on a ship, and its direction. However, it did not provide necessary information about the time somewhere else.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25