To learn one’s longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude – at that very same moment. The two clock times enable the navigator to convert the hour difference into a geographical separation. Since the Earth takes twenty-four hours to complete one full revolution of three hundred sixty degrees, one hour marks one twenty-fourth of a spin, or fifteen degrees.
-Dava Sobel
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