You have learned that Earth rotates 15 degrees every hour. This means that if you know the time difference between two locations, you can figure out the number of degrees of longitude that separate them. Furthermore, if you know the longitude of one of the places, then you can calculate the longitude of the other. This way of locating positions on Earth was very important to early navigators. In fact, the invention of an extremely accurate clock was a major breakthrough for navigating the open seas.
Imagine that you were traveling by boat from Le Havre, France (located on the prime meridian at a longitude of 0 degrees) to Miami, Florida. You traveled for several days, carrying a clock set to the time in Le Havre. Somewhere in the middle of the Atlantic Ocean, using the sun as a guide, you determine that it is 12:00 noon at your current position. Your clock shows that the time in Le Havre is 3:00 p.m.
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