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     The length of a degree of latitude is virtually the same where ever you happen to be (68.703 miles at the equator, 69.407 at the poles.  Longitude, however, is only the same for a single degree of latitude. At the equator a degree of longitude is 69.172 miles, at 40, 53 miles, and at the poles, 0 miles.
     Longitude begins from a 0 or prime meridian and is counted 180 east and west. The longitude of a location is determined by knowing the difference between local time and the time at the prime meridian. Ptolemy set his prime meridian at the Canary and Medeira Islands. Later map makers used the Azores, the Cape Verde Islands, Rome, Copenhagen, Jerusalem, St. Petersberg, Pisa, Paris, Philadelphia and London among others. In 1884 an international committee agreed that Greenwich, England would be the universal prime meridian.

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     The stamps were issued in 1984 to celebrate the centennial of the establishment of the universal prime meridian at Greenwich, England.

   Each degree of longitude is equivalent to 4 minutes of clock time. An accurate method of keeping time is important because an error or inaccuracy of 1 degree can mean an error of 60 nautical miles. Prior to the eighteenth century the sand glass or hour glass was used to keep time after local noon had been established. At night the time was determined by the use of the nocturnal. The  North Star is sighted through a hole, and the long arm of the device is turned until it lies along the line made by the two brightest stars in the Big Dipper.

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