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.
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.