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Tuesday, March 1, 2011
Map Coordinates Latitude Longitude
Lines of latitude and longitude form a grid from which the position of any point on the Earth’s surface can be measured. An obvious reference point is the equator and latitude is the angle of a place north or south of it as measured from the center of the Earth. Lines of latitude form horizontal rings around the Earth parallel with the equator. They become shorter as they get closer to the poles. Lines of latitude divide up the Earth through its poles, like the segments of an orange. Each line is a great circle going right round the Earth. Lines of longitude have no obvious reference point such as the equator, so English navigators used the longitude or their home port, Greenwich, on the Thames near London. Longitude is still measured east or west of the Greenwich meridian. Early navigators used the stars to find out their position. Latitude was calculated from the heights of the stars in the sky or the position of the Sun as it rose, set or reached its zenith. Longitude is also measured against the stars but it is necessary to know the exact time because the stars move east to west across the sky. It was only after the perfection of the marine chronometer by John Harrison in the mid 18th century that accurate longitude measurements, and hence accurate map making, become possible.