Geodesy is the science of accurately measuring and understanding the
             Earth's geometric shape, orientation in space, and gravity field.
             Pythagoras (580-490 BC) is credited with postulating that the Earth
             was spherical, and Eratosthenes (276-194 BC) estimated Earth's 
             circumference at 25000 miles. Today the Earth's shape is known to
             result from the competition of gravity, rotation, tides, plate
             tectonics, volcanism, fluid motion (ocean and atmosphere), and other
             forces. Rotation and gravity are themselves also variable.  All factors
             are measured repeatedly and modeled for both historical correlation and
             prediction.