|MadSci Network: Earth Sciences|
The plumb line points to the center of gravity of the Earth. However, local geology and surface features can affect the angle of the plumb line. If you were to take a measurement next to a mountain, the plumb line would tilt slightly towards the mountain because the extra mass has a higher gravitational attraction.
The equatorial diameter of the Earth is greater than the polar diameter. The effect of this irregular shape is that the gravitational force on a body at the equator is slightly less than at the poles.
The Coriolis Force applies to moving objects at or near the surface of the Earth. As a plumb line is stationary, the Coriolis force is not a factor. See http://www.windpower.dk/tour/wres/coriolis.htm for a demonstration.
Richard T. De Van
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