Superimposed on instrument drift is another temporally varying component of gravity. Unlike instrument drift, which results from the temporally varying characteristics of the gravimeter, this component represents real changes in the gravitational acceleration. Unfortunately, these are changes that do not relate to local geology, and are hence a form of noise in our observations.
Just as the gravitational attraction of the sun and the moon distorts the shape of the ocean surface, it also distorts the shape of the earth. Because rocks yield to external forces much less readily than water, the amount the earth distorts under these external forces is far less than the amount the oceans distort. The size of the ocean tides, the name given to the distortion of the ocean caused by the sun and moon, is measured in terms of meters. The size of the solid earth tide, the name given to the distortion of the earth caused by the sun and moon, is measured in terms of centimeters.
This distortion of the solid earth produces measurable changes in the gravitational acceleration because as the the shape of the earth changes, the distance of the gravimeter to the center of the earth changes (recall that gravitational accleration is proportional to one over distance squared). The distortion of the earth varies from location to location, but it can be large enough to produce variations in gravitational acceleration as large as 0.2 mgals. This effect would easily overwhelm the example gravity anomaly described previously.
**Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940.
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