Introduction to Earth Sciences I
1.3.2. The Earth Responds To The Moon
While it might seem incredible, the Earth as a whole responds to the pull of the Moon's gravity. The moon is, of course, responsible for the ocean tides (and we will study these a little later) but its gravitational pull is strong enough to distort the shape of the solid Earth also. The Earth's surface actually rises up a little when the moon is overhead. While the ocean tides have a range of several tens of feet in places and are commonly several feet, the surface of the solid earth moves only about 10 cm at most; much less in many places.
Obviously the moon is exerting the same pull on the oceans as it is on the solid earth, and the amount it yields is a measure of the strength of rock compared to water. In the same way, as we described for normal modes, we can calculate the yielding that would be associated with a fully solid Earth and one with a liquid core (i.e., a soft center). It is therefore possible to determine just how soft the center is by matching predictions and observations.
The distortion is described by so-called Love numbers after the same A.E.H. Love who studied normal modes. These Love numbers are a measure of the Earth's yield strength.