Drakkith said:
I'm saying that a satellite in a circular orbit experiences virtually no difference in gravitational pull anywhere in its orbit, because it is the same distance from the center of the Earth at all times. This is in contrast to someone at the equator or at the pole, both of which are on the surface and thus have to be at different distances from the center of the Earth.
Note that while it is true that over the equator there is more material closer to the satellite, it ignores that the bulge on the opposite side of the Earth is further away from the satellite than it would be if the Earth was a perfect sphere. This means that the distant bulge attracts the satellite with less force. The net effect of the bulges is that they cancel each other out as far as I know.
This cancellation effect only works for entire spherical shells: When you calculate the integral that gives you the gravitational attraction from any point outside of a spherically symmetric shell, it works out to exactly the same attraction as though all of the mass of the shell were concentrated at its center point. (Incidentally, from any point inside the spherical shell, the gravitational attraction to the entire shell cancels out to exactly zero.) Part of the balancing that makes this work is that, while the inverse square law means that mass close by is much more important than mass that is far away, the shape of a spherical shell means that from a point fairly close to its surface, the amount of mass that is far away is much greater than the amount of mass that is nearby, and the relative importance of the one effect happens to perfectly balance out the relative importance of the other effect.
If you have just two masses, however, similarly balanced in distance from the center, but not part of an entire spherical shell, the cancellation no longer works. Consider three masses, A, B, and C, all of the same mass, such that any pair of them experience a mutual attraction of one Newton at a distance of one kilometer. (This works out to masses of about 122.4 million kilograms each.) If B and C are both at a center point, and A is two kilometers away, then A will experience an attractive force of 1/4 + 1/4 = 0.5 Newtons in the direction of B and C. Now move B one kilometer closer to A (new distance: one kilometer) and C one kilometer further from A (new distance: three kilometers). The attraction that A now feels in the direction of B and C is 1 + 1/9 = 1.111… Newtons—more than twice the attraction that it felt when B and C were together! The inverse square law wins out because the amount of mass far away (at C) is not more than the amount of mass close in (at B).
The wonderful cancellation that happens in the spherical shell integral, that allows us to greatly simplify orbital calculations when we make the assumption of spherically symmetric bodies, becomes much more complicated and difficult once you work with more realistic geometries. In the case of an approximate oblate spheroid like the Earth, the net effect works out so that if two satellites are the same distance from the center of the Earth, but one of them is above the equator and the other is above a pole, the one above the equator feels a slightly stronger attraction—and a third satellite at a latitude somewhere in between will feel a net attraction, not to the exact center point of the Earth, but to a point somewhat closer to the near side of the equator.