# Earth bulge and axis precession

We know that earth is not spherical because it spins. Does every object that spins bulge? for example a rotating wheel bulges? Also how moon and sun create the torque that shifts the axis of rotation (precession) if gravitational force acts on center of mass therefore no torque?

Every object that spins must be accelerating. This means different forces (and usually stresses) from the same object at rest. Whether it bulges or not depends on the material and its response to that stress.

If you have a steel marble spinning at 1 revolution per minute, the rotational forces on it are tiny. Given the strength of the material, you probably would be unable to measure the deformation (or bulge) from the spin. As the object becomes larger, weaker, and faster, the deformation would increase.

...gravitational force acts on center of mass therefore no torque ?

Gravitational force acts across an entire mass, not just at the center. However, in some cases, the sum of these forces is identical to it acting only at the center. This is true when you have a uniform sphere or uniform spherical shell of material. So for things that are mostly spherical, the simplification that gravity acts only on the center of mass is reasonable, but perhaps not exact.

Because the earth deviates from a spherical shape, the sun and moon don't tug on it uniformly. The equatorial bulge gets pulled a bit differently and allows for a small torque that can cause precession. If the earth were exactly spherical, this torque would not exist.