The Earth's shape is a result of its spin, but why is it not a perfect sphere?

[semc]

okay i know everyone knows Earth is not a sphere but why is that so?has it got to do with the spin?
btw,i was wondering how did galileo found out that sun is not in Earth orbit but the other way round in the past?

Thanks

 

[Chi Meson]

Copernicus (before galileo) found that the retrograde motions of planets are easily explained if all planets, including earth, revolved areound the Sun. Galileo made observations of Venus that showed phases just like the Moon has. Simple analysis proved that these phases can only be due to revolution around the Sun, and not the Earth.

 

[arildno]

If we disregard the surface irregularities of the Earth that solid matter makes, the Earth is an oblate spheroid (a flattened sphere).
This shape is the natural one for an object revolving around its own axis.
Close to equator, the centrifugal effect works more strongly than close to the poles, thus when the centrifugal effects are balanced by the pull of gravity, the flattened shape is what you'll get.

With gravity alone and no revolution about any axis, you'd get a perfect sphere.