Rotating heavenly bodies bulge at their equators

Although all stars and planets are round, they are not necessarily perfect spheres. The surface of a rocky planet can be uneven, and when stars or planets rotate, they bulge slightly at their equators. Earth’s rotation means that it is slightly flat, as the distance from Earth’s centre to the Equator is 6,378 km, whereas the distance to the poles is only 6,357 km. The most extreme example of this phenomenon is the fast-rotating star Vega, which is 23 % wider than it is "tall".