Why does the Earth have an oblate shape instead of a perfect sphere?

Earth is not a perfect sphere because it spins on its axis about once every 24 hours. Rotation creates a small outward effect that is greatest at the equator, where the surface moves fastest, and weakest at the poles. Over billions of years, that effect helped Earth settle into a slightly flattened, oblate shape. The equatorial diameter is about 12,756 km, while the polar diameter is about 12,714 km, a difference of roughly 43 km. This shape is one reason scientists distinguish between mean sea level and a more precise reference surface called the geoid.