Why does the Earth assume the shape of an oblate ellipsoid?

The Earth is not a perfect sphere but an oblate ellipsoid (or oblate spheroid) due to its rotation on its axis [4]. As the Earth rotates, it generates a centrifugal force that is strongest at the equator and decreases toward the poles [2]. This outward force causes the Earth's mass to push away from the axis of rotation, resulting in an equatorial bulge where the equatorial radius is approximately 21 to 43 kilometers larger than the polar radius [1]. Consequently, the planet appears slightly flattened at the poles [3]. While the Earth's revolution around the Sun is responsible for seasonal changes, it does not dictate the planet's physical shape; rather, the long-term effects of rotation and the resulting centrifugal force are the primary drivers of its oblateness [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > The Shape of The Earth and Latitudinal Heat Zones > p. 241
2. [4] https://en.wikipedia.org/wiki/Equatorial_bulge
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 7: Tectonics > Forces Behind The Drifting Of Continents (According to Wegener) > p. 95
4. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 3: Interior of the Earth > Force for Drifting > p. 28