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Why is Earth flattened near the poles and wider at the Equator?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Earth is not a perfect sphere; it is an oblate spheroid, meaning it is flattened at the poles and bulges at the Equator. This shape is primarily caused by the rotation of Earth about its axis.123

As the planet spins, it generates a centrifugal force that acts outward from the axis of rotation. This force is strongest at the Equator and nonexistent at the poles. The outward push of this force causes the planet's mass to bulge at the Equator, resulting in an equatorial radius that is approximately 21 kilometers (13 miles) larger than the polar radius.2314

While the geoid serves as a more precise model for Earth's surface by accounting for irregular mass distributions and gravity anomalies, the oblate spheroid remains the standard geometric representation used in geodesy to simplify calculations.154