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How does Earth’s rotation contribute to the shape of the geoid?

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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.

The geoid is an equipotential surface that represents the figure of Earth, coinciding with mean sea level over the oceans. Its shape is defined by a potential function that accounts for the combined effects of Earth’s gravitational attraction and the centrifugal repulsion resulting from the planet’s rotation.12

Earth’s rotation generates a centrifugal force that acts on the planet’s surface. This force contributes to the slight bulging of Earth at the equator, causing the planet to approximate the shape of an oblate spheroid—a flattened sphere. Because the geoid is everywhere perpendicular to the local pull of gravity, this rotational effect is inherently integrated into its surface geometry.312

While the geoid is often modeled as a reference ellipsoid for simplicity in geodetic calculations, the actual geoid is irregular due to variations in Earth's internal mass distribution. Consequently, the geoid represents a more complex, physically accurate surface than a perfect geometric ellipsoid.14