What shape did Isaac Newton predict for the Earth?

Isaac Newton predicted that Earth is an oblate spheroid: slightly flattened at the poles and bulging at the equator.

Newton’s reasoning came from Earth’s rotation and his theory of gravitation. A rotating, self-gravitating body behaves differently at the equator than at the poles. Centrifugal effects are greatest around the equator, encouraging a small outward bulge, while the polar regions become compressed. Newton developed this argument in the Principia, published in 1687.

He modeled Earth as a rotating body with approximately uniform density and calculated a polar-to-equatorial axis ratio close to 689:692. That prediction was remarkably close in shape, although his assumed interior structure was too simple. Modern measurements give Earth a flattening of roughly one part in 298.

An oblate spheroid is not the same as a perfect sphere or a generic ellipsoid. An ellipsoid is the broader mathematical category; an oblate spheroid is the specific ellipsoid produced by flattening a sphere along its rotation axis. Earth’s irregular gravitational surface is more precisely described by a geoid.