To first approximation, the Earth is a sphere. To second approximation, the Earth is an oblate ellipsoid. What is its shape to third approximation? Is it still a surface of revolution?
EGM96 describes the shape of the Earth with spherical harmonics. What is the base shape: sphere or ellipsoid? If a sphere, does using just the first harmonic turn it into an oblate ellipsoid? I'm guessing not.
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