Historically, the Earth is best modeled as which of the following shapes?

• A. Oblate spheroid
• B. Prolate spheroid
• C. Spherical
• D. Ellipsoid with random flattening

Answer: (A)

Rotation causes Earth to bulge at the equator and flatten at the poles, so it is best described as an oblate spheroid. An oblate spheroid has a larger equatorial radius than polar radius, which matches measurements (equator about 6378 km, pole about 6357 km, flattening ~1/298). This shape is why geodesy uses ellipsoids that mirror this oblateness for maps and GPS. A prolate spheroid would bulge at the poles, which isn’t observed, a perfect sphere ignores the measured flattening, and an ellipsoid with “random flattening” isn’t a well-defined reference shape.