Empirical Earth · Gravity, Buoyancy & the Air

Earth is an oblate spheroid — the equatorial bulge

The claim

“They keep changing the shape — sphere, spheroid, geoid — so it’s made up.”

What is measured

Equatorial radius 6,378 km, polar radius 6,357 km. That is a ~21 km bulge, a flattening of about 1/298 (~0.3%). Newton predicted it from spin in 1687, and the French Geodesic Missions confirmed it by the 1740s.

What would show this is wrong

a degree of latitude that is the same length at the pole and the equator; an Earth with no measurable equatorial bulge; or satellite orbits that show no oblateness perturbation.

Sources

  1. The equatorial bulge, the numbers & Chimborazo. Equatorial radius 6,378.1 km vs polar 6,356.8 km (~21.4 km; flattening ~1/298); the bulge makes Chimborazo’s summit, not Everest’s, the point link
  2. Newton’s prediction & the French Geodesic Missions. Newton derived the equatorial bulge from rotation in the Principia (1687); the 1730s–40s missions to Lapland and equatorial Ecuador measur link
  3. The geoid and the “pear” term. The geoid departs from the reference ellipsoid by only about +85 m to −106 m (the Indian Ocean geoid low); the third-degree “pear” asymmetry, found by O’Keefe link
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