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 true

The words do get used loosely. Sphere, spheroid and geoid are three different things and they are often swapped about carelessly.

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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