Why is Chimborazo farther from Earth's center than Everest?

Everest is the highest point above mean sea level, at 8,848 meters. Chimborazo in Ecuador is the point farthest from Earth's center. The reason is that Earth is not a perfect sphere. It bulges at the equator, and Chimborazo sits about one degree south of the equator, where the bulge is greatest. Its summit is roughly 2.1 kilometers farther from the center than Everest's summit. [1] [2]

Earth is an oblate spheroid, a shape slightly thicker at the equator than from pole to pole. The National Ocean Service attributes this to the centrifugal force of the planet's constant rotation. A reader comment on the Library of Congress blog makes the same point against the post's own wording, which had credited gravity: the commenter says rotation, not gravity, causes the flattening. The American Alpine Club article from 1968 also describes the earth as flattened by centrifugal force from its spin. [1] [3] [4]

The effect is substantial. Mean sea level at the poles is 6,356.8 kilometers from the center, while at the equator it is 6,378.1 kilometers. That puts sea level at the equator about 21 to 22 kilometers farther out than at the poles. [2] [5]

Everest lies at nearly 28 degrees north, where sea level is closer to the center. Chimborazo lies at about 1.5 degrees south, where sea level is about 4.7 kilometers farther out. Everest rises about 2.6 kilometers higher above sea level than Chimborazo, which is not enough to make up the difference. One calculation, which models the Earth's cross-section as an ellipse, finds Chimborazo's summit about 2.1 kilometers farther from the center. A commenter noted that latitude is conventionally defined relative to the normal to the geoid, which changes the formula slightly but leaves the conclusion unchanged. [5]

Wikipedia gives about 6,384.4 kilometers from the center for Chimborazo and 6,382.3 kilometers for Everest. The sources differ slightly on the exact gap: figures of 2,072 meters, 2,163 meters and 2,168 meters appear, depending on the source and method. All agree it is roughly 2 kilometers. [6] [1] [3] [2]

Chimborazo is not the only mountain that beats Everest on this measure. Huascarán in Peru is a very close second, only about 10 meters closer to the center, and at least 26 other summits are also farther out than Everest. In 1968 Terris Moore noted that the contest between Chimborazo and Huascarán was close and might best be settled by gravity observations on their summits. Which mountain counts as farthest depends on how sea level is modeled, so the ranking of the top two rests on small differences. The NOAA page itself calls the question of the highest point debatable, since the answer depends on whether height is measured from sea level or from Earth's center. [2] [4] [1]