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The Earth is Not an Oblate Spheroid

Cosmically speaking, we're practically a perfect sphere.

(Neil deGrasse Tyson)

The Earth is a sphere. Well, most of us anyway. What we don't all know is that the Earth isn't a perfect sphere: it's slightly fatter at the equator because it's spinning. (This makes sense because of centrifugal force.) A sphere-like shape which is slightly flattened is called an oblate spheroid, so everyone who calls the Earth a sphere is wrong, yeah?

Well, the diameter at the equator is only about 43 kilometers fatter than at the poles. The Earth is 12,742 kilometers across. This means Earth is a sphere to within 99%. In fact, if the Earth was smaller, it would be round enough to be a legal regulation billiard ball. If we're going to start calling the Earth an oblate spheroid, and calling everyone wrong who calls it a sphere, then most of the spheres in our daily lives are going to cease to be spheres. Calling the Earth an oblate spheroid isn't technically wrong, and it may be useful to do so in some contexts. But it's truly neither a sphere nor an oblate spheroid: it's an Earth-shape, which is only approximated by a sphere or an oblate spheroid. Given that everyone knows what a sphere is, and nobody knows what an oblate spheroid is, it's unnecessarily pedantic to correct someone who claims the Earth is a sphere.

Even worse, Scientific American have suggested that not only is the Earth not a sphere, it isn't even round. Oblate spheroids are round shapes; they're referring to another fact about the Earth: it's surface isn't perfectly smooth.

It may seem round when viewed from space, but our planet is actually a bumpy spheroid.

Wow! The Earth isn't perfectly flat? This is important news!