We know that each ellipsoid has 3 diameters named $2a$, $2b$, and $2c$.
The Earth and all planets, in general, are ellipsoids (Saturn is the best example because it's the most oblate planet in the solar system).
But all we have read and heard are two types of diameters: equatorial diameter, and polar diameter.
So where's the third one?
I mean, the equatorial diameter itself should differ. One is 12756 km in the case of Earth. But what about the equatorial diameter that is perpendicular to the other equatorial diameter?