Which parameter is used to quantify Earth's flattening?

• A. Flattening
• B. Eccentricity
• C. Geoid height
• D. Polar radius

Answer: (A)

Flattening is the parameter used to quantify Earth's shape deviation from a sphere. It is defined as f = (a − b)/a, where a is the equatorial radius and b is the polar radius. Because Earth spins, the equator bulges, making a larger than b, so f is a small positive number (about 1/298). This single value directly describes how much the planet is flattened at the poles and is the primary descriptor for the reference ellipsoid used in geodesy. Eccentricity is related, since e^2 = 2f − f^2, but flattening is the specific measure of the flattening itself. Geoid height concerns gravity and sea-level irregularities, not the ellipsoid’s shape, and the polar radius alone doesn’t quantify flattening.