The compression of a circle along a diameter to form an ellipse or ellipsoid of revolution is described by which term?

• A. First eccentricity
• B. Second eccentricity
• C. Angular eccentricity
• D. Flattening

Answer: (D)

Flattening describes how much a circle is squashed along one axis when it becomes an ellipse or an ellipsoid of revolution. It’s quantified as f = (a − b)/a, where a is the equatorial (major) radius and b is the polar (minor) radius. When a equals b, the shape is a perfect circle (or sphere), so f is zero; as you compress along one diameter to reduce b, the value of f increases and the shape becomes oblate. Eccentricities measure how elongated the ellipse is, but they describe the distribution of shape rather than the explicit amount of axis shortening; they’re related to flattening but not the direct measure of compression itself.