Oblateness

Oblateness, or flattening, is the degree to which an object's shape deviates from a perfect sphere, typically due to rotation. In planetary science and astronomy, many bodies are well modeled as oblate spheroids, with an equatorial radius a larger than the polar radius b.

The flattening parameter f is defined as f = (a − b)/a. A value of 0 corresponds to a perfect sphere. For the Earth, a ≈ 6378.137 km and b ≈ 6356.752 km, giving f ≈ 0.00335 (about 1/298). Other planets vary: Jupiter's f is about 0.065, and Saturn's about 0.098, reflecting their faster rotation and internal structure.

Causes and consequences: Oblateness arises from centrifugal forces due to rotation, which push mass toward the equator. The degree of flattening depends on rotation rate and how mass is distributed inside the body. Oblateness influences the body's gravity field and orbital dynamics; it is often represented by a gravitational harmonic coefficient called J2.

Measurement and modeling: Oblateness is inferred from measurements of shape, gravity fields, and satellite orbits, using data from ground surveys, spacecraft tracking, and radar observations. Understanding oblateness aids geodesy, planetary formation studies, and the interpretation of rotation rates and internal structure.