Planetary radius

A planetary radius is the distance from a planet’s center to its surface or a specified reference level. Because planets are not perfect spheres, the value depends on latitude and on which surface or reference level is used.

Reference ellipsoid: A mathematical ellipsoid used to approximate a planetary body's shape and define coordinates or elevations. Its equatorial and polar dimensions provide standard reference radii for an oblate planet.
Bulk density: Mass divided by volume for an entire body, including its interior. Combining radius with mass gives the density used to infer a planet's composition.
Equatorial radius: The distance from a body's center to its surface at the equator. It is usually the largest conventional radius for an oblate planet.
Radius: A line segment from the center of a circle or sphere to its boundary, or its length. Planetary radius adapts this geometric distance to an irregular, rotating body.
Oblate spheroid: A spheroid flattened at the poles and widened at the equator. Rotation makes many planets' equatorial radius larger than their polar radius.
Planetary mass: The total amount of matter in a planet, commonly inferred from its gravitational effects. Mass and radius together determine bulk density and surface gravity.
Polar radius: The distance from a body's center to its surface along its rotation axis. It is smaller than the equatorial radius when a planet is flattened.
Diameter: The length of a straight line through a body's center joining opposite points on its boundary. A diameter spans the full body and equals twice a spherical radius.
Geoid: An equipotential surface of Earth's gravity field that closely approximates mean sea level. For Earth, this irregular reference surface differs from a simple ellipsoid.
Surface gravity: The gravitational acceleration experienced at a body's surface or specified reference level. A planet's radius helps set surface gravity for a given mass.

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