Oblate spheroid

An oblate spheroid is a three-dimensional surface formed by rotating an ellipse around its shorter axis. It is flattened at the poles and bulges at the equator.

Ellipse: A plane curve whose points have a constant sum of distances from two fixed foci. Rotating an ellipse around its shorter axis generates an oblate spheroid.
Geodetic datum: A reference system that defines coordinates and measurements for mapping locations on Earth. Earth’s oblate model underlies widely used coordinate reference systems.
Earth: The third planet from the Sun and the only known world with life. Earth’s rotation makes its equatorial radius larger than its polar radius.
Sphere: A three-dimensional surface whose points are all equally distant from its center. A sphere has no polar flattening or equatorial bulge.
Axis of rotation: A line about which a body or geometric figure rotates. The ellipse’s shorter axis becomes the spheroid’s polar axis.
Ellipsoid: A closed quadric surface whose planar cross-sections are ellipses or circles. An oblate spheroid is the rotationally symmetric special case of an ellipsoid.
Reference ellipsoid: A mathematically defined ellipsoid used to approximate a planet’s size and shape. Geodesy represents Earth with an oblate reference ellipsoid.
Prolate spheroid: A spheroid elongated along its axis of rotation and narrowed around its equator. It is flattened at the equator, opposite to an oblate spheroid.
Eccentricity: A dimensionless measure of how much a conic section departs from circularity. It quantifies the flattening of the generating ellipse.
Geodesic: A locally shortest path between points on a curved surface. On an oblate spheroid, geodesics differ from great circles on a sphere.

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