Earth Equatorial Radius

Earth's spin makes it an oblate spheroid: the equator bulges 21 km beyond the poles, so a mountain in Ecuador can be farther from the planet's centre than Everest is. Newton predicted the flattening from rotation in the Principia; the French Geodesic Missions to Lapland and Peru in the 1730s measured the length of a degree of latitude at both ends of the world and confirmed him against Cassini's rival prolate model.

The value here is a defining constant of the World Geodetic System 1984, not a measurement: the ellipsoid is fixed by convention so that coordinates are reproducible, and everything else about it (the polar radius, the mean radius, the flattening ratio 1/298.257223563) is derived arithmetic. Every GPS fix on Earth is expressed relative to this surface.