Earth Polar Radius

The polar radius is not measured independently; it follows exactly from the two WGS 84 defining constants as b = a(1 − f), with the flattening f = 1/298.257223563. The 21.4 km difference from the equatorial radius is the centrifugal bulge, and it is why sea level, orbital mechanics and map projections all need an ellipsoid rather than a sphere. Anyone quoting a single "radius of the Earth" is quietly choosing an average.

The oblateness has consequences beyond geometry. Its gravitational signature, the J₂ term, torques satellite orbits into a slow precession; mission designers exploit this deliberately to fly sun-synchronous orbits whose planes rotate once a year, so an imaging satellite crosses every latitude at the same local solar time on every pass.