Earth Equatorial Radius
| Value | 6,378,137 m |
| Status | Exact by definition — no uncertainty |
| Source | WGS 84 defining parameter |
| Categories | Astronomicalearthgeodesy |
| femtometer | 6.3781370e+21 fm |
| picometer | 6.3781370e+18 pm |
| nanometer | 6.3781370e+15 nm |
| micrometer | 6,378,137,000,000 μm |
| millimeter | 6,378,137,000 mm |
| centimeter | 637,813,700 cm |
| decimeter | 63,781,370 dm |
| meter | 6,378,137 m |
| kilometer | 6,378.137 km |
| inch | 251,107,760 in |
| foot | 20,925,646 ft |
| yard | 6,975,215.4 yd |
| mile | 3,963.1906 mi |
| nautical mile | 3,443.9185 nmi |
| astronomical unit | 0.000042635212 AU |
| light-year | 6.7416961e-10 ly |
| parsec | 2.0670134e-10 pc |
Learning zone
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.