Earth radius
Earth radius (represented as RšØ or [math]\displaystyle{ R_E }[/math]) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly 6,378 km (3,963 mi) (equatorial radius, represented a) to a minimum of nearly 6,357 km (3,950 mi) (polar radius, represented b).
| Standard: | astronomy, geophysics |
| Quantity: | distance |
| Symbol: | RšØ |
| Expressed in: | 1 RšØ = |
| SI base unit | 6.3781Ć106 m[1] |
| Metric system | 6,357 to 6,378 km |
| English units | 3,950 to 3,963 mi |
Earth radius is sometimes used as a unit of measurement in astronomy and geophysics. The International Astronomical Union recommends that the radius at the equator should be used.
A globally-average value is usually believed to be 6,371 kilometres (3,959 mi) with a 0.3% variability (Ā±10 km) for the following reasons. The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius (R1) of three radii measured at two equator points and a pole; the authalic radius, which is the radius of a sphere with the same surface area (R2); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R3). All three values are about 6,371 kilometres (3,959 mi).
Other ways to define and measure the Earth radius involve the radius of curvature. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topography or because they depend on abstract geometrical that are carefully thought about.
Earth Radius Media
A scale diagram of the oblateness of the 2003 IERS reference ellipsoid, with north at the top. The light blue region is a circle.
Geoid Height Models
Al-Biruni's (973 ā c.ā1050) method for calculation of the Earth's radius simplified measuring the circumference compared to taking measurements from two locations distant from each other.
Three different radii as a function of Earth's latitude. R is the geocentric radius; M is the meridional radius of curvature; and N is the prime vertical radius of curvature.
The length PQ, called the prime vertical radius, is N(\phi). The length IQ is equal to \, e^2 N(\phi) . R = (X,\, Y,\, Z).
Equatorial (a), polar (b) and arithmetic mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale)
References
- ā Mamajek, E. E; Prsa, A; Torres, G; Harmanec, P; Asplund, M; Bennett, P. D; Capitaine, N; Christensen-Dalsgaard, J et al. (2015). "IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties". .