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Concept
Earth radius
Summary
Earth radius (denoted as R🜨 or ) 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 (equatorial radius, denoted a) to a minimum of nearly (polar radius, denoted b).
A nominal Earth radius is sometimes used as a unit of measurement in astronomy and geophysics, which is recommended by the International Astronomical Union to be the equatorial value.
A globally-average value is usually considered to be with a 0.3% variability (±10 km) for the following reasons.
The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius (R_1) 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 (R_2); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R_3). All three values are about .
Other ways to define and measure the Earth's radius involve the radius of curvature. A few definitions yield values outside the range between the polar radius and equatorial radius because they include local or geoidal topography or because they depend on abstract geometrical considerations.
Figure of the EarthEarth ellipsoid and Reference ellipsoid
Earth's rotation, internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere. Local topography increases the variance, resulting in a surface of profound complexity. Our descriptions of Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate characteristics of Earth's surface, generally relying on the simplest model that suits the need.
Each of the models in common use involve some notion of the geometric radius. Strictly speaking, spheres are the only solids to have radii, but broader uses of the term radius are common in many fields, including those dealing with models of Earth.
Official source
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