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Earth radius is the distance from the Earth's center to its surface, about . This length is also used as a unit of distance, especially in astronomy and geology, where it is usually denoted by . This article deals primarily with spherical and ellipsoidal models of the Earth. See Figure of the Earth for a more complete discussion of the models. The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947–3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of . While "radius" normally is a characteristic of perfect spheres, the term as used in this article more generally means the distance from some "center" of the Earth to a point on the surface or on an idealized surface that models the Earth. It can also mean some kind of average of such distances, or of the radius of a sphere whose curvature matches the curvature of the ellipsoidal model of the Earth at a given point. An early report on the circumference of the Earth was given by Aristotle at 400,000 stadia.〔Aristotle On the Heavens, Book II 298 B〕 The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC. Estimates of the accuracy of Eratosthenes’s measurement range from within 2% to within 15%. ==Introduction== (詳細はEarth's rotation, internal density variations, and external tidal forces cause it to deviate systematically from a perfect sphere.〔For details see Figure of the Earth, Geoid, and Earth tide.〕 Local topography increases the variance, resulting in a surface of unlimited complexity. Our descriptions of the Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate the Earth's surface, generally relying on the simplest model that suits the need. Each of the models in common use come with some notion of "radius". Strictly speaking, spheres are the only solids to have radii, but looser uses of the term "radius" are common in many fields, including those dealing with models of the Earth. Viewing models of the Earth from less to more approximate: * The real surface of the Earth; * The geoid, defined by mean sea level at each point on the real surface;〔There is no single center to the geoid; it varies according to local geodetic conditions.〕 * An ellipsoid: geocentric to model the entire Earth, or else geodetic for regional work;〔In a geocentric ellipsoid, the center of the ellipsoid coincides with some computed center of the earth, and best models the earth as a whole. Geodetic ellipsoids are better suited to regional idiosyncrasies of the geoid. A partial surface of an ellipsoid gets fitted to the region, in which case the center and orientation of the ellipsoid generally do not coincide with the earth's center of mass or axis of rotation.〕 * A sphere. In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called ''"a radius of the Earth"'' or ''"the radius of the Earth at that point"''.〔The value of the radius is completely dependent upon the latitude in the case of an ellipsoid model, and nearly so on the geoid.〕 It is also common to refer to any ''mean radius'' of a spherical model as ''"the radius of the earth"''. On the Earth's real surface, on the other hand, it is uncommon to refer to a "radius", since there is no practical need. Rather, elevation above or below sea level is useful. Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (≈3,950 – 3,963 mi). Hence, the Earth deviates from a perfect sphere by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earth radius」の詳細全文を読む スポンサード リンク
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