"Earth radius" is the distance from the Earth's center to its surface, about 6,371 km. This length is also used as a unit of distance, especially in astronomy and geology, where it is usually denoted by R_\oplus . 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 . Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 km. 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. 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%. Earth's rotation, internal density variations, and external tidal forces cause it to deviate systematically from a perfect sphere. 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. Wiz Science™ is "the" learning channel for children and all ages. SUBSCRIBE TODAY Disclaimer: This video is for your information only. The author or publisher does not guarantee the accuracy of the content presented in this video. USE AT YOUR OWN RISK. Background Music: "The Place Inside" by Silent Partner (royalty-free) from YouTube Audio Library. This video uses material/images from https://en.wikipedia.org/wiki/Earth+radius, which is released under Creative Commons Attribution-Share-Alike License 3.0 http://creativecommons.org/licenses/by-sa/3.0/ . This video is licensed under Creative Commons Attribution-Share-Alike License 3.0 http://creativecommons.org/licenses/by-sa/3.0/ . To reuse/adapt the content in your own work, you must comply with the license terms.

• published: 02 Sep 2015
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