medelradie

Medelradie, or mean radius, is a size descriptor used for three-dimensional objects, particularly in astronomy and planetary science. It provides a single characteristic radius for bodies that are not perfect spheres, allowing easy comparison of their overall size.

The most common definition defines the mean radius as the radius of a sphere with the same volume as the object. If the object has volume V, the volume-equivalent mean radius is r = (3V/4π)^(1/3). For an ellipsoid with semi-axes a, b, and c, the volume equals (4/3)πabc, yielding a volume-equivalent mean radius r = (abc)^(1/3). For oblate spheroids with equatorial radius a and polar radius c, this becomes r = (a^2c)^(1/3). This radius is widely used in catalogs and analyses because it reduces complex shapes to a single, comparable quantity.

Other radii definitions are sometimes used in specific contexts. A surface-area-equivalent radius r_area = sqrt(A/4π) equates the sphere's surface area to that of the object. In some cases, a simple arithmetic mean of principal radii is used, though this is less standard and can be misleading for highly irregular bodies.

Examples include the Earth, with a mean radius of about 6,371 kilometers, and other planets or asteroids where the volume-equivalent mean radius is reported when detailed shape models are available. Medelradie thus serves as a practical shorthand for comparing size across diverse celestial bodies.

See also: radius, volume, surface area, ellipsoid.