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n. (context geometry English) In the International System of Units, the derived unit of solid angle; the solid angle subtended at the centre of a sphere of radius ''r'' by a portion of the surface of the sphere that has area ''r''2. Symbol: sr


n. the unit of solid angle adopted under the Systeme International d'Unites [syn: sr]


The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three- dimensional geometry, and is analogous to the radian which quantifies planar angles. The name is derived from the Greek stereos for "solid" and the Latin radius for "ray, beam".

The steradian, like the radian, is a dimensionless unit, essentially because a solid angle is the ratio between the area subtended and the square of its distance from the vertex: both the numerator and denominator of this ratio have dimension length squared (i.e. L/L = 1, dimensionless). It is useful, however, to distinguish between dimensionless quantities of a different nature, so the symbol "sr" is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W┬Ěsr). The steradian was formerly an SI supplementary unit, but this category was abolished in 1995 and the steradian is now considered an SI derived unit.

Usage examples of "steradian".

A complete steradian sweep showed nothing the size of a spaceship out there.

But given that, and assuming that everything else goes linearly, the best guesstimate I can make says we get hit with everything emerging from a solid angle of one two-hundredth of a steradian of Alpha C.

That power radiated equally in all directions would be distributed over a sphere, which is divided into four pi units of solid angle called steradians.