Steradian

SI derived unit of solid angle

The steradian or square radian is the unit of solid angle in the International System of Units (SI). It is used in three dimensional geometry, and is analogous to the radian, which quantifies planar angles. A solid angle in steradians, projected onto a sphere, gives the area of a spherical cap on the surface, whereas an angle in radians, projected onto a circle, gives a length of a circular arc on the circumference. The name is derived from the Greek στερεός stereos 'solid' + radian.

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