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The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. The symbol is sr.
The steradian is defined as "the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r2." Since the surface area of this sphere is 4πr2, then the definition implies that a sphere measures 4π steradians. A steradian can also be called a squared radian.
A steradian is also equal to the spherical area of a polygon having an angle excess of 1 radian, to 1/4π of a complete sphere, or to 57.2957795 degrees squared.
The steradian was formerly an SI supplementary unit, but this category was abolished from the SI system in 1995.
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