# Solid angles

*Solid angle. Measure of a solid angle. Steradian.*

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A solid angle
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is a part of a space, concluded inside of one part of a conic surface with a closed directrix. A solid angle is measured by a
piece of spherical surface ( ABCDEF, Fig.98 ).

A measure of a solid angle is a ratio of an area ABCDEF to a square of radius of the ball:

A measure unit of a solid angle is a
*
steradian
*
– a solid angle, cutting out a spherical surface an area, equal to an area of square, a side of which
is equal to radius. This measure of solid angles is similar to the radian measure of plane angles.