# explain steradian and solid angle

Solid angle is a three-dimensional analogue of an angle, such as that subtended by a cone or formed by planes meeting at a point. It is measured in steradians.

In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unit called a steradian (symbol: sr).

Any area on a sphere which is equal in area to the square of its radius, when observed from its center, subtends precisely one steradian.