Solid angle (in spherical coordinates)

Description

In geometry, a solid angle 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 . Any area on a sphere, totaling the square of its radius and observed from its center, subtends precisely one steradian.