Let the center of a sphere be O and the radius be r. We consider an area element dA on its surface. If the points on the boundary of this area element are joined with the center O of the sphere, then the lines so drawn subtend a solid angle dΩ at the center O.

Since the spherical area dA is directly proportional to the square of the radius r^{2}, the conclusion dA/r^{2} is a constant. This result is called the subtend solid angle dΩ by the area dA at the center O of the sphere.