On the solid angle subtended by a circular disc

The solid angle subtended by a circular disc from a point is approximated by the analytical expression for the solid angle for n-sided, regular polygon of area equal to that of the disc. When the point is farther from the plane of the disc than 0.1 the disc radius, polygons of 20 and 36 sides are required for approximating the solid angle subtended by the disc to within 1 and 0.1% respectively. In addition, the average solid angles subtended by circular discs from circular discs are calculated and given in tabular form for a range of disc sizes.