Question

Find the solid angle for a semisphere a radius R.​

Answer 1

The angle of a full circle is defined via the circumference of a circle with radius r=1: Hence, 360∘=2π rad; accordingly, 1 rad=(180∘/π)≈57.3∘. ... Hence, the solid angle of the full sphere, e.g. the entire sky, is given by 4π sr. ...