A solid angle
The solid angle is very similar to the plain angle with the only difference being that it is three-dimensional rather than just two-dimensional. It can be described as the ratio of the area of the surface of the cone over the radius of the sphere.
Imagine a sphere with a radius of 1 metre. A cone that covers an area of 1m² on the surface of the sphere encloses a solid angle of 1 stearadian. A full sphere has a solid angle of 4Pi steradian.
A solid angle
A round object that appears under an angle of 57° subtends a solid angle of 1sr. In comparison, the sun covers a solid angle of only 0.000,06sr. This equates to a plane angle of 0.5°.