Mathfags of /sci/ I seek your expertise. A theoretical light source creates a perfect sphere of light of which we explore the solid angle ? subtended from the center of the sphere with a radius of r by a portion of the sphere's surface having an area of r2, into which light projects. What does ? equal to in a two-dimensional world?

I get an answer of 0.5rad × 180/? = 28.648°

Is this correct?