A Lecture on Solid Angle, by Ben Kravitz
"In order to talk about solid angles, we fi rst need to talk about projections. There are lots of diff erent kinds of projections, some of which you already know. We should begin with a very simple defi nition of what a projection is, and then we'll get more complex as we go along. A projection is the transformation of points and lines in one plane onto another plane by connecting corresponding points on the two planes with parallel lines".