/ic/ - Artwork/Critique

Ok i'm missing some real freaking basic concepts here and i'm mad and frustrated as fuck right now.
Look at the figure 1. Once drawed the position of the light source and the circle, how am i supposed to draw the ellipse inside the cirle that represents the section of the sphere perpendicoular to the light source?
Of course i could do it at eye, like the others 4000 shitty videos on youtube, but i want to learn foundamentals for christ's sake, why i can't find a rational geometric explanation of this in thousands of google images results?