1. The problem statement, all variables and given/known data Assume two stars are in circular orbit around a mutual center of mass and are separated by d = a. Assume angle of inclination is i and stellar radii are r1 and r2. Find an expression for the smallest angle of inclination that will just barely produce an eclipse. 2. Relevant equations None that I know of. Question asking for you to invent one. 3. The attempt at a solution I drew a diagram and used trigonometry to find acos(i) = ??? I can't find either of the legs of the triangle because those legs will change depending on the position of the star in back, or the size of the star in back (the star having its light blocked for the eclipse), but what variables could I set that equation equal to so that the size of the star in back is accounted for? Also, the size of the star in front matters too, so we must assume that the star in back is smaller or of equal size to the star in front, correct? Or can an eclipse occur with something bigger in back? The solution manual sets the equation equal to r1+r2 but I don't understand the reasoning behind that. That would imply the adjacent leg of the triangle is equal to r1+r2, which I don't understand at all. Thanks.