I am trying to figure out the measure of the angle labeled "?" in the figure. All the labeled variables besides "?" are known. I have been staring at this for the past 2 hours and haven't found anything leading to a solution. As this figure is derived from a separate problem (approximation of the geometric apparent radius of Earth's penumbra at a given distance -- ignore if this makes things confusing), I may have forgotten a known value somewhere though it seems like there is only one unique figure for a given set of variables. Any ideas?

a, b and "?" are angles

r and d are distances

Thanks.