Nice explanations of the mathematics of rainbows. A "Round" Rainbow will always be observed when the sun is directly behind you and the water droplets are directly in front of you and you are at an altitude that enables you to see the circle of light. I'm not entirely sure if the distance you are from the water droplets determines the size of the circle, I'll have to investigate more. The "circle" is a "Halo" effect similar to the effect you get on a foggy night when the moon is at its fullest.