360 degrees

Somewhere in US (Dec 12, 2005)

Have you ever seen a full circle rainbow? I am not talking about a rainbow that you can see reaching the ground on both sides of its arch, but literally a rainbow that closes on itself to form a perfectly circular 360 degrees bull’s eye figure. More, two of them concentric one in the other.

Well, it can happen if you are flying and the sun is at the right angle between your airplane and the clouds below. I saw it today while flying from Boston to Dallas, on my way to Tucson for an observing run at the MMT telescope. In fact, while we were still gaining altitude and we were passing through the clouds there was a fleeting moment, just a few seconds, in which the airplane shadow was clearly seen at the bull’s eye center. It was very impressive, but didn’t last enough for me to take a shot. This is what I got, though.

I think the scientific name for the phenomenon is “glory”, and I am certainly not the first one capturing it with a camera. It is often seen from the summit of mountains, forming on the clouds below. The physics behind it is non trivial, and wasn’t fully understood until the seventies. It involves complex calculations in scattering theory, in part because of reflections from the interface between air and water, and in part because of multiple (ten) internal reflections inside single drops of water. The light coming from the glory is intensely polarized, with a complex pattern. The fact that multiple rings are visible apparently indicates that the droplets of water in the clouds are of very uniform size.