22° halo

I was going to write about Martin Rees winning the Templeton Prize, but he was trumped in interested-ness by an observed optical phenomenon.

Just before midday there was a full 22° halo around the sun. There was a lot of high cloud, which is formed of ice crystals. So the halo forms as the sunlight is refracted in what are in effect hexagonal prisms. The angle of minimum deviation is almost 22°, a bit less for the red end of the spectrum, and a bit more for the blue. I took a photo of part of the halo. It isn't brilliant as I didn't want to point the camera at the sun and risk frying it. But you can just about see that the sky is darker inside the halo, and that the inner edge is reddish and the outer edge is bluish.