A sign of the Apocalypse, or...

Today, walking to lunch I saw the weirdest thing.

There was a rainbow in a perfect circle around the sun. My question is what would cause this and what is it called?

It was a bright sunny day with whisps of clouds. Any answers?

It’s called a “glory”.

http://www.newscientist.com/lastword/answers/539seaside.jsp?tp=seaside1

http://www.polarization.com/rainbow/rainbow2.html#specter
http://www.badastronomy.com/bad/news/marilyn3.html

I’ve always heard the term “glory” used for a small ring of light around an observer’s shadow, when looking down at clouds. In other words, it’d be opposite of the Sun. What you describe would be a halo. They’re pretty common, but you were lucky to see one in a full circle: You usually only see an arc.