This circle has 22 degrees
It looks like the lunar halo I saw two nights ago isn't all that rare, and my 15° estimate was off by 32%. This whole ring around the moon bit makes me wonder what other kinds of phenomena go on out there that I haven't noticed because I've been too busy sitting in a classroom or in front of a CRT...

For further educational purposes, I didn't in fact see moon dogs as my Walt Whitman wannabe friend suggested (although the halo and paraselenae are related). It's a balmy 15°C outside right now, and that's because it's 19:00.