You've seen pictures of the planet Saturn, surrounded by its system of rings, but would you be surprised if you looked up at night to see a ring around the moon?
In fact, it's sometimes possible to see just this. There's even a folk rhyme about it: When there's a ring around the moon, rain or snow is coming soon. Is there really, sometimes, a ring around the moon?
An Optical Illusion
Unlike Saturn, which has actual, physical rings, the ring you can sometimes see around the moon is merely an optical illusion.
It's an effect of our own atmosphere that meteorologists call a "halo effect," because diffracted light rays create a halo around a bright object.
How It Works
Moon halos are caused by tiny ice crystals that have gathered twenty thousand feet above the ground, as thin, wispy clouds. These clouds are so thin, you might not notice them at night, if it weren't for their effect on the moonlight. Incoming light rays from the moon are bent, or diffracted, by these ice crystals at an angle of 22 degrees.
This means that in addition to the direct moonlight, you will also see diffracted moonlight in a circle 22 degrees away from the moon. This is about the distance of your fist, held at arm's length.
Like a rainbow, this halo can even be slightly colored; red on the inside, and blue on the outside.
Yes, it can mean that rain or snow is coming soon. Those high, wispy clouds could be the forerunners of storm clouds right behind them.