Your Own Private Halo

Lunar halo at midnight, with Jupiter on the left side

If you happened to be wandering around western Montana this morning, in the wee hours after midnight, you probably noticed a glowing halo around the 99% full moon. With Jupiter riding shotgun at ten o'clock high, it made for an impressive sight. But what does it mean?

Lunar halos are optical illusions that occur when moonlight (reflected sunlight) passes through high, thin cirrus clouds floating in the night sky at more than 20,000 feet above your frozen nose. The light gets refracted (bent) and reflected (bounced) by millions of tiny, six-sided ice crystals that form these clouds. Refracted light is bent at a 22 degree angle, so the halo will invariably occur 22 degrees away from the light coming straight-line from the moon to your eyes. That's about the width of your fist when held at arm's length.

Because moonlight is so dim, compared to direct sunlight, lunar halos are usually colorless. But under optimum conditions you might see lunar halos with red on the inside and blue on the outside - the familiar rainbow color gradient.

Cirrus clouds often form the leading edge of a low pressure system - what we call a storm - and precede the storm by a day or two. So there really is a little science behind the adage, "Ring around the moon means rain soon." During last month's 100% full moon, sky watchers on the east coast saw a lunar halo the night before Hurricane Sandy made landfall. On the night of Sandy's landfall, people in the Midwest saw a lunar halo as the same low pressure system headed their way.

The hexagon-shaped ice crystals must be oriented just so, relative to your eyes, for the bent light to glance in your precise direction. That means the lunar halo you see is created from different ice crystals than the halo seen by the shivering person standing next to you. In other words, every lunar halo is unique to each viewer - your own private halo.

You can see more examples of cool, natural light effects here.