Engaging Rings

By Bob Berman|Thursday, July 01, 1993
While it may mean nothing to a mathematician, 0 is a favorite shape for summer skygazers. From the gaseous Ring nebula now nearly overhead in the constellation Lyra, to beautiful Saturn rising in the southeast, rings are a recurring celestial theme. Closer to home, Earth’s own sky has some spectacular specimens, too: the bright and beautiful halos that often encircle the moon and sun.

The standard halo has an imposing 22-degree radius, closely matching the part of the sky encompassed by an outstretched hand with thumb and little finger extended. A chorus line of 88 moons would be needed to span the halo’s full width. Sometimes an even larger but dimmer ring with a 46-degree radius appears, one of nature’s most stunning apparitions.

Solar and lunar halos appear against thin cirrus clouds several times each month from most locations, making them as common as onion rings. Red always appears on the inside of the arc, followed by orange, yellow, and then a whitish band tinged by violet at the outer edge. Countless microscopic hexagonal ice crystals in the clouds act like prisms, splitting sunlight or moonlight into colors. Some light simply reflects off the crystals, creating the white part of the halo. Lunar halos, however, often look white simply because of the eye’s insensitivity to color in dim light.

Halos, according to folklore, are harbingers of bad weather. This traditional wisdom has some meteorological basis. Although halos don’t signal a definite change in the weather, the accompanying cirrus clouds are a sign of air movement high in the atmosphere. If they form rapidly, the high, icy clouds often herald the approach of a warm front, and they are followed some hours later by lowering and thickening cloud layers.

If the sun or moon is encircled by a disk with red on the outside and whose interior is glowing, then it’s not a halo at all; it’s a corona, created by tiny water or ice droplets in stratus or cumulus clouds a little lower in the atmosphere. Coronas can sometimes be seen around Venus and even the bright star Sirius. While always smaller than halos, coronas can display a wide range of sizes, depending on the size of the clouds’ droplets.

If the glow is fuzzy and poorly defined, with a smearing of pale blue, yellow, or brown, it’s an aureole. Aureoles are most pronounced during the brightest lunar phases, which in 1993 means the first and last weeks of July.

Coronas and aureoles are frequent and lovely, but unlike halos they have no talent for predicting bad weather. By the time they appear, the lower clouds have already arrived. Still, the show’s not over even if the clouds reach the ground, for on foggy nights the diffraction process allows halos to materialize around streetlights. Interestingly, eye irritation produces the same effect, as anyone who has just come from a chlorinated pool can attest. More ominously, colored halos can also be produced by serious eye problems such as glaucoma.

So besides providing an optics demonstration, an aesthetically lovely apparition, and a weather forecast, a sky ring offers a good news, bad news situation: you’re either going blind or enjoying a spectacle of nature. A quick test is to ask companions whether they see the ring, too.

A yes is always reassuring.
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