Autumn Acorn

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

Colors Around the Moon

If you ask people who are generally observant of nature whether they have ever seen lovely colors in clouds passing by the Moon, some of them will say yes. But how many people could name those colors and correctly recall the arrangement of the hues? Very few people know the facts about lunar coronas.

These beautiful patterns of color must be distinguished from "halos," rings of light which are far larger and which are caused only by ice crystals in cirrus clouds. The most famous halo is the huge "ring around the Moon." A lunar corona occurs in a disk-shaped area right up close to the Moon. Closest to the Moon is a greenish or bluish color, bounded on the outside by a thick band of reddish light. This is the first or innermost set of colors in a lunar corona, but sometimes there are fainter repetitions of the sets of colors outside of the first set (blue or green then red, blue or green then red, and so on.) Lunar coronas can occur in almost any cloud which is not so opaque as to hide the Moon altogether, but most of them are caused by clouds with water droplets, only some of them by high clouds with ice needles. And, instead of being caused by reflection and refraction (bending) of light by ice crystals (as is the case with halos), coronas are caused by a process called "diffraction." Diffraction occurs when a tiny aperture (like a pinhole) or particle (like a cloud droplet or ice needle edge) is similar in size to the wavelengths of light. Different colors of light are produced by different wavelengths, and (to simplify) cloud particles can block light from certain positions around the light source and intensify others. (Interestingly, the colors we sometimes see on roads after a rain are formed by the diffraction of light from incredibly thin deposits of oil on the road surfaces.)

Sometimes clouds are too scattered or too far from the Moon to reveal the concentric bands of a fully formed corona, and we just see patches of color here and there on the clouds as they pass the Moon. We call these "iridescent clouds":

As October gives way to November, the weather in much of the U.S. becomes much cloudier. Many locations have their cloudiest conditions of the year in November and December. But one consolation for sky-watchers is the appearance of numerous lunar coronas. The bigger the corona, the smaller the droplets or ice needles causing it. The more uniform the size of the droplets or needles (all else being equal), the more intense and pure the colors. But the pastels of coronas are beautiful. So is the changeability of coronas—and the way it often seems to be the Moon itself that is moving, wading through a sea of surrounding color.

RSS | Comments | E-mail | Print