“0” may mean “nothing” to a mathematician, but it’s a favorite shape for September sky-gazers. From the gaseous Ring Nebula now overhead to the beautiful Saturn system rising in the southeast at nightfall, rings are a recurring celestial theme. Yet most nature-lovers are oblivious to the most spectacular examples: the bright and beautiful halos that often encircle the Moon and Sun.
They may simply be too big to catch our attention. The standard halo has an imposing 22-degree radius, closely matching an outstretched hand with thumb and little finger extended. A chorus line of 88 moons would be needed to span its breadth.
Halos appear against thin cirrus clouds several times each month, making them more common than onion rings. Red always forms the inside of the arc, followed by orange, yellow and then a whitish band. Green and blue are rare. The colors are produced by prismlike refraction, while white halos are reflection phenomena. The reliable thumb-to-pinky radius is a result of the angle at which light enters and leaves the cloud’s countless tiny six-sided ice crystals.
Halos are the poor person’s Weather Channel: a time-honored harbinger of bad weather. This ancient folklore has a sound meteorological basis, for the high icy clouds spawning halos often signal the approach of a warm front, with lowering and thickening layers a good bet a few hours later.
If Sun or Moon is encircled by a much smaller disk with red on the outside and whose interior is glowing, then it’s not a halo at all; it’s a corona – a different animal. Caused by an entirely separate optical effect, a corona’s colored rings are the interference patterns of lightwaves on water droplets or ice. While always smaller than halos, coronas can display a wide range of dimensions depending on the size of the cloud’s droplets. Just as quiet people are often the most profound, smaller drops produce bigger rings.
If the glow is fuzzy and poorly defined, with a smearing of pale blue, yellow or browns, it’s an aureole – not the cookie, despite the possible chocolate resemblance. An aureole’s diffraction-based alchemy has the ability to mix colors, cooking up pastel hues not seen in either halos or rainbows. These are most pronounced during the brightest lunar phases of fat gibbous and full, which will occur at midmonth.
Coronas and aureoles are frequent and lovely, but unlike halos, they have no talent for predicting bad weather. When they appear, the lower clouds are already here. Still, the show’s not over even if the clouds reach the ground, for on foggy nights the diffraction process allows colored rings to materialize around streetlights.
Eye irritation produces the same effect, as anyone who has just come from a chlorinated pool can attest. Such rings around lights, caused by ocular edema or inconsistencies in the cornea and lens, clear up spontaneously. But, more ominously, colored halos can also be produced by serious eye problems such as glaucoma.
How to tell if the colored ring around Moon or streetlight is really there, or merely an artifact of your own eye? Easy! Block out the light source with an outstretched thumb. If the halo vanishes, your eye manufactured it. If it remains, it’s for real. So, besides providing an optics demonstration of refraction, reflection and diffraction, an aesthetically lovely apparition and a weather forecast, sky-rings offer 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 it too. A yes is always reassuring.
Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.