Jimmy Westlake: Ring around the moon
Did you see the magnificent ring around the full moon this past Thursday evening? Colorful rings, or halos, around the moon are not particularly rare, but conditions must be just right to produce one, and Thursday night’s halo was unusually good.
The December full moon traditionally is known as the “Moon before Yule” or the “Moon of the Long Night,” because it occurs near the time of the winter solstice and the longest night of the year. If you thought it looked a little bigger and brighter than usual, you weren’t imagining it. This year’s “Moon of the Long Night” happened to occur when the moon was at its closest point to the Earth, called perigee. According to http://www.spaceweather.com, this month’s full moon was 14 percent larger and 30 percent brighter than other full moons in 2008. Perhaps it was the extra intensity with which it shone that contributed to the surprisingly vivid halo.
Lunar halos long have been considered reliable forecasters of an impending snowstorm, and there’s good reason for that. Thin, wispy cirrus clouds often precede approaching weather fronts. These feathery clouds are made of tiny, pencil-shaped, hexagonal ice crystals adrift in the high atmosphere. Just as sunlight is broken into a rainbow of colors by a prism dangling in a window, moonlight can be dispersed into a colorful ring by the natural little prisms in the cirrus clouds. The specific shape of the ice crystals causes the moonbeams to bend at a 22 degree angle, creating a ring that is always 44 degrees in diameter, centered on the moon. Usually, lunar halos appear colorless to the naked eye because they aren’t bright enough to trigger the eye’s color receptors. Similar halos sometimes are seen around the sun in the daytime sky and appear much more colorful because they are brighter. Conditions were perfect Thursday evening to create a bright, colorful lunar halo, which might not be seen again in these parts for a long time.
Old-timers might tell you that the number of stars visible inside of the ring-around-the-moon tells you how many days the snowstorm will last, or the number of days until the snow begins. Apparently, old-timers differ on their interpretation. At any rate, I counted two stars inside of the ring (Capella and Aldebaran), and when I awoke Friday and looked at the weather forecast, guess what I found? In two days it’s supposed to snow for : two days! It seems the old-timers are correct on both counts. Time will tell.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA’s “Astronomy Picture of the Day” Web site, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His “Celestial News” article appears weekly in the Steamboat Pilot & Today. His “Cosmic Moment” radio spots can be heard on local radio station KFMU. Also, check out Jimmy’s Web site at http://www.jwestlake.com.
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