Wednesday, 12 October 2011

A lunar halo, or ring around the moon, over D.C., Oct. 10, 2011

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A jumbo-sized ring around the moon, as seen from downtown D.C. late Monday night. (Photo courtesy of James Joslyn)
UPDATE: The moon ring has caught the attention Devon Lucie, meteorologist for WJLA, who thinks it might not be the more common 22-degree halo discussed below. "I think it was the extremely rare 46-degree halo last night because it was HUGE!" Lucie says.
The larger 46-degree halos are created by the same process of reflected sunlight filtering through ice crystals, but with slight differences in the structure of the ice crystals. How rare are they? After studying the skies for 10 years, the German Halo Research Group concluded that 22-degree halos occur an average of 100 days each year, while 46-degree halos materialize only about four days a year. So if you saw this thing last night, consider yourself lucky.
More reading/pictures here and here.
ORIGINAL: The D.C. skies have rewarded night owls lately. First, there was theimmense fogbank hovering over the Potomac on Thursday – a dreamlike phenomenon that reappeared last night, turning Key Bridge into a brume-washed set piece fromThe Legend of Sleepy Hollow.
And then there's this colossal sky doughnut that swelled around the nearly full Hunter's Moon on Monday night. The refulgent ring was in full effect at 11:30 p.m., casting a perfect circle around the moon that looked like it must've been hundreds of thousands of miles in diameter. WJLA's James Joslyn caught it as it loomed over downtown D.C., as you can see above.
These "lunar rings," quite different from luna rings, actually have their beginnings more than 93 million miles away, on the sun's boiling surface.
Rays of sunlight shooting into space hit the almost-full moon, which acts like a nearly perfect reflecting disc and bounces the light down to earth. The light then passes through cirrostratus clouds floating high in the atmosphere. The hexagonal-shaped ice crystals that compose these clouds bend the light at a 22 degree angle, casting a luminous ring that is 44 degrees in diameter. It's math, it's chemistry and it's gorgeous.
You can see other moon rings herehere and here. And just for the heck of it, here is amoonbow, or a rainbow shining at night. (We don't get many of those around D.C.)