The moon, here overexposed to show the corona as it really looked to the eye, put on a fine show last night as high cirrocumulus clouds passed over. The tiny droplets or ice crystals in the clouds diffracted the light into a series of colored disks. Details: 35mm lens at f/4.5, ISO 400 and 1 1/2 second exposure. Photo: Bob King
Last night I spent some time between passing clouds looking at the nova in Eridanus and the moon. When it became clear that the clouds were gaining the upper hand, I stored away the scope and went inside to warm up. My daughter Maria arrived home a half hour later and casually mentioned that there was nice ring around the moon. I walked downstairs and opened the door for a look. WOW! A bullseye of colored rings centered on the moon made me forget both the temperature and my coat. No matter. I quickly attached camera to tripod and took a few pictures as the rings expanded from two to three and even four. It was such a beautiful sight.
This corona around the sun is distorted into a spiral shape. Coronas shine an iridescent blue-green on the inside of the rings and red on the outside. Credit: Andrew Kirk
What we saw was a corona, or series of small concentric rings of color caused by the diffraction of light by millions of microscopic droplets or ice crystals in the clouds. When the droplets or crystals are all very small and nearly uniform in size you get mulitple-ringed and vividly-colored coronas like last night’s. Coincidentally, Andrew Kirk of Bishop, Calif. had just sent me his photo of an unusual, distorted corona he nicknamed "the white hole". Since droplets can vary in size and uniformity across a moving cloud, coronas will sometimes look temporarily squished or stretched.
Although the full moon will be near the Seven Sisters star cluster tonight, you probably won’t see them because of glare. Easier to find will be Aldebaran, Taurus’ brightest star, and Capella in Auriga the Charioteer. Created with Stellarium.
Tonight’s moon is the first of two full moons in December. The actual moment of full-as-it-gets happens at 1:30 tomorrow morning (Central time). Because this full moon occurs at the very opening of December and the time between any two similar lunar phases is 29.5 days, we get a bonus "blue moon" at month’s end on December 31st.
Moonrise today is nearly an hour before sunset but tomorrow the moon will rise only about 10 minutes after sunset (for Duluth-Superior and region). Since moonlight and twilight balance out nicely around sunset, this is a great time to photograph a moonrise while including an interesting scene in the foreground. Even if all you own is a simple point-and-shoot camera, you should be able to get a few hand-held pictures of the nearly full moon. Find a place with a view of the horizon to the northeast to catch it early. If you wait too long, the moon’s light will overpower everything else and you’ll record little more than a bright blip against a black sky. You can use the camera’s automatic setting but also try a series of different exposures on manual to get just the effect you want. Check the monitor on the back of the camera to make sure you’re in the ballpark.
Full moons in the winter months pass through the constellations of Taurus and Gemini, the same ones the sun occupied six months ago during summer. Winter full moons follow the same high path across the night sky that the sun did. Expect moonlit landscapes to be considerably brighter now than during the warmer months. Throw in some snowcover — which we sadly still don’t have — and you’ve got a photon extravaganza.
This week Venus becomes difficult if not impossible to see from mid-northern latitudes while Mars becomes the third brightest "star" in the nighttime sky, surpassed only by Sirius and Canopus. Illustration: Bob King