February 26, 2019

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Moondog Mysterie

Originally published December 18, 2009 LPOD-Dec18-09.jpg
image by Chuck Hall

I have never seen a moondog and thus was excited when Chuck Hall submitted two photos that he took of a display. When I made a simple mosaic and greatly enhanced it the surprising view above emerged. The camera appears to have been rotated between shots for presumably the the two faint dogs to the right and left of the overexposed full Moon should be on a straight line. Moondogs are like sundogs in being bright spots from moonlight refracted by ice crystals in cirrus clouds. Moondogs and related arcs are much less common than sundogs because the Moon is much fainter than the Sun. This great image captures a number of rarely seen features of moondogs. Closer to the Moon than the dogs is a circular halo and a second much smaller halo is close to the Moon. Note the hint of color in the big halo and the dogs. Radiating from the Moon are two bright streaks that lead to the dogs. If the large halo is the 22° halo, the smaller one is 5°-6° wide, and the dogs have a diameter of about 28°. Based on nomenclature for sundogs the horizontal streaks connecting the moon dogs would be part of the paraselenic circle, The displacement of the moondogs from the 22° halo seems to be rare but related to the altitude of the Moon above the horizon. Solar halos aren't report as small as the 5°-6° circle seen here, so it is probably a corona formed by diffraction of light by small water droplets or ice crystals. But it does look more like the 22° halo than a typically colored corona. Is this a newly recognized feature? We need a halologist.

Chuck Wood

Technical Details
Chuck Hall writes: On 14 Sept 2008 around 10pm EDT, I took a break from work and walked outside to see what was in the sky. I was astonished to see a halo around the moon with moon dogs on both the right and left sides. While I had seen sun dogs before and had read about them, the moon dogs were something new! I went inside and got my Olympus Stylus 400 digital camera and took pictures. I had to use whatever was available to brace the camera because I could not locate my tripod. These fotos were taken with a 2 sec exposure.

Yesterday's LPOD: Coming Soon To an Eyepiece Near You

Tomorrow's LPOD: Chunky Moon


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