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Moon Doggies

Bad Astronomy
By Phil Plait
Sep 18, 2008 12:00 AMNov 5, 2019 12:26 AM


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Reading NoisyAstronomer's tweets, I saw she linked to a great picture of a halo around the Moon (a small version is on the left, click to enlargenate) taken by Rachael Beaton, a grad student at UVa. Halos form when ice crystals in the air refract the light from a bright source, bending it back toward you. They're common around the Sun, and a bit less common around the Moon because the Moon is fainter (though the sky is darker at night, giving you better contrast). But the little rainbow flare you can see off to the side is much rarer in Moon bows. Called parhelia (or sundogs), they have a similar origin as the bow, but are due to flat, hexagonal-shaped ice crystals. I don't think I've ever seen a Moon dog! And that little blue gizmo off to the side is a new one on me as well! Cool. Also cool, in that picture, is that you can see the tube of the old 6" telescope at UVa, built over 100 years ago and still a fine piece of work. Back in 1994 I used that 'scope to observe the scars left on the top of Jupiter's atmosphere by the impact of Shoemaker-Levy 9, a comet that broke into pieces and repeatedly slammed Jupiter. I could see them as small but perfectly formed black spots on the planet. It was awe-inspiring, to see impact remnants larger than Earth, knowing that impacts like that on our own planet would have wiped us out as a species. ... hmmm... it's cool, where your mind wanders, and where a single picture can take you.

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