When The Moon Gets All Bent Out Of Shape

The Great Lakes Trader / Joyce L. Van Enkevort cuts through the gleaming reflection of the moon on Lake Superior as it arrives in Duluth Thursday evening. Credit: Bob King

Watching a moonrise can be filled with surprises. Especially if you can see clear down to the horizon. Last night, I was out with a group of folks from the local radio club and we thrilled to a pumpkin-orange moon rising over Lake Superior. Heck, it even looked like a pumpkin — bigger around its equator than poles. But there was more to see, both with the naked eye and through the telescope.

Just like a chubby-cheeked baby. The top of the moon appears pinched as it rises last night. You’ll also notice that the top edge or limb is green-blue and the bottom, red. This separation of white light into colors is called dispersion. You can almost always see it near moonrise or moonset when the moon is near the horizon. It’s also visible with bright stars that are low in the sky. Credit: Bob King

The squished moon is familiar to many skywatchers, and it’s caused by the air near the horizon being denser or thicker than the air just above. That thicker air acts like a prism and “bends” the bottom half of the moon into the top half, smooshing it into a melon or pumpkin shape. Sometimes thin layers of air of different temperature and density stack up like noodles in lasagna. As the moon rises through them, sections of the disk get pinched or stretched according to which layer of air is doing the refracting. Last night’s moon developed two lovely dimples in each side of the disk that moments later filled out and disappeared.

Streaky clouds stripe the last night’s moon, one day past full. In this sequence, you can see dispersion and a small slice of the bottom of the moon appear to pinch off (left) and separate (center) before disappearing in the final frame at right. It all happened in seconds. Moonlight shining through varied layers of air cause the effect. Credit: Bob King

Perhaps the most amazing moment occurred just when I was taking pictures through the telescope with a cellphone. See something odd about the moon’s top and bottom edges? One’s green, the other red. This is no camera effect – you can see it with your own eyes when viewing the rising or setting moon through binoculars or a telescope. It’s that thick air again. Not only does it refract or “lift” the moon up, each color is refracted to a different degree. Blue is bent more than red, so one edge of the moon appears fringed in blue and the other fringed in red. It’s called dispersion, and this diagram will help you understand it better. Wild, isn’t it?

By the time the moon rose high enough to round out and color to pale, a ship cut through its gleaming reflection on Lake Superior.

For much more about the moon and all the crazy stuff you can see up there without equipment, pick up a copy of my recently published book Night Sky with the Naked Eye at Amazon.com or Barnes and Noble. Here’s a recent review.