Monday, October 22, 2007

Marquee Moon

If you're a total weirdo who stares vacantly into the sky like me, last month you would have noticed the full moon looking completely amazing, kind of like this:

That gorgeous phenomenon was known as the Harvest Moon, and is the full moon closest to the autumn equinox (you know, when the day and night are equal in length). If you missed it, never fear, because this Friday is the equally lovely Hunter's Moon.

In fact, this year's Hunter's Moon is likely to be even more amazing than the Harvest Moon. Why? I thought you'd never ask.

The Harvest and Hunter's Moons are really fantastic to see for two primary reasons: their unusually large size and their reddish-orange coloring. Of course, since the moon doesn't actually change, they are both optical illusions created by the moon's low position in the sky. The times of sunset and moonrise change throughout the year, and in the fall the two times are particularly close together. This allows the moon to become bright and clearly visible while it is still early in it's rise and thus low in the sky.

Although both illusions are created by the same thing, they happen for very different reasons. The color is a result of seeing the low-hanging moon through a greater amount of atmosphere. See, the Earth's atmosphere scatters blue light (short wavelength light) more than it does other kinds of light. That's why the sky looks blue in the daytime and objects in the distance appear bluish and muted. When something luminescent, like the moon, is set against a dark background the blue light is basically scattered away into the darkness. That leaves the red light to make a beeline for your eyes and voila! The resulting red moon is sometimes known as the creepy-but-awesome Blood Moon.

The size thing, called moon illusion, is a lot more complicated, mostly because no one actually knows for sure why it happens. The thing everyone can agree on is that, unlike the color illusion where you are actually seeing real red light, the size illusion is a trick that occurs in your brain. The extra weird thing is that a small percentage of people actually don't see moon illusion. To them, the horizon moon looks just the same as it does at its peak.

One popular theory of moon illusion is this: most objects in the sky (clouds, birds, airplanes) appear larger overhead because they are closer, and smaller near the horizon because they are farther away. The moon, however, is the same size both overhead and on the horizon. Since you expect it to be smaller on the horizon, when it is not, it seems unusually large.

Don't believe your brain is tricking you? On Friday, try this: hold a coin up next to the moon when it appears large and close one eye. Compare the size. Now do the same when the moon is high in the sky and back to its usual size. It's the same! Also, the larger moon shrinks back to its smaller self when you view it upside-down, which is completely mind-boggling.

As for this year's especially amazing Hunter's Moon, that has to do with the angle of the moon's orbit. By a complete coincidence, the moon will be at its closest position to the Earth the very same night as the full moon, likely increasing the moon illusion effect.

The Harvest and Hunter's Moons aren't named because of their crazy visual effects. All full moons have a name, originally given to them by Native Americans. There are some variations, but here's a basic list:

January: Wolf Moon July: Buck Moon
February: Snow Moon August: Sturgeon Moon
March: Worm Moon September: Harvest Moon
April: Pink Moon October: Hunter's Moon
May: Flower Moon November: Beaver Moon
June: Strawberry Moon December: Cold Moon

Alright, so maybe the Sturgeon Moon and the Worm Moon aren't as glamorous as the Pink Moon (of Nick Drake fame), but it's supposed to be a functional reference for seasonal plant and animal life, not a beauty contest.

The moon rises at 5:11 p.m. on Friday night, and the sun sets at 5:45 p.m. So head outside around 6ish and admire that lovely thing in the sky.

Oh, and while you're at it, consider all the wonderful gifts you'd like to shower me with for my birthday on October 28th. My online wishlist is here. Cash donations are accepted.

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