Big moon rising

For the week ending January 15, 2011

Most people say the full moon looks bigger when it’s rising and setting. It looks bigger, but it’s not. I’m not talking about its physical size staying the same no matter where the moon is. I’m saying the moon takes up the same amount of sky in either place.

We’ve all heard about atmospheric effects that alter images. When the moon or sun appears to be just above the horizon, it is actually below it. You see it because the earth’s atmosphere bends the light.

However, it doesn’t magnify the image of either the full moon or the sun. That’s an illusion that happens entirely in the brain. If you can measure how much of the sky the full moon takes up when it rises or sets and compare that to its overhead “footprint”, you’ll get the same measurement.

I don’t have the equipment to take a proper measurement, and you probably don’t either. But I do know a few things you can do to make comparisons that will reveal any changes in apparent size from position to position.

You can take an object like a coin or a tiny rock. Hold it at arm’s length in front of the full moon when it rises on January 19th. How much of the object covers the moon? Remember that and make the comparison again an hour or two later. Do it again an hour or two after that. It’ll be the same.

You can look at the moon through a telescope at various times of the evening. Make a mental note of how much of the moon you can see with any one eyepiece. Use the same eyepiece to look later. You’ll see just as much of the moon. Or, if the moon took up only part of the viewing field, it will take up the same amount.

The easiest and fastest way to prove that the magnified moon is just an illusion is to look at the rising or setting moon upside down. While upright, look at how big the moon looks just above the horizon. Then, turn around, bend over, and look at the moon between your legs. It’s instantly smaller!

Nobody’s sure why this illusion happens. The most popular idea is that it’s important for humans to have a better view of things on the horizon than of things overhead, so the brain magnifies the images from over yonder.

Now that I’ve given you that fact about the moon’s apparent size, here’s another one. The moon’s distance changes during its orbit, so sometimes it actually does take up just a little more space in the sky. But the difference isn’t enough that you can tell by casually looking up at it.

At any given time, the moon is about a half degree wide in our sky. How convenient that the sun also takes up almost the exact same amount of space. That’s why the moon can neatly cover it during a solar eclipse.