The big moon illusion

¡SkyCaramba! Weekly astronomy blog for the week ending May 10, 2014

When the moon looks bigger as it rises or sets than when it’s overhead, it’s easy to think the air is magnifying the image. But it’s not. The big moon is just an illusion.

Find something you can hold at arm’s length in the moon’s direction so you can compare them side by side. It should be something of a fixed shape and size so you can use it again when the moon’s in a different place.

It doesn’t matter where the moon is, what phase it’s in, or if it’s day or night. You’ll see it’s always almost exactly the same apparent size! Even over the span of a month, when the moon’s distance changes a lot.

You can take pictures of the moon in two or more positions. Be sure to use all the same settings so you get a fair comparison.

Biologists and psychologists haven’t settled on an explanation for the moon illusion. But there are other situations where our sight doesn’t size things up right.

So the air isn’t magnifying the moon. It’s having another effect though. On the horizon, the moon looks a little squished. Combine that with the moon being farther away than when it’s overhead, and a rising or setting moon should look about 2% smaller.

There’s more. Earth’s air bends the light path so when you see the moon just coming above the horizon, you’re seeing it over the earth’s curve. It hasn’t really risen yet. Likewise, when you see the moon going below the horizon, it’s already set.

Adding to the mystery of how our brains work, some people can make the moon illusion disappear just by looking at the moon upside down. Others can’t. Try it yourself by lying down and tilting your head moonward. Or turn your back to the moon, bend over, and look at it between your knees. Make sure you’re with people you can trust if you try that.

It may be safer and easier just to curl up a piece of paper into a tube and look at the moon through that to instantly eliminate the big moon illusion.