I have fun talking with sky watchers about the moon. One question that always seems to pique their interest is: “Why does the moon appear so large when we see it rising in the east?”
The questions is simple, but not the answer.
One of the first things I do is suggest they try to recall the last time they saw a huge full moon rising over the horizon. Then I ask them to estimate the moon's size and compare it to a familiar object – say, their hand. I explain that their little finger, held at arm's length, appears to their eye to cover about 1 degree of the horizon. Their fist, also at arm's length, appears about 10 degrees across.
It's truly amazing the variety of estimates I get of the moon's size. This is partly because memory of such things is not particularly reliable, but mostly because they're trying to recall and measure in their minds an optical illusion.
Think about it right now yourself. How many degrees across would you estimate the full moon to be the last time you saw it rising in the east? One, 10, 20 degrees? The fact is, the full moon appears one-half degree across no matter when you see it.
Don't believe it? Well, shortly after sunset Saturday – which, on that day of a full moon, is also moonrise – go out and measure it yourself.
Hold out your hand at arm's length and use it to measure the moon's size. Hold up two fists, or one. Then hold up your little finger. Surprised to find that the “huge” moon fits neatly behind your little finger? You're not alone.
I'm still so amazed by the moon's apparent size that I take a measurement whenever I see it. What's even more unbelievable is that it will remain the same size as it rises higher into the sky, no matter how it appears to the eye.
This phenomenon is called the moon illusion, and it really is quite startling. To correct the illusion, look at the rising (large) moon through a loose fist – using your fist to block out all but the moon itself. The moon will appear to shrink, proving that the foreground somehow contributes to this bizarre perception. And for an even more startling demonstration, blink your eyes back and forth – looking through, and then around, your loose fist.
Quite an illusion!
© Creators Syndicate Inc.
