Sky watch: Lunar size at moonrise an optical illusion

I always enjoy talking with sky watchers about the moon. One question that never fails to pique their interest is "Why does the moon appear so large when we see it rising in the east?"

Simple question, but not so simple an 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. Try it yourself. Estimate the moon's size by comparing it to a familiar object, say, your hand. Your little finger held at arm's length subtends to our eye an angle of about 1 degree. Your thumb, also held at arm's length, appears about 2 degrees across, while your fist appears about 10 degrees wide.

It's truly amazing the estimates I hear. This is partly because our memory of such things is not particularly reliable, but mostly because people try to recall and "measure" in their minds something that's not even real; it's purely an optical illusion!

How many degrees across did you estimate the full moon to be the last time you saw it rising in the east? One? Ten? Twenty? The fact is that the full moon appears only one-half degree across no matter when you see it.

Don't believe it? Well, shortly after sunset on Oct. 22 or 23, go outside and face eastward, and measure it yourself.

Hold your hand out at arm's length and use it to measure the moon's size. Hold up your fist or your thumb. Then hold up your little finger. Surprised to find that the "huge" moon fits neatly behind your little finger? You're not alone. What's even more unbelievable is that the moon remains the same actual size as it rises higher into the sky, no matter how much smaller we perceive it to be.

This striking phenomenon is called the "moon illusion" and is caused not by the atmosphere, as many people seem to think, but by the brain. To prove this to yourself, make a loose fist and hold it up to your eye. Look at the rising (large) moon through it, using your fist to block out all but the moon itself. The moon will appear to shrink. For an even more startling demonstration, blink your eyes back and forth, looking through - and then around - your loose fist, and watch as the moon grows and shrinks!

Quite an illusion!