One of the most beautiful sights in nature is a massive, full moon rising over the eastern horizon. This is a treat everyone should experience at least once, and when you do, you’ll surely ask the same question as every other moon gazer: “Why does the moon appear large when it’s rising and smaller when it’s overhead?”
A simple question without a simple answer.
Many believe the Earth’s atmosphere somehow acts like a lens and magnifies the moon’s appearance, but this is just as wrong now as it was when Claudius Ptolemy suggested it 20 centuries ago. More likely is that it’s related to how we humans have evolved to perceive rigid objects appearing near the horizon.
Whatever the cause, try estimating the moon’s size the last time you saw it rising in the east, and compare it to a familiar object — say, your hand. For example, hold your little finger at arm’s length and it will subtend to your eye an approximate angle of one degree; your fist — also at arm’s length — will appear about 10 degrees across.
Unless you know the answer in advance, it’s amazing just how far off your guess will be. This is partly because memory of such things is unreliable but mostly because you’re trying to measure something that isn’t even real. It’s an optical illusion.
So how many degrees across would you estimate the full moon appeared the last time you saw it rising over the eastern horizon? One? Ten? Twenty? The fact is that the full moon appears only one-half degree across, no matter when and where in the sky you see it.
Don’t believe it? Well, shortly after sunset on Wednesday or Thursday, Aug. 14 or 15, you’ll have a chance to measure it for yourself.
Hold your hand out 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?
What’s even more unbelievable is that it will remain the same size as it rises higher into the sky, regardless that it appears to the eye as though it’s shrinking!
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!
Now, if you’ve got a camera, you can photograph the full moon just as it appears over the horizon and later when it’s overhead. You may be surprised to discover that the camera isn’t fooled at all: The moon appears the same size when it’s low against the horizon as well as when it’s overhead.
Yes, any way you look at it, the rising, full moon presents quite an illusion!
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at firstname.lastname@example.org and follow him on Twitter: @dennismammana. The opinions expressed are his own.