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Super Moon of 2012

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When the full moon rises around sunset on the evening of Saturday, May 5, it will appear much larger than usual.

Experienced sky watchers and regular readers of this column might know that a large-appearing moon is caused by an optical trick known as the "moon illusion." It's really quite a startling effect, and most people swear that the rising moon must actually be immense. I even have a tough time with this one myself.

It's quite easy, however, to prove that this is just an illusion created within your own brain. With only one eye, look at the gargantuan moon through a loose fist held near your face, using your fist to block from view all but the moon itself. You'll be amazed by what you see... the moon will seem to shrink back to "normal," proving that it's the foreground (blocked from view) that somehow contributes to the illusion.

For an even more remarkable demonstration, blink your eyes back and forth — looking through, and then around, your loose fist. The moon will appear to shrink, grow, shrink, grow... pretty weird!

But on the night of May 5/6, 2012, not only will the rising full moon appear larger because of the moon illusion, it will actually be larger. Wait a minute, how can this be? The moon is a solid body; it can't change its actual size. True, but it can change its apparent size.

This effect occurs because the moon doesn't orbit Earth along a circular path, but moves instead along an ellipse that at times carries it closer to us and at times farther from us during its monthly cycle.

Fortunately, this change isn't much, because, without the moon's relatively constant gravitational pull, the tides might have never made it possible for life to emerge from the seas eons ago.

Astronomers who accurately measure the moon's distance tell us that this is constantly changing. On the night of May 5/6, the full moon will lie at its nearest for the year — only 221,800 miles from Earth. That's 30,870 miles — or about 12 percent — closer than will be on the night of November 27/28, its farthest approach to Earth this year.

Now, one might think that this is reason enough for the full moon to appear to change size, but this difference is relatively minor. So will the rising moon appear especially huge on May 5... a "super" moon? Probably not. My guess is that, unless you've accurately measured the apparent diameter of the moon from time to time, you'll be hard pressed to prove that the moon really does appear larger.

First of all, our memory of such things is not terribly reliable and, secondly, there's that pesky moon illusion thing that confuses the issue even more.

Only experienced moon watchers might be able to detect this difference with the unaided eye. However, by comparing two full moon images — one taken when the full moon is at its nearest (perigee) and another at its farthest (apogee) — we can easily see a dramatic difference in its size.

But it'll sure be fun to go out at dusk next Saturday to check it out!

Visit Dennis Mammana at www.dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

COPYRIGHT 2012 CREATORS.COM




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