When the full moon rises around sunset on the evening of Thursday, Dec. 11, it will appear larger than normal.

Regular readers of this column know that this is due to an optical trick known as the "moon illusion." It's really quite a startling effect and most people swear that the rising moon appears immense. I know I have a hard time with this one myself.

But it's easy to prove that it's just an illusion created by your own brain. Look at the seemingly large rising moon through a loose fist, using your fist to block from view all but the moon itself. The moon will appear to shrink, proving that the foreground somehow contributed to this bizarre sight.

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

But on the night of Dec. 11, not only will the moon appear larger, it will actually be larger.

How can this be, you ask? The answer comes from the fact that the moon doesn't orbit the Earth in a circle, but orbits instead along an elliptical path that carries it closer and 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 never have made it possible for life to emerge from the seas eons ago.

But astronomers who study the moon can accurately measure the moon's distance and know that it's changing constantly. And on the night of Dec. 11, the full moon will lie at its nearest — only 221,560 miles from Earth. That's 30,967 miles, or about 12 percent, closer than it was on the night of May 19.

Now, one might think this is reason enough for the full moon to look larger, but this difference is relatively minor. It's actually not enough to detect with the unaided eye. By comparing two full moon images, however — one taken when the full moon is at its nearest, or perigee, and another at its farthest, or apogee —we can see a dramatic difference in its size.

So will the rising moon appear even more immense on Dec. 11 than it normally might? Probably not, but it'll be well worth stepping outside to check it out anyway.

To find out more about Dennis Mammana and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.