Week of August 3-9, 2014
If the full moon appears especially large when it rises over the horizon around sunset next weekend, Saturday and Sunday, August 9 and 10, that's because, well, it is.
Regular readers of this column know about the optical illusion that causes the rising moon to appear immense. It's called the moon illusion, and occurs because the landscape behind which the moon rises tricks the brain into thinking the moon is larger than it really is. Look at the rising moon through a loose fist or cardboard tube that blocks that landscape from view and the moon seems to shrink back to a normal size.
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!
Amazing, but true.
But that's not all for, you see, not only will this rising full moon appear larger because of the moon illusion, it will actually be larger. Now, wait a minute, you're thinking, how can this be? The moon is a solid body; it can't change its actual size. This is true, but it can change its apparent size.
This takes place because, during the moon's elliptical orbit around the Earth, it periodically becomes closer (perigee) and farther (apogee) from us, and this particular full moon occurs at the closest perigee of 2014 — when the moon lies 221,760 miles (356,896 kilometers) from the Earth.
It's such a full moon that the media, in recent years, has come to call a "super" moon. In fact, they've even begun calling full moons before and after perigee a "super" moon. And, if that's not enough, some are already touting 2015 as having six "super" moons! Where does it end?
But will this particular perigee full moon actually appear larger to the eye? At its closest, the moon will appear only 7.2 percent larger than its average size, so there really is nothing particularly "super" about it. In other words, if you ordered a 16-inch pizza, but were given a 17-incher, would you call it a super pizza? That's the difference we're talking about.
I'm guessing 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. Secondly, there's that pesky moon illusion thing that confuses the issue even more when the moon appears low in the sky.
Experienced lunar watchers might legitimately notice its larger size this month, but most folk will not see a difference — though many will be convinced they do. Now if you were compare two images of the full moon— one shot when the moon lies at perigee an another when it's at its average distance or even at apogee — well, the size difference would become quite obvious.
So, will the "super" moon be super this month? Only one way to find out ... go outside to check it out. It may be awhile until you see such a large moon again ... well, not for a few months anyway!
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.