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Big Moon A-Risin'


Last updated 9/14/2023 at Noon

Though the moon always looks closer when it rises, that's actually an optical illusion – except when it's not!

Make sure you're out at sunset on Wednesday, Aug. 30, to watch the beautiful full moon rise in the east.

If it seems particularly large when it rises that night, that's because, well, it is. Not the moon's actual size, of course; that is always 2,159 miles across. I'm talking about its apparent size – how big it appears to the eye.

I know what you're thinking: The full moon always looks large when it rises. That's true, but you may also know that's purely an optical illusion caused by a combination of foreground landscape along with some convoluted decision making by the human brain. This is known as the "moon illusion," and it produces quite a striking sight. I know I have a difficult time with this one myself.

Don't believe it's all in your head? The next time you see this grand spectacle, close one eye and check out the rising moon through a loose fist or cardboard tube that blocks the landscape from view. You'll see that the moon seems to shrink back to a "normal" size, proving that the effect is purely an optical trick played by the brain.

Amazing, but true! So why, then, might next week's rising full moon appear even larger? Because during the moon's elliptical orbit around the Earth, it occasionally becomes closer (perigee) and farther (apogee) from us. And in between, its apparent size change is even tougher to notice.

This particular full moon is the second full moon of this month, referred to as a "blue moon." It will occur only a few hours after its closest perigee of the year. Since it'll lie only 221,942 miles from us – 7.6% closer than average – it will be the nearest full moon of 2023.

Even though the moon will display a larger size than normal, will anyone even notice? Probably not, unless you either know it's supposed to appear larger (in which case you're cheating!) or you've measured its actual size from time to time. Otherwise, most folks would be hard-pressed to notice that the moon's disk appears larger than normal. That's because our memory of such things is not particularly accurate, and there's that pesky "moon illusion" thing that confuses matters even more.

While these apparent lunar size changes are relatively minor, we can see the difference easily by comparing two full moon photographs – one taken when the full moon is near perigee and another when it is at apogee.

You can create such photos for yourself. Use a camera with a long telephoto lens (300-400 mm will do nicely) and exposures similar to those for a bright, sunny landscape on Earth. Be sure to "bracket" your exposures by shooting some slightly overexposed and some slightly underexposed – just to make sure that one turns out.

Take one photo next Wednesday night and another on March 24, 2024, when the rising full moon will lie about 251,906 miles from us. When you compare the two images, you'll discover that the full moon of March will appear about 13.5% smaller – not sufficient for unsuspecting moon gazers to notice by eye, but enough that your photos will show a dramatic difference.

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