That's One Big Freakin' Moon

From Science@NASA:

bq.. The lowest-hanging full moon in 18 years is going to play tricks on you this week.

Sometimes you can’t believe your eyes. This week is one of those times.

Step outside any evening at sunset and look around. You’ll see a giant moon rising in the east. It looks like Earth’s moon, round and cratered; the Man in the Moon is in his usual place. But something’s wrong. This full moon is strangely inflated. It’s huge!

You’ve just experienced the Moon Illusion.

Sky watchers have known this for thousands of years: moons hanging low in the sky look unnaturally big. Cameras don’t see it, but our eyes do. It’s a real illusion.

This week’s full moon hangs lower in the sky than any full moon since June 1987, so the Moon Illusion is going to be extra strong.

What makes the moon so low? It’s summer. Remember, the sun and the full Moon are on opposite sides of the sky. During summer the sun is high, which means the full moon must be low. This week?s full moon occurs on June 22nd, barely a day after the summer solstice on June 21st–perfect timing for the Moon Illusion.

When you look at the moon, rays of moonlight converge and form an image about 0.15 mm wide in the back of your eye. High moons and low moons make the same sized spot. So why does your brain think one is bigger than the other? After all these years, scientists still aren’t sure why.

Maybe it’s the shape of the sky. Humans perceive the sky as a flattened dome, with the zenith nearby and the horizon far away. It makes sense: Birds flying overhead are closer than birds on the horizon. When the moon is near the horizon, your brain, trained by watching birds, miscalculates the moon’s true distance and size.

The best time to look is around moonrise, when the moon is peeking through trees and houses or over mountain ridges, doing its best to trick you.

p. Note that the best time to see this illusion in the North East is 7:25 PM on June 20, 8:39 PM on June 21 and 9:43 PM on June 22.

About Kevin

Kevin Jarnot is a technologist who lives just South of Boston, MA. He is currently employed as Chief Technology Officer at DebtX, a financial services technology company based in Boston.
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