Full Hunter’s Moon Sunday — Watch For The Moon Illusion

The full moon can appear outsized when it’s near the horizon. It’s an illusion likely caused by how our brains unconsciously compare the moon to familiar objects near it. Click the photo to find out when the moon rises for your city. Bob King

On Sunday evening get ready for the Full Hunter’s Moon. It will rise close to sunset, and depending on where watch, look larger than usual … or maybe not. Most people see a rising full moon as much larger than the same moon when it shines overhead. We know that’s physically impossible, and you can prove it by taking photos of the moon at both times. When you compare the images the moon’s size will be identical.

In fact, the moon at the horizon is actually about 4,000 miles (6,400 km) further away and 1.5 percent smaller compared to overhead. That’s because we have to look across half the planet to see it at moonrise. When it’s way up in the sky, it right above us (no Earth in the way) and 4,000 miles closer. Crazy, huh? Aristotle thought the atmosphere magnified the moon, but while it can change its color and bend its light, thicker air near the horizon does not magnify moonrises.

In the Ebbinghaus Illusion, the horizon moon (lower left) is surrounded by familiar objects (circles) and looks larger than when the same moon (upper right) is high up and surrounded by vast expanses of sky (large circles). Wikipedia

It comes down to our senses. We have no real sense of how far away it is, so our brains use environmental cues to size up the moon. When it’s near the horizon and especially if there are familiar foreground objects near the moon like trees, docks, homes and buildings, we mentally inflate its size. I’ve noticed this just like you.

But when the horizon is completely open with nothing in the foreground, say when viewed from a wide-open plain or lake, the full moon looks normal to me. Maybe even a bit on the small side. Removing the context gives my brain nothing to work with.

What is your experience? Another idea is that we think the horizon is incredibly far away. It sure looks that way compared to distant trees and other familiar earthly sights. When the moon is near the horizon we unconsciously assume that it must be huge to be so far away compared to a normal, overhead moon.

This is an example of the Ponzo illusion. Both of the horizontal yellow lines are the same length. We interpret the upper line as though it were farther away, so we see it as longer. A farther object would have to be longer than a nearer one for both to produce the same size image on your retina. Tony Philips, NASA

No single explanation — and there are many more — is perfect. But I cast my vote for the Ebbinghaus Illusion. To see it best (at left), stare at the bottom pattern first and then quickly look up at the top. It’s amazing how much larger the bottom center circle appears even though the two are identical.


The Shrinking or Disappearing Mill

For a wonderful example of how context affects the size of an object seen near the horizon, take a look at the brief video of the Shrinking Mill suggested by reader Carol Behan. From a distance the mill seems large tucked between the trees but as you approach it and the trees fall away, the transformation is amazing.

So now I need your help. I would love to hear your impressions of the rising or low moon when it comes up on Sunday evening (Oct. 13). Does it look bigger than normal? If you mask the foreground scene with your hands, does the moon’s size change? Drop me a comment and I’ll share your observations.

Oh, you’ll want to know you local moonrise time. Click here for that.

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