The Moon Illusion

We have record of people noticing that the Moon looks larger at the horizon than directly overhead dating as far back as 4 BC, but I have a feeling that people have noticed it for as long as people have noticed the moon – which is really hard to miss. Ever since then, we have told and re-told tales of possible explanations of the phenomena regardless of the truth behind the tale. One outrageous idea suggests that the Moon might actually be moving closer to or farther away from the Earth to make it appear larger or smaller in the sky. Another possible explanation suggests that the round shape of our atmosphere might act like a magnifying glass at the horizon, making the moon look bigger, which has since been proven to be untrue. In fact, scientists haven’t actually agreed upon one single explanation amongst themselves. As you might guess, the answer is a lot more complicated that anyone previously thought.

One thing scientists know for sure about the Moon illusion is that it’s definitely an illusion that manifests in our minds because the angular size of the moon is the same no matter where its position is in the sky. This can be made obvious by the use of a theodolite or by taking images of the moon at various positions in the sky. A much simpler yet less precise way to estimate the size of the moon in the sky is to hold your thumb out at arm’s length beside the moon and compare it to your thumb nail while it is at various positions in the sky.

A popular explanation has to do with how our eyes focus on close and distant objects. When we look at the moon directly overhead it lacks the visual cues to tell our brain that it is really far away so our eyes focus on it as if it were closer. Oddly enough, when our brain focuses on the horizon moon and we perceive it as being farther away it actually makes it appear closer. This seems backwards, but it can be demonstrated if you imagine you are looking down a street and there’s a transfer truck 100 yards away and a tiny sports car 10 feet away in the same visual field. The truck is still perceived as being larger even though it produces a smaller image onto your eye.

There are also quite a few optical illusions that are referenced when discussing the Moon Illusion, which include the Ponzo Illusion and the Ebbinghaus Illusion.

Example of the Ponzo Illusion

 

Example of the Ebbinghaus illusion

 

None of the explanations are exactly solid because the same moon illusion is experienced by airplane pilots and even astronauts on The International Space Station who never have long stretches of landscape surrounding the moon to give a visual comparison. It is also worth mentioning that because of the eccentricity of the Moon’s orbit it does appear about 11% larger during part of the year, but that’s hardly noticeable over that amount of time unless you’re specifically looking.

We may never pin down the exact reason why we perceive the moon as being larger at the horizon, but I am glad I get to witness such a grand illusion as frequently as this one!

Sources:

Moon Illusion – Wikipedia

Ponzo Illusion – Wikipedia

Ebbinghaus Illusion – Wikipedia

New Thoughts on Understanding the Moon Illusion – Carl Wenning 1985

Solstice Moon Illusion – Science@NASA

The Moon Illusion Explained – University of Wisconsin Whitewater

The Moon Illusion an Unsolved Mystery – Donald E. Simanek

Summer Moon Illusion – NASA

Explaining the Moon Illusion – Kaufman

Moonrise over Turkey – APOD

Apogee Moon, Perigee Moon – APOD

The Moon Illusion VIDEO – ASAP Science

About Dani Johnson

I am 26 years old and I live in a college town with my boyfriend, our 2 dogs and chinchilla, 4 additional room mates and the house cat. Since I share financial responsibilities with my boyfriend I am waiting on him to finish college before I go back (he's almost done!). I will then focus my studies on Science Writing. I want to write particularly about Astrophysics, Cosmology and Planetary Science. Until then, I spend my free time listening to various podcasts about science and skepticism to inspire deeper research on potential writing topics. I also enjoy sewing, drawing, writing fiction, spending time with the boyfriend and pets, amateur astronomy and some girly things like nails, hair and makeup.
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11 Responses to The Moon Illusion

    • Dani Johnson says:

      That’s very interesting, thanks for sharing. :) I don’t know that the idea would apply when the very same phenomena is experienced by astronauts in the International Space Station. Maybe it’s something different for them, though.

  1. Jon D Toellner says:

    The moon actually appears smaller on the horizon than when it’s high up in the sky. The average distance from Earth to Moon is 384,400 km (it varies because the moon’s orbit is elliptical.) The average radius of the Earth is 6,371 km (it too varies because the Earth is not a perfect sphere). When seen on the horizon the Moon is about one Earth radii further away; about 1.6%.

    • Dani Johnson says:

      Great information! While it’s true that the moon projects a smaller image on your retina when it is on the horizon but our brains interpret things that are farther away as being bigger (as suggested by the transfer truck/small car analogy in the original post) so we perceive the moon as being larger. That’s one possible explanation, at least. We still don’t know the exact reason.

  2. Stephen Propatier says:

    Awesome. Always thought its was related to time delay and atmospheric lensing. Shows what I know. I think what you are proposing is, our brain and the way it interprets the information, is why we see it as bigger.
    One question the 11% is gradual but is it discernible?

  3. Freke1 says:

    We see with the brain, and we see what we expect to see. That’s why magicians have such an easy time with us. And that’s why eyewitnesses can’t really be trusted (if You ask me). I’ve never noticed this illusion even though I knew about it, but I’ll sure go look for it.

  4. curious says:

    Do you have a link to astronauts describing their experience of the Moon Illusion?

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