The Moon Illusion: Why Does The Moon Appear So Big?

~6 min
The Moon Illusion: Why Does The Moon Appear So Big?

The huge Moon hanging near the horizon is captivating. But what if we tell you that part of its beauty is just an optical trick? The Moon illusion has puzzled humanity since the 4th century B.C. Let’s learn about it and see if the Moon can still cast its spell on us.

Contents

Why is the Moon so big tonight?

The Moon may seem bigger for two reasons:

  1. It’s a Super Full Moon – the Full Moon at or near its closest approach to the Earth;
  2. It’s the Moon illusion – the trick of our brains makes the Moon appear larger when it’s on the horizon.

To learn more about the first reason, see our Supermoon guide. Here we will discuss the Moon illusion.

What is Moon Illusion?

The Moon illusion is an optical illusion in which the Moon appears larger when it’s near the horizon than when it’s at the zenith. The Sun and constellations are also subject to this effect. It was first mentioned in “Meteorology” (350 B.C.E.) by the Greek philosopher Aristotle who attributed the phenomenon to the reflection of light. As of yet, there is no consistent explanation for the Moon illusion.

How to prove the big Moon is just an illusion?

One thing we know for sure about the Moon illusion is that it’s in our heads. And here is how to see through it:

  • Take a photo of the large Moon near the horizon and the small lunar disk at the zenith with the same camera settings. When you compare the two pictures, you’ll see no size difference. (We’ll discuss later how to make the Moon seem huge in a photo).
  • Roll a piece of paper and tape it so that the large Moon fits the size. Wait for the Moon to rise high in the sky and look at it through the paper tube — you’ll see that the lunar disk fills the same space.
  • The funniest way to prove the Moon illusion is to look backward between your legs. Your brain won’t perceive the Moon’s surroundings as familiar ones and won’t create the illusion.

You don’t even need to compare the low-hanging Moon with the zenith Moon to break the illusion momentarily. If you just look at the “huge” Moon through a paper tube or in the photo you’ve taken, you’ll be immediately disappointed because the illusion only works when you see the Moon in real-time with all the surroundings.

Moon Illusion theories

The Moon seems bigger because of an optical illusion, but the question is — which one? There is no solid answer yet, so scientists continue to search for it. Let’s have a look at several common theories.

Ancient theory: magnifying atmosphere

In the 4th century B.C., Aristotle mentioned that celestial objects appear bigger on the horizon. He assumed that the Earth’s atmosphere acts like a water lens magnifying the image of the low-hanging Moon, Sun, and stars. The Greco-Egyptian astronomer Ptolemy and Greek astronomer Cleomedes suggested similar theories in the 2nd century A.D.

Flattened dome theory

In the 11th century, an Arab scholar Ḥasan Ibn al-Haytham suggested a theory by which we don’t perceive the sky as a hemisphere but as a flattened dome. According to this idea, the distance to the highest point in the sky appears much shorter than the distance to the horizon. That’s why the lunar disk seems closer at the zenith and is perceived as a smaller object, while the low-hanging Moon appears to us further away, hence we presume it to be bigger. Our brains just can’t comprehend that the Moon’s distance from us doesn’t change much from its position in the sky.

Ebbinghaus illusion

According to the illusion, discovered in the 1890s by the German psychologist Hermann Ebbinghaus, we perceive objects’ size relative to the context. The typical illustration shows two circles — the first surrounded by large circles and the second surrounded by small circles. The first one seems to us smaller, even though the circles are equally sized. As for the Moon, it may appear larger when the trees, houses, and mountains surround it than when we only see the huge expanses of the sky around.

Ponzo illusion

As the Italian psychologist Mario Ponzo demonstrated in 1911, if we place two identical objects over a pair of converging lines, the upper one will look bigger than the lower one. The phenomenon is also called “railway lines illusion.” The houses, trees, and other objects may serve as “railway lines,” creating the linear perspective where the Moon is far from us, therefore big.

Convergence micropsia

As one of the newest theories called “Convergence micropsia” suggests, our brain judges the distance to objects and their apparent size by the focus of our eyes. When looking at the low-hanging Moon, we focus far off into the distance perceiving the Moon as a big faraway object. High in the sky, though, there is nothing to focus on, so our eyes set for a default focus which is just a few meters, and our brain takes the Moon to be close and small.

Why are all of these theories unconvincing?

As interesting as all of these theories may sound, there are arguments against each of them:

  • Ancient theories of “magnifying atmosphere” fail right away when you compare the size of the low-hanging and the zenith Moon and see they are identical.
  • Ponzo and Al-Haytham’s theories suggest the Moon should look further away on the horizon, but most people say that, on the contrary, the lunar disk seems to them closer at that point.
  • Ebbinghaus’s theory doesn’t explain why astronauts and airline pilots see the Moon illusion, though there are no smaller objects around to compare.
  • Even the new theory of Convergence micropsia doesn’t satisfy all scientists. They argue that the Moon illusion works for elderly people and persons with eye lens implants who have no eye accommodation, which is crucial for Convergence micropsia.

How to photograph the big Moon?

To make the Moon seem huge in a photo, you also need to create an illusion of sorts but using technical methods. Here is a simple guide:

  • Use a camera with a long-zoom telephoto lens — not surprisingly, the more you zoom in, the bigger the Moon will appear.
  • Place other objects (trees, buildings, people) in the foreground to compare the Moon’s size with something. It puts to work the Ebbinghaus illusion we explained above. The Moon will look more impressive rising behind a house than in a blank black sky.
  • Control your exposure and use a tripod to take a sharp shot.
  • Plan your photo beforehand. To see when the Moon will look best in a particular landscape, use the app Ephemeris — the tool will help you choose the perfect conditions for your photoshoot.

Interesting facts

  • The Moon turns yellow or orange on the horizon. The Moon’s light has to pass through more air near the horizon. As a result, the shorter blue wavelengths of light are scattered by the air, while the longer red wavelengths still reach our eyes. It’s the only real effect the Earth’s atmosphere has on the low-hanging Moon.
  • The low-hanging Moon is actually about 1.5% smaller than an overhead Moon. It’s about 6,400 kilometers (4000 miles) farther away from our vantage point than when it is high in the sky. However, this size difference is intangible to the eye.
  • The planetariums have to make the Moon’s image near the horizon twice as large to make it look like the real sky. The Moon illusion only works for the real Moon, so the planetariums had to sacrifice accuracy for the sake of realism.
  • The Moon Illusion is often mixed up with a Supermoon. However, Supermoons are not as impressive as their name suggests. A Supermoon looks about 7% larger than the average one, while the Moon illusion makes the lunar disk appear to us about twice the size.

Bottom line: The Moon seems bigger near the horizon because of the “Moon illusion.” This optical trick still doesn’t have one solid explanation, but we can continue to search for an answer while enjoying the Moon’s beauty.

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