What is the Mysterious Moon Illusion?

Have you ever noticed that the Moon looks bigger when it’s near the horizon than when it’s high in the sky? This phenomenon is called the moon illusion, and it has been puzzling humans for thousands of years. The moon illusion is not a physical effect of the atmosphere or the distance of the Moon, but a psychological trick of our brains that makes us perceive the Moon differently depending on its position in the sky. In this article, we will explore what causes the moon illusion, how to measure it, and some of the possible explanations for it.

What causes the moon illusion?

The moon illusion is an optical illusion, which means that it is a misinterpretation of what we see by our visual system. The Moon does not actually change its size or shape as it moves across the sky, but it appears to do so because of how our brains process visual information.

One of the factors that influences how we perceive the size of an object is its angular size, which is the angle that it subtends at our eye. The angular size of an object depends on both its actual size and its distance from us. For example, a coin held close to our eye will have a larger angular size than a coin held far away, even though they are both the same size.

The angular size of the Moon is about half a degree, which means that it occupies about half a degree of our field of view. This is true regardless of where the Moon is in the sky, as we can verify by measuring it with a ruler or a camera. However, our brains do not always judge the size of an object based on its angular size alone. We also use other cues, such as perspective, context, and comparison, to estimate how big or small something is.

When we see the Moon near the horizon, we tend to perceive it as larger than when we see it high in the sky. This is because we have more reference points on the horizon, such as buildings, trees, mountains, or clouds, that give us a sense of scale and depth. We also have a mental model of how objects appear smaller as they get farther away from us. Therefore, when we see the Moon near these familiar objects, we assume that it must be very large and very far away to have such a large angular size.

On the other hand, when we see the Moon high in the sky, we have fewer reference points and less depth perception. We also have a mental model of how objects appear larger as they get closer to us. Therefore, when we see the Moon in isolation, we assume that it must be smaller and closer to us to have such a large angular size.

These assumptions are based on our everyday experience with terrestrial objects, but they do not apply to celestial objects like the Moon. The Moon is about 240,000 miles (384,000 kilometers) away from us on average, and its distance only varies by about 10 percent during its orbit. This means that its actual size and distance do not change significantly as it moves across the sky. However, our brains do not take this into account when we look at the Moon, and instead rely on our faulty heuristics to judge its size.

How to measure the moon illusion?

The moon illusion is not easy to measure objectively, because it depends on subjective perception and individual differences. However, there are some methods that researchers have used to quantify how much bigger or smaller people perceive the Moon at different positions in the sky.

One method is to ask people to compare the apparent size of the Moon with another object of known size, such as a coin or a finger held at arm’s length. By measuring how far away people hold these objects from their eyes to match them with the Moon’s diameter, researchers can calculate how much larger or smaller people perceive the Moon at different elevations.

Another method is to ask people to adjust the size of a circle on a screen until it matches their perception of the Moon’s diameter. By measuring how many pixels people use to create these circles at different elevations, researchers can calculate how much larger or smaller people perceive the Moon at different elevations.

Using these methods, researchers have found that people typically perceive the Moon as about 1.5 times larger near the horizon than at its zenith (the point directly overhead). However, this factor can vary depending on individual factors such as age, gender, culture, mood, and familiarity with astronomy.

What are some possible explanations for the moon illusion?

There is no consensus among scientists on what causes the moon illusion. There are many theories that have been proposed over time, but none of them can fully account for all aspects of this phenomenon. Some of these theories are:

• The apparent distance theory: This theory suggests that we perceive objects near the horizon as farther away than objects high in the sky because of atmospheric and perspective cues. Therefore, we perceive the Moon near the horizon as larger because we assume that it must be larger to have the same angular size at a greater distance. This theory is supported by some experiments that show that the moon illusion is reduced or eliminated when people view the Moon through a telescope, a window, or a hole in a card, which eliminate the horizon cues.
• The relative size theory: This theory suggests that we perceive objects near the horizon as larger because we compare them with other objects on the horizon that are familiar to us. Therefore, we perceive the Moon near the horizon as larger because we compare it with buildings, trees, mountains, or clouds that are smaller than the Moon. This theory is supported by some experiments that show that the moon illusion is reduced or eliminated when people view the Moon in isolation, without any horizon cues.
• The angular size contrast theory: This theory suggests that we perceive objects near the horizon as larger because they contrast with the smaller angular sizes of other objects on the horizon. Therefore, we perceive the Moon near the horizon as larger because it contrasts with the smaller angular sizes of buildings, trees, mountains, or clouds that are farther away from us. This theory is supported by some experiments that show that the moon illusion is enhanced or induced when people view the Moon near objects that have smaller angular sizes than the Moon.
• The oculomotor theory: This theory suggests that we perceive objects near the horizon as larger because they require more eye movement to focus on them than objects high in the sky. Therefore, we perceive the Moon near the horizon as larger because it requires more eye movement to focus on it than when it is high in the sky. This theory is supported by some experiments that show that the moon illusion is reduced or eliminated when people view the Moon with one eye closed, which reduces eye movement.
• The cognitive theory: This theory suggests that we perceive objects near the horizon as larger because we have learned to associate them with largeness from our experience and culture. Therefore, we perceive the Moon near the horizon as larger because we have learned to associate it with largeness from myths, legends, stories, and media. This theory is supported by some experiments that show that the moon illusion varies across cultures and contexts.

Frequently Asked Questions about the Moon Illusion

Here are some of the most common questions that people ask about the moon illusion:

• Is there a difference between a supermoon and a moon illusion?

Yes, there is a difference between a supermoon and a moon illusion. A supermoon is a term used to describe a full moon or a new moon that occurs when the Moon is at its closest point to Earth in its orbit (called perigee). A supermoon can make the Moon appear up to 14 percent larger and 30 percent brighter than an average full moon or new moon. A supermoon is not an illusion, but a physical effect of the Moon’s orbit.

A moon illusion is a term used to describe an optical illusion that makes the Moon appear larger near the horizon than high in the sky. A moon illusion can occur at any phase of the Moon and at any distance from Earth. A moon illusion is not a physical effect of the Moon’s orbit, but a psychological trick of our brains.

• Does the moon illusion affect animals?

There is not much evidence to suggest that animals experience the moon illusion. Most animals do not rely on vision as much as humans do, and they may not have the same mental models and expectations that humans have when they look at celestial objects. However, some animals may be affected by other factors related to the Moon’s position in the sky, such as its brightness, color, or phase.

• How can I avoid or enhance the moon illusion?

There are some ways to avoid or enhance the moon illusion depending on your preference. To avoid or reduce the moon illusion, you can:

• View the Moon through a telescope, a window, or a hole in a card, which eliminate or reduce horizon cues.
• View the Moon in isolation, without any horizon cues.
• View the Moon with one eye closed, which reduces eye movement.
• Measure or compare the apparent size of the Moon with another object of known size, such as a coin or a finger held at arm’s length.

To enhance or induce the moon illusion, you can:

• View the Moon near objects on the horizon that are familiar to you and smaller than the Moon, such as buildings, trees, mountains, or clouds.
• View the Moon near objects on the horizon that have smaller angular sizes than the Moon.
• View the Moon with both eyes open, which increases eye movement.
• Imagine or recall stories or images that associate the Moon with largeness.