Why The Moon Looks Bigger As It Rises

Any curious observer has noticed that the moon often looks larger at the horizon and smaller overhead. This occurrence is termed the Moon illusion and has been noticed and recorded by many cultures since ancient times. Even with all this time to solve the mystery, the explanation for this illusion is still debated and largely unresolved. No real breakthroughs in thought have developed since long ago. This optical illusion is not restricted to the Moon either, but it also occurs with the Sun and various constellations.

In reality, the Moon should appear at least 1.5% smaller when near the horizon because it is farther away than when in the sky. It is further by at least the distance of one radius of the Earth when at the horizon. In addition, atmospheric refraction should make the Moon smaller in the vertical axis, therefore making it look smaller and not bigger.

If these scientific reasons for proving that this can only be an optical illusion are not convincing enough, then there is another simple way to test the fact. Hold a small object like a quarter out at arm’s length and close one eye. When the moon is higher in the sky, it is the same size relative to the coin as when it is rising over the horizon.

One hypothesis to explain the illusion suggests that humans perceive size based on the relative size of objects around it. The horizon moon therefore looks bigger because of the detail surrounding it. All of the trees and houses give a scale to the Moon so that it looks massively larger than its surroundings. In the open sky, there is nothing but a large open space to compare the Moon too, so it subsequently looks smaller. In the classic Ebbinghaus illusion, two circles of equal size are put in perspective. One is surrounded by a ring of small circles, and the other with a ring of large circles. The one with smaller circles in comparison looks much larger, even though it is the exact same size. The one with larger circles looks smaller, just like the Moon does when fixed in an expansive sky.

The Apparent Distance Hypothesis proposes that the horizon moon looks bigger than the zenith moon because it looks farther away. A bird overhead looks bigger because it is closer than that same bird is as it flies away. Due to this daily event, humans supposedly expect the Moon as well to recede as it approaches the horizon. It does not though, as the experiment with comparing the Moon to an outstretched coin proved. The hypothesis then assumes that because the size stays the same at the horizon even though no other bird does, human brains attempt to compensate for this oddity and assume that the Moon is larger. The problem with this idea is that most people perceive that the Moon is closer and bigger at the horizon, not further. Unfortunately for the curious people out there, the moon illusion has yet to be fully and adequately explained, even after years of debate.

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