The observation that the Moon’s size appears to shift dramatically in the sky is a common experience, particularly when it hangs low near the horizon. This perceived change is caused by two separate phenomena. The vast difference in apparent size is a trick of human perception, known as the Moon Illusion, where the brain misinterprets the Moon’s true scale. There is also a separate, more subtle, physical variation in the Moon’s size due to the mechanics of its orbit around the Earth.
How Orbital Distance Changes Apparent Size
The Moon orbits the Earth in an ellipse, not a perfect circle, meaning its distance constantly fluctuates over its approximately 27-day orbit. When the Moon is closest to Earth, known as perigee, its apparent size is maximized. Conversely, when it reaches its farthest point, called apogee, it appears smallest.
This difference in distance causes the Moon’s angular diameter to vary by approximately 11% to 14%. This variation is measurable but is not the dramatic effect people observe near the horizon. When a full moon coincides with perigee or apogee, the terms “Supermoon” and “Micromoon” are used. The change in brightness is more pronounced than the change in size, with a perigee Moon appearing up to 30% brighter than an apogee Moon.
The Psychology of the Moon Illusion
The Moon Illusion makes the Moon look gigantic near the horizon and smaller when high overhead. This perceptual phenomenon is rooted entirely in how the human brain processes visual information. The Moon’s actual angular size remains nearly identical regardless of its elevation, demonstrating that the illusion is a product of cognitive scaling.
Apparent Distance Hypothesis
The Apparent Distance Hypothesis suggests the brain misinterprets the distance to the Moon based on surrounding visual cues. When the Moon is low, terrain features like trees and houses provide a sense of depth, making the horizon appear far away. The brain attempts to maintain “size constancy,” a mechanism that scales an object’s perceived size based on its perceived distance.
Since the Moon’s image size on the retina does not change, the brain compensates for the seemingly greater distance by perceiving the Moon as larger. When the Moon is high, the brain lacks these terrestrial cues, and the sky is often perceived as a flattened dome. This makes the Moon seem closer and thus smaller, as the brain attempts to apply size constancy scaling to a celestial object.
Relative Size Hypothesis
The Relative Size Hypothesis focuses on comparing the Moon to nearby objects. Near the horizon, the Moon is framed by foreground objects like trees or buildings, which appear tiny due to their distance. The brain compares the Moon’s size to these small cues and scales the Moon up, making it appear immense.
When the Moon is high, it is surrounded by a vast, featureless expanse of sky, offering no small objects for comparison. Without these relative size cues, the brain does not inflate the Moon’s perceived size, and it maintains its normal appearance.
Atmospheric Distortion and Viewing Conditions
While the dramatic size change is an illusion, the Earth’s atmosphere does alter the Moon’s appearance, though not by magnifying it. When the Moon is near the horizon, its light must travel through a much greater column of air than when it is overhead. This passage through the dense atmosphere causes two physical effects: refraction and scattering.
Atmospheric refraction, the bending of light through varying air density, actually compresses the Moon’s image vertically. The lower edge is refracted more strongly than the upper edge, causing the lunar disc to appear flattened or squashed near the horizon. This effect contradicts the perception of a larger, rounder Moon, confirming the atmosphere is not the source of the illusion.
The long path through the atmosphere also causes light scattering, which is responsible for the Moon’s distinctive color when it is low. Shorter, bluer wavelengths are scattered away by air molecules, leaving longer, redder wavelengths to reach the observer’s eye. This process gives the horizon Moon its characteristic yellow, orange, or reddish hue, enhancing the visual drama without causing the perceived size increase.
Distinguishing Real Change from Perception
The most effective way to understand the Moon Illusion is to use a simple test that bypasses the brain’s perceptual scaling. A direct measurement of the Moon’s angular size consistently shows that its physical size in the sky does not change as it rises.
One easy method is to hold a small object, such as a pencil eraser or a pinky finger, at arm’s length and cover the Moon when it is low on the horizon. Repeating this measurement later, when the Moon is high overhead and appears smaller, reveals that the same object still covers the Moon completely. This demonstrates that the apparent size difference is purely psychological.
Another method is to bend over and view the low Moon backward between your legs. This removes the terrestrial cues and often causes the illusion to instantly vanish. While the real change in the Moon’s size due to its elliptical orbit is minimal, the dramatic size change felt at Moonrise is a perceptual experience occurring inside the observer’s head.