The Moon's massive horizon appearance is actually a trick of the mind
In short: When the Moon rises over a cityscape or mountains, it can look breathtakingly huge. You might assume the Earth's atmosphere is magnifying it, but this is a persistent myth. In reality, the Moon's width remains constant; the perceived change is an illusion rooted in how our brains process visual information.
The phenomenon where the Moon appears much larger near the horizon than when it is high in the sky is known as the Moon illusion. Despite humans observing this for thousands of years, scientists still lack a fully satisfying explanation for exactly why this occurs. While it is a common misconception that the atmosphere magnifies the Moon, photographic evidence proves that its width remains the same regardless of its position in the sky. This debate has persisted for several millennia. Even Aristotle discussed this concept in the fourth century BC.
In fact, the atmosphere can actually make the Moon appear slightly squashed in the vertical direction when it is low on the horizon, acting like a weak lens. When the Moon is near the horizon, it often appears to loom over trees, mountains, or buildings. However, you can debunk the illusion by looking at the Moon through a paper tube or by comparing two photographs taken at different altitudes with the same zoom settings; in both cases, the Moon's actual width is identical. You can test this yourself by holding up your index finger. Alternatively, try peering through a paper tube.
There is one physical difference in the Moon's appearance when it is low in the sky: its color. Near the horizon, the Moon often takes on a yellow or orange hue. This happens because the light must travel a longer distance through the atmosphere to reach your eyes. During this longer journey, shorter blue wavelengths are scattered away, leaving behind the longer, redder wavelengths that reach the observer. This colour shift occurs due to light scattering. This process leaves more long-wavelength light for us to see.
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Sources
- Summary by Intellect · source: Why Does the Moon Look Bigger on the Horizon?
- Also: The Moon Illusion: Why Does the Moon Look So Big Sometimes? - NASA Science
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