Moon Illusion (Why the Moon Looks Bigger at the Horizon)

A moon that grows on the horizon

A full moon rising over the skyline can look enormous, far bigger than the same moon a few hours later when it's high in the sky. But if you photograph it at both times with the same lens and settings, or hold a small object at arm's length against it, the two moons turn out to be the same size. In fact the horizon moon is very slightly smaller in the photo, because when it's on the horizon you're further from it by about the Earth's radius, roughly 6,400 kilometres. The effect happens entirely in your perception.

No single settled explanation

Unlike most of the illusions on this site, the moon illusion has no single agreed explanation, even though people have been writing about it for more than two thousand years (Ross & Plug, 2002; Hershenson, 1989). Several accounts are still being debated, and the fairest answer I can give you is that more than one of them probably contributes.

Something to compare it with

One account treats the horizon moon like the Ponzo and Ebbinghaus illusions. On the horizon, the moon sits next to buildings, trees and hills, things whose size you know, and gets judged against them. High in an empty sky, there's nothing nearby to compare it with. A flat sea horizon offers very few objects for comparison, though, and the illusion is still seen there, so comparison can't be the whole story.

A flattened sky

A second account, which goes back centuries, says that the sky doesn't look like a true dome to us. It looks more like a shallow bowl, with the horizon seeming further away than the point directly overhead. Lloyd Kaufman and Irvin Rock found that it's the terrain between you and the horizon that makes the horizon seem far away, and that the illusion largely disappears when the terrain is hidden (Kaufman & Rock, 1962). An object of fixed size that seems further away gets scaled up by the same size constancy that stops distant people looking tiny, so the horizon moon looks bigger. One puzzle with this account is that when many people are asked which moon looks further away, they say the horizon moon looks closer, perhaps because once it looks bigger, it looks nearer too (Kaufman & Kaufman, 2000).

The angle of your eyes

A third account looks at your eyes instead of the landscape. In 1940, Alfred Holway and Edwin Boring found that the moon looked smaller when observers had to raise their eyes to see it and larger when they looked straight ahead, even when they lay down so that the overhead moon was straight ahead of them (Holway & Boring, 1940). Kaufman and Rock later argued that the effect of eye angle was too small to explain the full illusion, and most researchers now see it as a minor contributor at most.

See for yourself

Whatever the cause turns out to be, the moon illusion is easy to test for yourself. A pencil's eraser held at arm's length is about the same size as the moon, so compare the two when the moon is on the horizon and again when it's high, and you'll find the match is the same. Or try looking at the horizon moon through a rolled-up tube of paper that hides the landscape: for most people the moon shrinks, which supports the role of the terrain. The Ponzo and Ebbinghaus pages have tests that measure how strongly context changes your own size judgements, if you'd like to put a number on it.