The bizarre Moon illusion

Does anyone know why the Moon appears so big near the horizon? Anyone?


Now, this one’s trickier than it sounds. You may have noticed that a rising or setting Moon on the horizon appears larger than an overhead Moon, but scientist and psychologists still can’t agree on why it occurs. Even NASA can’t fathom it, so just what is going on?

There are two main theories behind what’s been dubbed the ‘Moon illusion’. We know the size of the Moon doesn’t actually change so we can safely assume that it’s a trick of the mind. Also, while the Moon’s distance from Earth gets regularly shorter and longer, the difference in size from this change is negligible to the human eye. No, to find out the answer to this one we have to delve into the human mind.

One idea suggests that we instinctively attempt to judge the distance to a rising Moon (it’s hard to comprehend 400,000km) based on visual objects, such as trees and houses in the distance. These objects seem near the Moon, giving a distorted point of reference, making it appear bigger. However, this theory can be called into question as pilots have also seen the illusion despite no point of reference against the ground.

Our brains often incorrectly perceive the sky as a flattened dome rather than the spherical shape it really is. This might mean our brains are tricked into thinking an overhead Moon is closer than a Moon on the horizon, and thus they compensate to make the latter bigger.

The second theory has to do with the fact that we tend to think of the sky as a flattened dome, rather than the hemisphere it is, and therefore perceive things overhead (birds and planes) as much lower, or nearer the things we see on the horizon. And so although the Moon may well be the same size whether it’s above your head or off on the horizon, you perceive the Moon to be much larger.

The Ponzo track, above, is an illusion in which two lines of the same length appear to be different sizes due to the positioning of the railway lines. Check them with a ruler if you don’t believe us – they’re the same length, although they might not look it.

Whichever theory holds true, either way your brain has been tricked. It’s just not quite clear which one is right… What do you think? Let us know below.

Images courtesy of Jodrell Bank and NASA.

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