Goodnight Moon

Mostly copied from Palmer’s Vision Science: Photons to Phenomenology (2000) pg. 322-324.

The Moon illusion is the universal perception of the moon as being larger when it is located low in the sky (near the horizon) than when it is high in the sky. Just so you know, the moon is about 450,000km from earth and it isn’t actually bigger near the horizon.

There is no agreed upon explanation of why this is so, but here is the best one I’m come across:
We use depth cues available from the surrounding context of an object we are viewing to determine distance. When the moon is seen on the horizon, it is seen in the context of the ground terrain extending into the distance. When it is at its zenith, it is seen only in the context of stars and empty space. The “apparent distance theory” (Ptolemy to Rock & Kaufman, 1962) hypothesizes that this makes the moon appear further away at the horizon than at its zenith, as though the sky were flattened not spherical. When the same-sized retinal image is seen as farther away (horizon), it will be perceived as larger than when it is seen as closer (zenith). But, if asked when the moon looks closer, people usually say the horizon moon, which the theory hypothesizes to be registered as farther. The response to this challenge is to state that people are consciously judging the moon’s distance from its apparent size, after the unconscious process has done nearly the reverse. This would mean it is a two-stage process: an initial unconscious one and a later conscious one.

Support
1) On a moonless night, people judged an imaginary point at the horizon farther away than one at the zenith of the sky.
2) When observers viewed artificial moons (whose size could be controlled) through a screen that occluded the ground terrain, the standard illusion disappeared.
3) Using optic tricks, Kaufman and Rock were able to reverse the visual context of the two moons, projecting the horizon moon with its contextual ground terrain upward into the zenith position and the zenith moon with its surrounding empty sky down into the horizon position. The moon illusion then reversed.
4) They showed that a smaller appearing moon is always judged to be farther away, regardless of whether it is at the horizon or zenith. That supports the proposal that the moon’s apparent size governs the answer to questions about its distance.


Hope that helps a little. I'm sure we've all thought about it at some point. :)