In his book, Bad Astronomy, Philip Plait argues against the long-held myth that the moon appears larger when it's near the horizon. You've no doubt noticed this yourself when a full moon rises and looms ridiculously large, only to shrink an hour later. Scientists don't know exactly what's going on in our brains, but they do know that the whole thing is an illusion.

If you don't believe them, conduct your own test. Hold a pencil eraser at arm's length, and compare its size to that of the moon as it moves from the horizon and up into the sky. Your arm might get tired, but the moon will not shrink.

The illusion of a big moon is all in our heads and has to do with the fact that most people (whether they know it or not) perceive the sky as having a boundary. We imagine it's shaped more like a shallow cereal bowl than half of a true sphere. And so our brains tell us that the sky is farther away at the horizon than it is directly overhead. Our brains are further confused when confronted with two objects that appear to be the same size—the moon and the trees and buildings we see at the horizon, for example—the more distant object is interpreted as being larger. This is called the Ponzo Illusion.

A combination of these two phenomena lead us to believe that the moon is larger when it rises and the sun is larger when it sets.