The Moon Illusion

When you see the moon rising or setting over the landscape, it seems so big and close that you could reach out and touch it. Then a couple of hours later when it's high in the sky, it seems so much smaller! Why does the moon look so huge on the horizon? The moon stays the same, but your brain experiences an optical illusion.

One of my favorite brain-benders is the Ponzo Illusion. You’ve seen it: the simplest case is with two short horizontal lines, one above the other, between two slanting but near-vertical lines. The upper line looks longer than the lower line, even though they’re the same length.

The illusion works because our brains are a bit wonky. The slanted lines make us think that anything near the top is farther away; the lines force our brain to think those lines are parallel but receding in the distance (like railroad tracks). The two horizontal lines are physically the same length, but our brain thinks the upper one is farther away. If it’s farther away, then duh, our brain says to itself, it must be bigger than the lower one. So we perceive it that way.

See examples of how this works at Bad Astronomy Blog. Link