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Have you ever noticed that the Moon sometimes looks bigger when it is low on the horizon? Do you know why? Well, to be honest, scientists don’t really know exactly why this happens, either. This “Moon illusion” has perplexed people for millenia: it was described in early Greek and Chinese writings, and Aristotle wrote about it in 350 BCE. There are some theories and speculations, but one idea seems to make sense: it is purely an optical illusion.
Our mind puts images into size and shape categories in order to interpret them. This is referred to as shape and size constancy. Shape constancy can be described like this: look at any door frame and you can see that it is rectangular. If you were to sketch the same door frame from the angle that you are looking at it, most likely you would sketch a trapezoid. Your mind adjusts the door so that you perceive it as a rectangle from whatever angle you look at it.
Shape constancy reads out like this: Take two exact duplicate objects. Place them at opposite ends of a hallway. The objects are the same size, but you perceive the one in back to be bigger; because your mind sees the hallway and decides that the back object is farther away than the front, so your mind adjusts the size to make up for the increased distance.
The angle that the full Moon subtends at an observer’s eye can be measured directly to show that it remains constant as the Moon rises or sinks in the sky. A simple way of demonstrating that the effect is an illusion is to hold a small object, like a coin(quarter maybe) at arm’s length with one eye closed, and position it next to the Moon. Do that when the Moon is on the horizon and when it is high in the night sky and you will see that there is no change in size.
We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.