But when the moon is rising over a distant horizon, we perceive it to be farther away and therefore larger. In the 11th century, Arab mathematician Ibn Al-Haytham developed the first plausible theory for how the moon illusion works, suggesting that the size difference has to do with how our brains perceive distance, and then how we automatically adjust an object’s apparent size to match.Īl-Haytham suggested that when the moon is overhead, we perceive it to be closer and therefore smaller. Still, the actual angular size of the lunar disk during a single moonrise will always be the same. That changes by a minuscule amount between lunar cycles, with the moon’s apparent size getting up to 14 percent larger than normal during its closest approach to Earth. The moon always occupies roughly 0.52 angular degrees on the sky, or about the size of a thumb tip held at arm’s length. You’ll find that the moon never shrinks or expands inside that circle. Tape the rolled-up paper at that size and use it to view the rising moon. Or, when the moon rises, roll a piece of paper so that when you look through the tube, the paper just hugs the humongous moon. You’ll see that there is exactly zero difference. When the moon is high in the sky, go back and compare the size of the moon’s disk in your photos. Set a camera on a tripod and snap multiple images of the huge moon rising. More recently, psychologists began to appreciate that the huge horizon moon is a true trick of the imagination-and it’s relatively easy to see for yourself that things are not as they seem. It may change the color of the moon, depending on how particles bend and filter moonlight, but that’s about all it does. However, we now know that Earth’s atmosphere does nothing of the sort. The Greco-Egyptian astronomer Ptolemy suggested something similar in his famous treatise Almagest, published during the second century A.D., as did Greek astronomer Cleomedes around the same time, though they both also ascribed the phenomenon to a change in the moon’s apparent distance. Lunar Trickeryīack in the fourth century B.C., Greek philosopher Aristotle suggested that Earth’s atmosphere might be enlarging the image of the horizon moon, just as water can make immersed objects seem magnified to our eyes. Since long before the moment Neil Armstrong took his "one small step" in 1969, humans have been mesmerized by the moon.
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