The question of why the Moon looks bigger on the horizon than when it’s high in the

sky has been debated for several thousand years. One popular myth, dating all the way

back to Aristotle in the fourth century BC and which still endures somewhat today, is

that it is simply a case of magnification caused by the Earth’s atmosphere.

While a “magnification” effect is taking place, it actually is going the other way

and is more of a compression. Atmospheric refraction causes the Moon to appear slightly

smaller in the vertical axis when it is near the horizon vs. when it is high in the sky.

This refraction, combined with the fact that the Moon is about 4,000 miles farther away

when it is on the horizon, causes it to appear 1.5% smaller, if you were to measure very

precisely its apparent size on the horizon vs. higher in the sky.

So if it’s not magnification from the Earth’s atmosphere, what is going on here? In short,

the Moon appearing bigger near the horizon is nothing more than an optical illusion.

It’s really as simple as that. You can verify this fact by taking a pair

of calipers, or even just a ruler, and measure the diameter of the Moon on the horizon; later

that night, when it is higher in the sky, measure it again. (Be sure and hold the measuring

device at the same distance away from your eyes each time to get accurate results.) If

you do it precisely enough, you’ll find that it actually will measure about the same

size both times, despite appearing nearly twice as big to your brain when it is on the

horizon. Exactly what is going on in our brains to

cause this optical illusion is still somewhat up for debate, but it seems to center around

size consistency, where our brains are trying to grapple with the size of an object vs.

how far away it thinks it is. For instance, when you see people very far away from you

and their heads look incredibly tiny, your brain doesn’t think for an instant that

the people and their heads are actually tiny. Your brain adjusts your perception based on

what else is in your vision to gather that third dimension of depth.

Something of this same effect is thought to be happening with the Moon, only this time

your brain is getting tricked into thinking that the Moon is farther away when it is on

the horizon, making it appear bigger to you. This is known as a Ponzo Illusion, named after

Italian psychologist Mario Ponzo. Mario Ponzo first demonstrated the “Ponzo

Illusion” in 1913. In this experiment, Ponzo drew two converging vertical lines on a piece

of paper. He then drew two horizontal lines crossing these lines, one at the top, and

one at the bottom. These two horizontal lines are the same length, but it appears that the

one at the top is much longer because it looks farther away. (See image in the show notes)

This is due to our brains interpreting the two converging lines as parallel lines that

only appear to be converging because they are getting farther away. Thus, if both horizontal

lines are making the same length “imprint” on our eyes, but one is farther away, then

the one that is farther away must be much larger, so our brains perceive it as larger

than it actually is. So, in the end, it’s a matter of our brain’s

getting tricked in terms of the distance from the Moon to us when it’s on the horizon

vs. high in the sky. When it’s on the horizon, the brain has reference points to compare

and judge the distance, and similarly adjust the apparent size based on that perceived

distance. When it is high in the sky, there is nothing useful to compare it to, so the

apparent size changes based on how far away our brain thinks it is at that point, namely,

thinking that it is closer to us high in the sky and farther away on the horizon.