Why Does the Moon Look Bigger on the Horizon?
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.