Why does the moon look bigger near the horizon?

On Sun, 5 Mar 2000, nathaniel wrote:

> Hello. A friend of mine and I are having a violent arguement over the moon
> illusion. One of us thinks that the size of the moon changes due ot a
> matter of perception, while the other thinks it is due to light refracting
> throughtthe atmosphere. Who is right, and why? If the atmosphere doesn't
> alter the apparent size of the moon, why not?

Hello Nate,

The exact cause of the moon illusion is not due to refraction, but due to
a psychological perception of how large the moon appears to be. If
refraction were the cause, then the moon's size would change. But if you
are to carefully measure the moon's size, say, with your thumbnail, you
would see that the moon does not change size as it moves from the horizon
to overhead.

The atmosphere DOES refract the moon's light, and at the horizon the
moon's light is refracted quite a bit. But to make the size appear
different, the top of the moon and the bottom of the moon would have to be
refracted differently. The moon's angular size is small enough that this
differential refraction is small. Sometimes this differential refraction
is noticeable; for instance, the sun often appears squashed at sunset due
to this effect.

The real cause of the moon illusion (which, by the way, holds for
constellations, too) is psychological, but the exact origins of our
perception are unknown. Most of the theories for this effect deal with
the fact that, when the moon is at the horizon, we have other objects
(mountains, buildings, etc.) to compare it with, but overhead there are no
reference points. But the discussions quickly get complicated, and I've
never bothered to follow them.

So, in short, the moon illusion is truly just an illusion. The moon is
always the same apparent size (if accurately measured), whether it is on
the horizon or overhead.

Thanks for writing!

Sincerely,
Kurtis Williams

Back to the Ask An Astronomer page.