Run for cover, it's the MOON!

Why does the moon (and the sun for that matter) loom so large on the horizon yet seem smaller once it’s found it’s proper place overhead? Is there some sort of atmospheric refraction going on? And would this effect work on on my … appendage?

Here’s an experiment:

When the moon is near the horizon, hold a quarter in front of your face so that it just barely covers the face of the moon. Do this again when the moon is high in the sky. You’ll find that the quarter is the same distance from your eyes, no matter when you check it. The moon’s relative size does not change.

The reason it looks larger close to the horizon is that you have something to visually compare it to, like trees and buildings. When it is higher up there is nothing around it but that huge empty sky.

Papabear has it right (as usual).

Cecil covered this in depth in More of the Straight Dope. Unfortunately, I can’t find a link to the column in the Classic columns on this site.

The moon illusion has been observed at sea, where there’s nothing on the horizon to compare it to. Last I read about it, which I’ll grant you was years ago, the illusion was unexplained.


Love Poems -

Yea, I considered that one, PB, but I notice it mostly on the boat out at sea. Well, out at lake, actually.

Say it isn’t so, coffeecat. Unexplained phenomena in the land of Cecil? Where are you, UnDead Dude?

Excuse me, the sea is on the horizon. When you look out at the sea, you get an impression of how distant it is (largely due to the ripples which would be visible due to the moon’s shine). Same effect.

As I recall Cecil saying about the experiment, (paraphrasing) try it and if you’re still convinced the moon is larger on the horizon, then continue to make your case. He recommend making calipers out of a paperclip, but it should work the same as PapaBear’s quarter.

Carpe hoc!

“And would this effect work on on my … appendage?”

Of course it does.
Do not use your appendage during a full moon.

The explanation seems to be something like this.

We all know that the Earth is round, that it rotates on its axis, and that space goes on out there for what might as well be infinity. But when we’re out in the back yard, what we see is a flat world, a sun and moon that go around it about once a day, and a thing we call the “sky” that’s hung over us like a roof.

Now, for some reason or other, we don’t perceive this roof thingie as a hemisphere. Instead, we see it as something much flatter, more like a sports-arena dome. Now, this being the case, when the moon is near the edge of this sky-roof, it’s logically much further away from us than when it’s up high. But it looks the same size (if you measure it). Therefore, it must “logically” be the case than when it’s lower, it’s bigger.

It’s ultimately the same principle as any number of false-perspective illusions you can draw on paper, but in this case it’s the imaginary sky-dome, rather than an imaginary perspective, that’s at fault.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

Cecil offered this proof it’s an optical illusion: look at the moon near the horizon through a cardboard tube. The illusion disappears whether you’re on land or on the sea.

The naysayers need to read Cecil’s explanation before they argue any of these points.

Nick, I haven’t been able to find the column in the archives where Cecil covered this. Have you had any luck?.

>>Now, for some reason or other, we don’t perceive this roof thingie as a hemisphere. Instead, we see it as something much flatter, more like a sports-arena dome>>johnwkennedy

For a pretty good, if rudimentary, explanation of the Ponzo Illusion and the “flattened dome” effect (and why it is not just “objects are next to the moon on the horizon”), see:

No, it does not appear to be there.
(Shameless plug)
One more reason to buy the books, kids!

In a nutshell, Cecil says it’s a psychological optical illusion entirely dependent on visual cues provided by the terrain when the moon is near the horizon, and the lack of visual clues when it’s high in the sky. He offers various proofs such as the paper clip calipers and the cardboard tube I mentioned earlier.

One rascal argued it was really a magnifying effect of the atmosphere, but Unca Cece shut his ass down as willfully ignorant in refusing to go out and look at the incontrovertible proof those two simple devices reveal.

Thanks for thinking of me, but it looks like this was well covered. It really does come down to perspective. The light of the moon is refracted as it enters the atmosphere also, but any size change caused by that would be miniscule in comparison with the illusion that people see.

Well, if I haven’t said it before, I’ll say it now: Thanks folks!

Well, you’re welcome, but the credit should go to Papabear who answered the question thoroughly and correctly in the very first post, as MKIA pointed out.

While you are slinging honorable mention, some should be slung in the direction of the astute Mr. Kennedy for introducing the “flattened dome” concept, which is an important link to understanding this phenomena, and is not wholly related to trees and buildings on the horizon.

Sorry. An honorable mention to the flattened dome analogy of JWK, which was indeed cited in Cecil’s column as having been first posed by Ptolemy in the 2nd century A.D.
Unca Cece relegated it to “Ok, as far as it goes.”

This particular thread reminds me of an auto commercial I saw several times where a very large moon asks about the SUV this couple is driving. I always thought it should end by the moon snatching the couple up with a shiny black prehensile tongue and eating them.

Now back to your regularly scheduled thread.

Cave Diem! Carpe Canem!