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Jody Gomez

"Moon Illusion"

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These past few nights, as I've enjoyed the weather of approaching spring, and sat on my balcony watching the full, or near full moon rise, I've been reminded of the "moon illusion".

Many people think that when the moon is near the horizon, it appears larger due to the same reasons the setting sun does. In fact, it does not "appear" larger at all, it is our mind playing a trick on us. Its true apparent diameter does not change at all, as multiple-exposure photography shows us.

seattlemoon_stephens_strip.jpg

Some credit this to the Ponzo Illusion, but this explanation doesn't wholly fit, and we really don't know what causes this trick of the mind.

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Many people think that when the moon is near the horizon, it appears larger due to the same reasons the setting sun does.

But the reasons are the same.

In fact, it does not "appear" larger at all, it is our mind playing a trick on us.

Now I'm not sure about English/American usage, but I thought that "appear" could very well mean that our mind is playing a trick on us, that it seems to be larger, while it isn't.

Its true apparent diameter does not change at all, as multiple-exposure photography shows us.

In fact the setting moon is even somewhat smaller than a moon high in the sky, due to the greater distance between the observer and the moon. The effect is too small to be detected by the naked eye, however. The same is true for the setting sun, but here the effect is of course still much smaller and probably not measurable, due to the distance which is much greater than the distance earth-moon.

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Dragon-

You are correct about the sun, and I had intended to go back and add to that, but forgot to. I was talking about refraction and the distortion, or squishing of the sun in the vertical dimension due to the lower light traveling through more air than the light at the top of the sun.

As for "appear", I see what you are saying, and I think you understand the point I was trying to make. I used "appear" to refer to the information that the eyes receive. The information our eyes receive is not wrong, it's our brain that gives us the error.

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Jody,

Hmm, you made me think of an interesting point: is it truly an error? I guess this is an epistemological point, but in what respect is it an error? The sizes of the heavenly bodies look different in the sky and near the earth, but maybe the one near the earth gives a more actually realistic proportion of the other body to earth, perspective-wise. On the other hand, in the middle of the sky, the heavenly bodies are without context or reference to the size of anything else.

I don't know the answer!

Cheers,

Marsha

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Hmm, you made me think of an interesting point: is it truly an error? I guess this is an epistemological point, but in what respect is it an error? The sizes of the heavenly bodies look different in the sky and near the earth, but maybe the one near the earth gives a more actually realistic proportion of the other body to earth, perspective-wise. On the other hand, in the middle of the sky, the heavenly bodies are without context or reference to the size of anything else.

I don't know the answer!

But I do... It is an error while we conclude that the angular size of the moon at the horizon is larger than high in the sky, and that conclusion is incorrect. As I've said before, it's even slightly smaller when it's near the horizon. We're talking about a measurable characteristic. If we have the impression that A is bigger than B, but measurement reveals that B is bigger than A, then our impression was false, period.

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Many years ago I came across an article assessing the various theories about this, and decided (as did the author, I believe) that the following is the correct explanation (I don't remember who the article said first thought of it):

When you look up at the sky, you automatically envision it as bowl-shaped, the sky directly above your head seeming closer than the sky near the horizon. This is because the earthly objects at the horizon "push" the sky away from you visually, forcing you to see it as it is--something that is really miles away at least. This effect is absent above your head, unless some skyscraper is included in your visual field when you look up. (Somewhere, Ayn Rand mentions this: she refers to buildings "pushing the sky up" for one of her characters.)

Thus, when you see the moon against the bare sky above you, you think it is closer than when it is near the horizon, and since it occupies the same area in your visual field in both cases, the effect is that the moon is bigger on the horizon.

EDIT: Oh, I see this is the aforementioned "Ponzo illusion," but anyway I think it is correct. I'll leave this in, in case it might sway any remaining doubters.

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Marsha,

I'm sorry I missed your post here, it was another of those that did not show the orange, new-post blip on my radar screen. I understand your point, and I don't know the answer either! ;) For some perspective and further thought though, try this experiment...next full moon you happen upon, if your up for some sky gazing, when the moon is close to the horizon and appears larger, hold your index finger at arms length and impose its tip over the moon, noting the apparent size of the moon to the tip of your finger---then, when the moon is high in the sky perform the same experiment. The size will be the same. This is what I meant by error. This error of our perception of it's size.

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Everyone,

"Ignore the punk Jody, the guy was always full of shit and also soused with the fermented grape anyway."

And payback is a mother! :D/

And the moon is made of cheese. Brie I believe. Goes well with burnt kitten and a nice 2002 Renwood Voignier! (AKA - The Gomez Effect)

gw

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Never trust a man who chooses Brie. And if you need proof of my statement, just look to the french! Pick the wine first, then the cheese.

I thought you were going to send me a case of good Lemberger gw. Stop sending your trucks out in search of kittens, but instead send them east with some good "blood of jove."

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