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Big Bad Voodoo

Why Moon Appears Larger Near the Horizon

8 posts in this topic

http://www.technologyreview.com/view/509966/moon-illusion-new-theory-reignites-debate-over-why-moon-appears-larger-near-the-horizon/

The second is that it does not explain why the effect disappears in photographs and videos.

Then there is the question of why the illusion reportedly disappears when the world is viewed upside down, ie standing on your head. Having not tried this, I cannot vouch for its veracity. But come the next full moon, I fully intend to test it. So don’t be surprised to see some observers of the next full moon acting rather strangely.

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I always thought that it had to do with how the human eye processes the three dimensional picture, that's why it doesn't show up in pics or vids, because our eyes don't have the three dimensional 'field' involved when actually viewing the scene first hand.

Edited by Hasina

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This subject has been discussed before here, but it is an ongoing puzzle. I've read all sorts of explanations. One I like is, the horizon appears nearer to us than the sky overhead.

Sort of like this:

post-50472-0-87044000-1358557139_thumb.j

So the moon on the horizon would appear to be closer and larger than when it's higher in the sky.

I don't know the actual explanation, I'm not sure anyone does.

Edited by StarMountainKid

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I'm sorry, but I don't think that is a good article. It's not a hotly contested topic imo, and the use of statements like "Some people report.." is a giveaway to a poorly written piece. "Some people"' also report that the Moon is an alien base...

And I'm sorry, but that second point about videos and images is just wrong. Of course the illusion disappears in videos or images. In flat 2D images, you have no 3D depth perception, PLUS everyone knows that videos and still images are taken via a lens, and may be cropped - in other words, the amount of magnification is unknown. A long focal length (telephoto) lens setting will create a HUGE moon, while a short focal length (wide angle) one will create just a small dot. Examples:

Long telephoto (from http://cumbriansky.w...2/12/huge-moon/

moonrise.jpg?w=468&h=358

Compare that to this wide angle view (one of mine..):

gallery_95887_14_10722.jpg

(The bright spot is the Moon - it seems a little smaller, don't you think..? No illusion there, just different magnifications..)

Your EYE cannot zoom or use interchangable lenses! So of course any illusion will 'vanish' in an image where the observer doesn't know what magnification/zoom was used.

In simple terms:

- your brain has NO distance clues to the Moon's real distance, so it automatically compares it to other things in the immediate surrounds.

- when the Moon is near the horizon, there is 'stuff' in your field of view, so the brain makes an assumption about its size/distance.

- when the Moon is near zenith, there is nothing to compare it to, so it looks smaller.

Measuring the moon's angular diameter by eye so you can compare the 'difference' (there isn't any) is ridiculously simple (just use the width of your pinkie..!), so it is both patently obvious and easily testable that it is a simple illusion... I'd be most surprised if the illusion vanished if you were upside down but no, I'm not testing that one either..

Besides, there is a FAR better 'illusion'. Just as/after the Sun sets, if the Moon is also visible.. draw an imaginary line at right angles to the terminator (the edge of the illuminated area on the Moon) back towards the source of the illumination (ie the Sun). You'll note that if you draw a perfectly straight line in the direction it should be coming from, it will miss the Sun's actual location by a considerable margin... {cue Twilight Zone theme}

I'm happy to provide the explanation for this Moon Tilt Illusion (later!).. but let's just say that the effect is *not* actually an illusion...

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I'm sorry, but I don't think that is a good article. It's not a hotly contested topic imo, and the use of statements like "Some people report.." is a giveaway to a poorly written piece. "Some people"' also report that the Moon is an alien base...

And I'm sorry, but that second point about videos and images is just wrong. Of course the illusion disappears in videos or images. In flat 2D images, you have no 3D depth perception, PLUS everyone knows that videos and still images are taken via a lens, and may be cropped - in other words, the amount of magnification is unknown. A long focal length (telephoto) lens setting will create a HUGE moon, while a short focal length (wide angle) one will create just a small dot. Examples:

Long telephoto (from http://cumbriansky.w...2/12/huge-moon/

moonrise.jpg?w=468&h=358

Compare that to this wide angle view (one of mine..):

gallery_95887_14_10722.jpg

(The bright spot is the Moon - it seems a little smaller, don't you think..? No illusion there, just different magnifications..)

Your EYE cannot zoom or use interchangable lenses! So of course any illusion will 'vanish' in an image where the observer doesn't know what magnification/zoom was used.

In simple terms:

- your brain has NO distance clues to the Moon's real distance, so it automatically compares it to other things in the immediate surrounds.

- when the Moon is near the horizon, there is 'stuff' in your field of view, so the brain makes an assumption about its size/distance.

- when the Moon is near zenith, there is nothing to compare it to, so it looks smaller.

Measuring the moon's angular diameter by eye so you can compare the 'difference' (there isn't any) is ridiculously simple (just use the width of your pinkie..!), so it is both patently obvious and easily testable that it is a simple illusion... I'd be most surprised if the illusion vanished if you were upside down but no, I'm not testing that one either..

Besides, there is a FAR better 'illusion'. Just as/after the Sun sets, if the Moon is also visible.. draw an imaginary line at right angles to the terminator (the edge of the illuminated area on the Moon) back towards the source of the illumination (ie the Sun). You'll note that if you draw a perfectly straight line in the direction it should be coming from, it will miss the Sun's actual location by a considerable margin... {cue Twilight Zone theme}

I'm happy to provide the explanation for this Moon Tilt Illusion (later!).. but let's just say that the effect is *not* actually an illusion...

Then how do you explain that people from airplane said that they seen big moon near horizon?

Also when moon is almost in zenith and you stand in bottom of building still looks small. How come?

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Measuring the moon's angular diameter by eye so you can compare the 'difference' (there isn't any) is ridiculously simple (just use the width of your pinkie..!), so it is both patently obvious and easily testable that it is a simple illusion... I'd be most surprised if the illusion vanished if you were upside down but no, I'm not testing that one either..

Many years ago Patrick Moore conducted an experiment to demonstrate exactly this, to see whether the effect was genuine or simply one of perception. The experiment was elegant in its simplicity.

Volunteers on a beach (near Sir Patrick's house in Selsey if I remember correctly) were asked to look at the moon when it was near the horizon. They were then asked to pick up a pebble from the beach which, when held at arms length, was the same diameter as the moon appeared. These pebbles were collected in a bucket

A few hours later, when the moon was high above the horizon, they repeated this and placed these pebbles in a second bucket.

Even though the volunteers believed the moon had looked bigger when close to the horizon, when the pebbles in the two buckets were compared they were found to be the same diameter.

What this experiment showed is that it is our perception which is at fault and the moon actually appears the same size no matter how high in the sky it is.

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Then how do you explain that people from airplane said that they seen big moon near horizon?

Umm, what?? Isn't that exactly what we are discussing - that when near the horizon the Moon looks (but ISN'T) bigger..

I already explained that. The brain/eye combination perceives stuff, it doesn't record accurately. That's why optical illusions exist.

Also when moon is almost in zenith and you stand in bottom of building still looks small. How come?

Again simply because your perception is at work. What's more, everyone's perceptions will differ according to their life experiences, their training, their knowledge. Your brain KNOWS how big the Moon looks at zenith, so that's probably how you will perceive it, be it from open desert or near a building. But perceptions are very personal - it's a very complex topic, and throwing one-liners at it, and simply asking people what they think, is useless - especially if you are not accurately quantifying anything - may I repeat that:

As soon as you actually measure the moon size, whether by finger widths, pebbles, a ruler or a highly accurate telescope, the 'illusion' disappears. There isn't much point trying to analyse people's differing opinions if - when you introduce some meaningful measuring methodology - the difference completely vanishes.

I note you didn't comment on the issue of magnification in video or stills?

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I noticed the moon did this when looking at the "Supermoon" last year. Good to finally have an answer. :yes:

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