
Why does the Moon look so huge on the horizon? - semmons
http://blogs.discovermagazine.com/badastronomy/2010/05/13/why-does-the-moon-look-so-huge-on-the-horizon/
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torrenegra
As far as I knew the illusion was caused by light scattering and refraction:

"The distance from the surface to 300 mb averages 9,300 gpm. When the sun is
directly overhead, radiation passes through 9,300 meters of atmosphere between
the surface and 300 mb. When the sun is at an angle, it must pass through a
greater length of the atmosphere before reaching the surface. When the sun is
passing through the atmosphere at a 45 degree angle, the radiation must pass
through 13,000 meters of atmosphere between the surface and 300 millibars.
When the sun is passing through the atmosphere at a 5 degree angle, the
radiation must pass through 107,000 meters of atmosphere between the surface
and 300 millibars.

"The amount of scattering and refraction differential depends on how far the
sun's radiation must pass through the atmosphere. As the length of passage of
light through the atmosphere increases, the amount of scattering and the
refraction differential increase. When the sun or moon are within about 10
degrees of the horizon, they have the illusion of looking much bigger than
they do higher in the sky and the coloring looks different. This increase in
scattering is also the cause of sun sets and sun rises giving a red color on
the horizon. Although the size looks different when close to the horizon, it
is mostly an illusion."

Taken from <http://www.theweatherprediction.com/habyhints/215/>

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TallGuyShort
That's what I thought - but the illusion in question doesn't work if you take
a photograph. It's purely psychological.

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xtacy
Yes, if you take a photograph, a dollyzoom like effect comes into place.

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dedward
It's a purely psychological effect.

When it's way overhead - it's the moon. You've seen it all your life. It's
just there.

When it's on the horizon - your automatic distance-judging machinery (we're
talking more than just parallax and depth perception here - I mean the whole
process) says "hey that thing is really far away, and way, way past the
horizon, way further away than those mountains, so it's obviously really,
really big by comparison (which is completely true).

Humans perceive the world more or less on a horizontal plane - so we interpret
things in that plane differently than we do things above/below us.

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jonsen
A camera is not susceptible to psychological effects i believe. Will it be
bigger on a picture?

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mcantor
The camera isn't, but our brain still is when we process the picture taken by
the camera.

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Sthorpe
But the key point is having some other objects in the picture that will give
you the perspective change.

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qwzybug
If you're into this sort of thing, I highly, highly recommend M. G. J.
Minnaert's Light and Color in the Outdoors: <http://www.amazon.com/Light-
Color-Outdoors/dp/0387979352>

The harvest moon is one of his favorite illusions, and there just tons of
little nuggets like this. It reads like an optical Wittgenstein: a series of
short, numbered observations about the way light and vision work that you'd
never have thought of, but once you read, you can't stop noticing.

E.g.: you can always predict exactly where a rainbow will be, you can tell
leaded glass at a glance, wire meshes only cast vertical shadows, the green
flash is very real, sunsets look _exactly_ like sunrises, you can navigate by
a polarized lens, hills look steeper from above for the same reason that
steeples look shorter than the shadows they cast on a cloud (related to the
harvest moon), and the line of the sun on the ocean always points right to
you. Have you ever noticed atmospheric perspective, or how drastically colors
change with distance? You will after reading this book.

Minnaert makes some interesting claims about the harvest moon in particular.
For example, lie on your back and look at the sky for a while: the flattened
dome effect tends to wash out rather quickly, and the moon looks roughly the
same size in any direction. He also claims that binocular vision is closely
related to the perceptual flattening of the celestial dome, and one-eyed
people do not perceive the harvest moon. I'm not sure if he's totally correct,
but it's interesting to think about, isn't it?

It changed the way I look at the world. I believe scientific consensus has
moved on in certain areas in the years since it was originally composed, but
Minnaert's careful thought has aged superbly. Definitely worth it.

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Groxx
This explanation works, but there's another: On the horizon, you have things
to compare it against. It looks the size it _is_. In the middle of the sky,
there's nothing but a large black field with a small circle: it looks smaller
than it is, because it's overwhelmed by the larger field. It's a _very_ well
known effect in the art world.

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sumeetjain
The next time you see the moon like that, turn your head sideways: It won't be
so big anymore.

The psychological effect depends on the horizon. Tilting your head to the side
alters the horizon's alignment, so the moon next to "some straight, vertical
line" doesn't appear as big.

