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Ufonaut99
2006-Jul-13, 10:59 AM
A friend asked me about why the moon looks bigger on the horizon rather than overhead. I replied about seeing reference objects like trees, etc.

Next, of course, I thought I'd check the source at BadAstronomy (http://www.badastronomy.com/bad/misc/moonbig.html), where this explanation goes into the "Bad Astronomy" bin :doh:

The nearest to a "real" explanation, apparently, is that we expend more effort on looking around us (survival technique) rather than "up" (where there's less danger).

So, my question is :
Do astronauts on ISS,etc - since they don't have an "up" - get the "big moon" illusion when they see the moon coming over the Earth's horizon vs. when its hanging clear?

jseefcoot
2006-Jul-13, 04:12 PM
The explanation described is a new one to me; I had always figured it to be a trick of the brain's perception, one of the explanations first offered to me for this phenomenon and the one that appears to me to have the most merit.

When the moon is high in the sky, we lose practically all visual context -- there is no basis for comparison regarding size. When it is near the horizon, it is close to items that we are very close to and are very familiar with, whose sizes are well known. I think that when the moon is high in the sky, and it lacks the context of ground features nearby, it just looks smaller because our brain literally perceives it as being much more distant than it actually is. Depth perception, I have been told, can be extremely reliant on peripheral vision. A high-in-the-sky moon offers very little for your peripheral vision to pick up on, and so might alter your brain's perception of its true distance, and hence, its true size.

Following this (probably flawed) logic, I would imagine that the moon always appears to be the same size to the astronauts aboard the ISS -- their context never deviates, they are always looking through a window, and the Earth and moon are always at a constant distance. The only possible exception I can imagine is for those who may be on a spacewalk, which changes the observer's context. However, If the moon appeared larger when it was near to the Earth and smaller when it was 'hanging clear', it kind of throws this approach out the window. I wonder if there are any observations to this effect made by astronauts aboard the ISS? I would think that if they did perceive a size difference in the moon based on its relative location to other objects in the sky, it wouldn't be because of their perception of what is 'up'. And doesn't the ISS spin to simulate gravity? What is 'up' in such a situation?

However, not being a true scientist, I can't really say. Perhaps someone on the forum can offer something more concrete?

Eventual
2006-Jul-14, 12:47 AM
A really cool link that explains the big moon illusion. (doesn't answer the question about what astronauts see, though)

http://science.howstuffworks.com/question491.htm

Jeff Root
2006-Jul-14, 04:16 AM
I think that when the moon is high in the sky, and it lacks the
context of ground features nearby, it just looks smaller because
our brain literally perceives it as being much more distant than
it actually is.
You meant "closer", not "more distant". If it were perceived
as more distant, then it would look larger.

-- Jeff, in Minneapolis

Glutomoto
2006-Jul-14, 07:14 AM
The astronauts see a different moon illusion, See space station science picture of the day where the moon looks like it is inside the earths atmosphere. (http://science.nasa.gov/ppod/y2003/06jun_moonset2.htm) There are also links to other moon illusions as seen from ISS.

wOw

:)

Bob B.
2006-Jul-14, 09:55 PM
So, my question is :
Do astronauts on ISS,etc - since they don't have an "up" - get the "big moon" illusion when they see the moon coming over the Earth's horizon vs. when its hanging clear?
I had the opportunity to speak with Larry DeLucas, who flew as payload specialist on STS-50 in 1992, about his trip in space. I recall him commenting that he thought the Moon looked smaller when viewed from orbit, however he was speaking generally. He did not specifically comment about the Moon's appearance near the horizon versus far from it.

jseefcoot
2006-Jul-17, 08:35 PM
You meant "closer", not "more distant". If it were perceived
as more distant, then it would look larger.

-- Jeff, in Minneapolis


????? The farther away something is the smaller it appears. Please explain to me how the moon would look larger if it were farther away.

Joff
2006-Jul-17, 09:02 PM
If you hold your arm out straight, your little fingertip will be the "same size" - occupy as much of your field of view - as the moon. Because you know your finger tip is closer, it looks smaller than the moon - rightly so, because your finger is in fact smaller than the moon.

There's three different things going on here:
- visual size; the subtended angle of view that the object occupies
- perceived distance; how far away we think it is
- perceived size; how big we infer the object is based on the first two.

and I've just made these terms up, so if there are better ones let's use them.

So using these defined terms:
Given that we all see the moon as the same visual size, the perceived size changes only according to the perceived distance. If the perceived distance increases - we think the moon is further away - perceived size also increases. If the perceived distance decreases - we think the moon is close (to take it to extremes, if our extended fingertip goes behind the moon) - perceived size decreases.

The words here are very tricky - I'm sure there must be an elegant phrasing....

George
2006-Jul-17, 09:56 PM
The astronauts see a different moon illusion, See space station science picture of the day where the moon looks like it is inside the earths atmosphere. (http://science.nasa.gov/ppod/y2003/06jun_moonset2.htm) There are also links to other moon illusions as seen from ISS.
wOw
:)
Wow, is right. Looks more like a lunatrick for a lunatick. Very interesting. Of course, I doubt it looks that small.

publius
2006-Jul-18, 05:14 AM
Try looking at the moon on the horizon by bending over fowards and look at through your legs. That makes the size illusion go away for many, and it does for me. The moon on the horizon looks like it does high in the sky.

By looking at it upside down, you're playing tricks on your brains sky vs ground processing somehow, and this does tend to go against the "comparison to foreground objects" hypothesis, as you'll see the same foreground, just upside down.

-Richard

Jeff Root
2006-Jul-18, 08:29 PM
jseefcoot,

Joff did a fine job of answering your question, but since you asked
me, I'll answer, too.

Look at an object. It takes up a certain amount of your field of
vision. Suppose you can't tell how far away it is, but you guess
that it is ten feet away. In that case, you say, the object must
be a foot across. But suppose you guess that it is is a hundred
feet away. In that case, you will say that the object must be
ten feet across. The more distant you think the object is, the
bigger you think it is.

-- Jeff, in Minneapolis

George
2006-Jul-18, 11:24 PM
Yep. Just look at any real fisherman's picture of their big fish. The fish is always a little further in front than it should be. [We'll ignore the BA's recent bass picture. :)]

dvb
2006-Jul-19, 03:36 AM
Try looking at the moon on the horizon by bending over fowards and look at through your legs. That makes the size illusion go away for many, and it does for me. The moon on the horizon looks like it does high in the sky.

By looking at it upside down, you're playing tricks on your brains sky vs ground processing somehow, and this does tend to go against the "comparison to foreground objects" hypothesis, as you'll see the same foreground, just upside down.

-Richard
Richard has it right. It's more than just our perception of nearby objects. You can demonstrate the effect by looking at any object on the horizon while upside down looking between your legs. All objects will appear smaller.

Think I first read about this in an astronomy book by Terrence Dickinson. Very good author for anyone getting started in astronomy.

George
2006-Jul-19, 03:43 AM
Think I first read about this in an astronomy book by Terrence Dickinson. Very good author for anyone getting started in astronomy.
I first saw it in the BA's book Bad Astronomy. It didn't work, however, when I tried.

publius
2006-Jul-19, 03:51 AM
I first saw it in the BA's book Bad Astronomy. It didn't work, however, when I tried.

Yep, it doesn't work for everybody, but it does for a lot. That demonstrates how things like this vary with the individual. For me, upside down through the legs, it looks as small as it does high in the sky.


-Richard

Joff
2006-Jul-19, 04:06 PM
Perhaps, George, you can read upside down. Of course if publius can too that wrecks the theory.

George
2006-Jul-19, 05:08 PM
Perhaps, George, you can read upside down. Of course if publius can too that wrecks the theory.
Hold on, let me verify. *flips monitor* The moon looks the same, but I am having trouble with this thread. ;)

jseefcoot
2006-Jul-19, 06:35 PM
jseefcoot,

Joff did a fine job of answering your question, but since you asked
me, I'll answer, too.

Look at an object. It takes up a certain amount of your field of
vision. Suppose you can't tell how far away it is, but you guess
that it is ten feet away. In that case, you say, the object must
be a foot across. But suppose you guess that it is is a hundred
feet away. In that case, you will say that the object must be
ten feet across. The more distant you think the object is, the
bigger you think it is.

-- Jeff, in Minneapolis

That is so much more clear, thank you. I didn't really get it with Joff's explanation, but I didn't exactly realize that you were both saying the same thing at first. Like Joff said, the phrasing is tricky. . . . .I thought you were saying "the more distant an object IS, the bigger it looks" when you were actually saying "the more distant an object APPEARS, the bigger it looks". Big difference to overlook, thanks again for setting me straight.