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View Full Version : Why do celestial objects appear larger close to the horizon?



AK
2004-Apr-08, 04:04 AM
In particular, the moon.

I've heard various explanations for this phenomenon but none that satisfied me.

Brady Yoon
2004-Apr-08, 04:12 AM
Well, there's a whole page on the main site about this, so if you haven't already, take a look. :D

The fact of the matter is that we don't know why. It's a very powerful illusion.

http://www.badastronomy.com/bad/misc/moonbig.html

The link has more links that might help shed some light on the matter.

Ut
2004-Apr-08, 05:11 AM
I've always been a fan of the following theory:

When you (well, when I do anyway) look at the sky, the horizon seems to be farther away than the point directly above your head. If you pretend the Earth's flat for a moment, then you can almost picture the sky in your head as being a sort of flattened semi-sphere. So, you'd expect to see things on the sky near the horizon as being smaller than you would when they're overhead (since, you know, they're essentially 2d objects attached to the sky). The Moon looks the same size at all altitudes, though.

So, let's assume that the sky at the horizon is 2x as far away from you as the sky directly above your head. If you see one Moon on the horizon, and a second Moon directly over head, both visually the same size, then you assume that the one on the horizon is 2x as large as the one overhead (since it is ON the sky, and thus has to be twice as far away). So, your brain tells you it's bigger.

That's just one of the ideas that's out there. I think it probably holds as much weight as any of the others, but it's the one that makes most sense to me. So it's my favourite.

milli360
2004-Apr-08, 08:36 AM
Last week, just before the full moon, my daughter and I were driving due East and I saw the moon low on the horizon. I said "wow, look at that moon," and she said "whoa! it looks huge!"

It looked ordinary to me. I was a little jealous. :)

Ricimer
2004-Apr-08, 03:07 PM
in brief, its due to the mechanisms by which the brain relates size and distance.


The moon has an angular diameter of about half a degree, regardless of where you see it in the sky.

Now, on the horizon you tend to see familiar, every-day objects, like, say, a tree.

A tree, on the horizon, is really quite small. But you know it to be pretty big when you get close to it.

The moon, at the horizon, dwarfs the tree in comparison. And as such, the moon is much, much bigger than that tree! So it must be huge you mind says.


When its straight up, there's nothing to compare it to, and so it's measly 1/2 a degree seems small.

skrap1r0n
2004-Apr-08, 03:12 PM
hmm I always thought that the atmospere acted as a lens thus magnifying images. I haven't read the BA page on this yet, but thats the impression I have been under.

Normandy6644
2004-Apr-08, 03:32 PM
hmm I always thought that the atmospere acted as a lens thus magnifying images. I haven't read the BA page on this yet, but thats the impression I have been under.

I don't the effect is particularly noticeable though. IIRC the magnification is negligible compared to the actual size of the moon. I don't quite agree entirely with the whole "perspective" explanation, but it's the best one so far.

Ricimer
2004-Apr-08, 03:39 PM
some examples of the perspective based illusions:

http://www.yorku.ca/eye/perspect.htm

milli360
2004-Apr-08, 04:02 PM
hmm I always thought that the atmospere acted as a lens thus magnifying images. I haven't read the BA page on this yet, but thats the impression I have been under.
The distortion by the atmosphere is not to any great extent. Now, where's that BA page again...

I think he mentioned the possibility of a website search function on the BABB pages. That'd be nice.

Dgennero
2004-Apr-08, 04:25 PM
Oddly, while to me the moon doesn't look much bigger when near the horizon (maybe I "taught" my brain to ignore this as an illusion), when I see Jupiter through 16 or 20 power binos, though it *should* appear half the size of the moon as visible with the naked eye, it looks much smaller to me. Same with crescent of Venus.

milli360
2004-Apr-08, 04:46 PM
Oddly, while to me the moon doesn't look much bigger when near the horizon (maybe I "taught" my brain to ignore this as an illusion), when I see Jupiter through 16 or 20 power binos, though it *should* appear half the size of the moon as visible with the naked eye, it looks much smaller to me. Same with crescent of Venus.
Does a marble (http://www.mastersgames.com/rules/marbles-rules.htm) look a fifth of the size of a tennis ball (http://hypertextbook.com/facts/2000/ShefiuAzeez.shtml) to you?

Ut
2004-Apr-08, 04:59 PM
in brief, its due to the mechanisms by which the brain relates size and distance.


The moon has an angular diameter of about half a degree, regardless of where you see it in the sky.

Now, on the horizon you tend to see familiar, every-day objects, like, say, a tree.

A tree, on the horizon, is really quite small. But you know it to be pretty big when you get close to it.

The moon, at the horizon, dwarfs the tree in comparison. And as such, the moon is much, much bigger than that tree! So it must be huge you mind says.


When its straight up, there's nothing to compare it to, and so it's measly 1/2 a degree seems small.

Actually, there's been at least one study that contradicts this idea. I don't know how strongly it made its point, or how strong its legs were, but it seems that super-imposing a horizon on a picture of the Moon when it's high in the sky doesn't change how we see it. Granted, I personally don't see super-imposing images on a tv/computer screen as a fully valid method of falsifying the idea. It just kind of throws out a "it could be any number of things" into the discussion.

Ricimer
2004-Apr-08, 06:05 PM
Oh, I'll be one of the first to agree, its likely more complicated and involved than just pure perspective. The brain processes things in some odd ways afterall.

But the perspective is IMO a significant piece of the puzzle (probably the majority).

But I've also seen perspective demonstrations fail. There are a few out there that the two objects always appear the same, despite a lot of the classic tricks. Why, I don't know, there's probably some subtle clue there letting me key in on the fact.

The Bad Astronomer
2004-Apr-08, 10:30 PM
I believe there is a sufficient explanation of this.

1) If you have two objects of the same angular size, the one that the brain perceives as farther away will look bigger. This is called the
Ponzo Illusion (http://www.google.com/search?q=ponzo+illusion).

2) We do not perceive the sky as an inverted hemisphere, but more of a bowl shape. The horizon is interpreted as farther away than the zenith. Cloudy days reinforce this illusion, as clouds overhead are much closer than clouds on the horizon.

3) When the Moon is on the horizon, the brain interprets it as farther away. But due to the Ponzo Illusion, it also interprets it as being much bigger.

Voila. By the way, I wrote an article on this very topic for the premier (May 2004) issue of Night Sky Magazine (http://www.nightskymagazine.com), an astronomy magazine for beginners to the hobby. 8)

George
2004-Apr-08, 10:42 PM
...
Voila. By the way, I wrote an article on this very topic for the premier (May 2004) issue of Night Sky Magazine (http://www.nightskymagazine.com), an astronomy magazine for beginners to the hobby. 8)

What you wrote in your Bad Astronomy book was excellent. 8) [That reminds me, I haven't put my head between my legs yet as I promised but my home is in a little valley :) ]