[Brendan Wiltse]

Brendan Wiltse asked the Naked Scientists:

I recently listened to a radio program where they were discussing why the moon (or sun) looks bigger on the horizon.  It is also interesting that it
doesn't always appear larger on the horizon, only sometimes.  It seems as
though this is a debated topic, do you have any ideas or explanations as to what is going on?

What do you think?

[techmind]

The "moon illusion" is well-known, and it is caused by the way the brain interprets what we see, rather than any real physical principle. As to why we interpret the horizon moon as being larger, this is still hotly debated.

There's a Wikipedia page here: http://en.wikipedia.org/wiki/Moon_illusion

And I found some other explanations (or refuting of explanations) here: http://www.lhup.edu/~dsimanek/3D/moonillu.htm


In my personal experience, the moon seems significantly larger when low in the sky only when there's known objects very nearby, eg houses just across the street.

Without reading anything about it, my guess would be that the moon seems a fairly small object in the sky when high up because the open sky is a vast thing to take in (I get a sensation of not knowing quite where to look), and it's not broken up by anything - the moon is only a small part of it. But for terrestrial objects we're used to directing our attention to quite small angular detail (such as bricks within a chimney of a nearby house) and from this frame-of-reference the moon seems large. (Our brain cannot comprehend how huge the moon really is, or how far away it is.)
In other words, for the open sky, our frame of reference is huge (uncomfortably so) whereas our visual system is much more inclined to concentrate on only a fairly narrow field of view for terrestrial objects and structures. When the moon is low on the horizon (near other buildings/trees etc) we latch on to angular field-of-view which contains a "managable" amount of structure (detail), and compared to this reference, the moon seems larger. Just my two-penny-worth.


It might be interesting to do an experiment with an eye-tracker when an observer is viewing a high vs horizon-moon against a realy sky/horizon. I would hazard a guess that the eye lingers longer over a smaller angular field of view for the horizon than for the sky. But this is only my personal guess...

[techmind]

The moon, as recorded a few minutes ago:

 [ Invalid Attachment ]


but it's high in the sky and this is how small it is when you see the whole photo (at max optical zoom on my camera):

 [ Invalid Attachment ]

(ok, proves nothing, but an excuse to take some pictures  [])

[wiltseb]

I was the one that originally posted this question.  I have read/heard about this illusion, but it seems as though there is little documentation of it.  Also, what is the threshold of the size/distance of the object(s) that have to be in the same field of view to make the moon or sun look larger?  Are their other instances beside the moon or sun where our brain will perceive a distant object as larger than it really is?