# Thread: Since we think of the moon as being farther away than the trees and buildings seen wi

1. ## Since we think of the moon as being farther away than the trees and buildings seen wi

This might need some scientific knowledge, and my first feeling was "If the moon is compared to small objects around it on the horizon, due to the relative size, it may look bigger" but this is saying "the farther the moon, the larger it looks", which is unclear to me. If someone understands it clearly, please let me know.

ex)An illusion that has puzzled man for hundreds of years is the moon illusion - the fact that the moon looks larger near the horizon than when it is overhead. In actuality, of course, the moon does not change size, and the image cast on the retina is the same whether the moon is overhead or at the horizon. What, then, is the cause for such a difference? One theory holds that when the moon is seen in context - that is, in relation to its background - objects in the background provide distance cues. Since we think of the moon as being farther away than the trees and buildings seen with it on the horizon, we perceive it as larger. In other words, if the retinal image of an object remains constant, an increase in apparent distance will produce a corresponding increase in perceived size.

st73

2. ## Re: Since we think of the moon as being farther away than the trees and buildings see

I don't think this explanation is very good. We perceive the moon as larger when we see it compared to other objects on the horizon. Out by itself overhead, the moon is a small part of the picture we see. On the horizon, it can look the same size as a building (or trees) off in the distance. But we know the moon is much farther away than the building. Since the building and the moon are about the same size (but we know the moon is farther away), the moon seems much larger than when it is overhead.

(And personally, I don't "think of the moon as being farther away," I know it is.)

3. ## Re: Since we think of the moon as being farther away than the trees and buildings see

Is it because the trees and buildings are big enough to give the same-sized moon a bigger image than when it's along with small stars around it? I mean, big or small things around the moon can affect its illusion? Maybe the explanation is awkward...

4. ## Re: Since we think of the moon as being farther away than the trees and buildings see

Originally Posted by keannu
Is it because the trees and buildings are big enough to give the same-sized moon a bigger image than when it's along with small stars around it? I mean, big or small things around the moon can affect its illusion? Maybe the explanation is awkward...
It's a bad explanation. I don't think you need to make sense of it.

PS: Any explanation that calls the trees and buildings the moon's "background" doesn't warrant too close a reading.

5. ## Re: Since we think of the moon as being farther away than the trees and buildings see

It is poorly worded. The point they are trying to make is... if you saw an object located behind a building that you know to be in the distance, you would intuitively know that this object must be bigger than the building. Therefore the moon "must be" (i.e. perceived) bigger with context, compared to no context high above.

I've never understood or believed this explanation, as the moon looks the same size to me when it's high or low in the sky.

6. ## Re: Since we think of the moon as being farther away than the trees and buildings see

Originally Posted by BobSmith
I've never understood or believed this explanation, as the moon looks the same size to me when it's high or low in the sky.
But that would seem to be beside the point. The moon looks bigger closer to the horizon to me, and to a whole lot of other people. That needs explanation regardless of whether you suffer from the same illusion. But no explanation of the illusion itself will explain why some people are not prone to it.

7. ## Re: Since we think of the moon as being farther away than the trees and buildings see

With so much more atmosphere to work through at the horizion, doesn't diffraction actually make the size of the shape we see larger there than when the moon is overhead? My college physics was a long time ago, but I think this is the case.

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