labelle@hplabsc.UUCP (WB6YZZ) (02/21/84)
There has been some discussion on the net recently regarding the
apparent increase in size of the moon when viewed on the horizon as
opposed to viewing overhead.
Can someone clear the air on this one? Does the moon appear larger
when viewed near the horizon due to an atmospheric affect, or is it
an optical illusion, or...?
thanks GEORGEken@ihuxq.UUCP (ken perlow) (02/23/84)
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The "moon illusion" is known to be an illusion. There is no
atmospheric effect. Angular measurements of the moon's diameter
show that it is just an illusion. The reason for the illusion
is presumed to involve context, that is, the relationship of the
moon to objects or references around it. See any text on the
psychology of perception for details and analogous phenomena.
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..ihnp4!ihuxq!ken *** ***leblanc@ucbvax.UUCP (Emile LeBlanc) (03/01/84)
Why does the moon appear to be larger on the horizon than when it is overhead? The answer I heard is that it is purely psychological (i.e. I agree with the previous answers); and I was told a test that shows this is so (I think so). Look at the moon between your legs (while standing up). Then when it is on the horizon, it will not look any bigger than usual. Why? The explanation that I've heard is that the mind thinks of the sky as a dome with a very high center, higher than it is wide. So naturally when the moon is at the horizon (a closer point, according to the mind) it looks bigger, since closer objects are bigger. When you look at the moon upside down, the dome effect leaves. I really don't know if this is a valid psycholgical explanation, but it's what I've heard. Emile LeBlanc (ucbvax!leblanc) (leblanc@berkeley)
stewart@ihldt.UUCP (R. J. Stewart) (03/02/84)
> So naturally when the moon is at the horizon (a closer point, > according to the mind) it looks bigger, since closer objects are > bigger. The way I understand, the effect is just the opposite from this. When the moon is high in the sky, there is nothing to compare it to. The mind tends to just pick a frame of reference for it. On the other hand, when the moon is close to the horizon, the mind says something like: "Gee, the moon is farther away than that building, and that tree, and everything else I can see. Since it's so far away, it must be really big to cover that much of the sky". In other words, for a constant apparent size, the perceived size gets larger as the perceived distance gets greater. This effect can be created in the laboratory in several different ways. Bob Stewart
jlg@lanl-a.UUCP (03/03/84)
There have been several articles on this illusion in past issues of Scientific American. I'll see if i can find some references. Also, Scientific American sometimes publishes some bibliographic info with thier articles ... maybe they did this time.