Date: 1999 - 2000
Why does the moon appear large over the horizon, but
small in the sky? What are the hypothesis that scientists come up with
to answer this question?
First of all, the moon does not appear larger at the horizon than
high in the sky. If you don't believe this, *measure* it with a
protractor next time you have the opportunity.
What you are experiencing is an optical illusion. Why is the human
brain subject to this one? Probably because we judge the size of
objects by what is near them, the size of which we know. When you see
the Moon high in the sky, nothing is near it, and you have no real
clue to its size. Your brain assigns it a size based on some
"default" distance at which it assumes faraway things lie, which is
probably not much farther than your binocular vision is good for,
about 20 to 40 feet. When the Moon is near the horizon, you see it
next to mountains, trees, or buildings the size of which you know (or
rather the visual processing part of your brain knows). Of course the
Moon is bigger than they, and so the Moon suddenly looks much bigger.
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Update: June 2012