Name: Geoff P.
In basic terms, explain the "Moon Illusion"
Why does the moon appear larger on the horizon but smaller overhead?
I have read the many atmospheric theories and the scientific retinal
aberrations of various individual's eyesight. Some people claim not to be
aware of any such deviations in size. However, to the astronomical
novice, the Moon as viewed on the horizon is considerably larger yet when
photographed we are very much disappointed.
This observation tends to prove the visual line of varying eyesight to be
a contributing factor.
What you are seeing is basically an optical illusion. If you were to actually sight the moon
with your thumb as you stretched out your forearm and closed your eye, you would find that it
is no bigger at the horizon than it is at it's zenith. It is just that we have all these
houses and buildings on the ground and we can do a little comparisons of sizes. Truly, the
moon is no bigger up high than down low. Try it, it is pretty neat.
I believe the correct explanation for the moon appearing larger when it is close to the
horizon is a psychological one. When it is close to the horizon, it is easier to compare
the moon with terrestrial objects. Then the moon is clearly very far away but still large.
The mind then realizes it must be a very large object and somehow influences the brain to
make it appear even larger.
For a fuller explanation, contact a psychologist, who (I would hope) understands the human
mind much better than I do.
Best, Dick Plano...
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Update: June 2012