From A. R. Constable
I was interested to read that Lloyd Kaufman has further experimental evidence
for the now generally accepted explanation he put forward in the 1960s for the
illusion of a large Moon on the horizon
(15 January, p 12). That is, the Moon
looks large when it is full and close to the horizon because our sky dome is not
a true hemisphere and we perceive the sky above to be closer to us than the
horizon.
It is easy to remove the horizon signals from the brain by closing one eye
and viewing the Moon with the other eye through a small hole, such as can easily
be formed with the root of the thumb and first finger. When you look at the Moon
in this way it suddenly returns to its normal, smaller size and distance.
For a more interesting and less well-known phenomenon, keep one eye looking
through the small hole and then open the other eye to look directly at the Moon.
The effect is dramatic—you can see two entirely different sized Moons
simultaneously. Go slightly cross-eyed and you can play with the two Moons,
sliding them together or separating them at will.
Whatever tricks your brain is playing to create the illusion of a large Moon
it can, at one and the same time, discard them. I strongly urge readers who have
not seen this extraordinary phenomenon to look for it within a few days of a
full Moon. The next full Moon is on 19 February.
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