The Moon Illusion
The moon illusion is one of
the most famous of all illusions. Stated simply, the
full moon, when just above the horizon, appears much
larger than when it is overhead. Yet the moon, a quarter
of a million miles away from the earth, always subtends
the same angle wherever it is in the sky, roughly 0.5
The first problem is for photographers.
A wonderful picture presents itself, with the full moon
just rising above a spectacular horizon. Click, the
picture is taken. Yet the result is disappointing. The
moon seems much smaller in the photograph than it did
when viewed with the naked eye. Even professional photographers
fall for this one. Yet on a normal lens, 50mm on a 35mm
camera, the field of view is around 50 degrees, and
the width of the moon, subtending an angle of 0.5 degrees,
will be 100th of the width of the photo! Many photographs
that you see in magazines, containing both a moon and
a landscape, will be composites. The landscape will
be taken with a normal lens, the moon taken with a telephoto
lens, to get a bigger image.
How does this illusion come about? Since the moon always
subtends an angle of 0.5 degrees, the image on the retina
must always be the same. Clearly the problem is one
of interpretation. One simple experiment shows this
to be so. A full moon just above the horizon will not
appear so large to the human eye if a piece of paper
is held up to that eye with a hole in it, so that only
the moon can be seen through the hole and not the horizon.
If the other eye is open at the same time, viewing both
the moon and the horizon, the two eyes will each see
different sized moons!
The explanation is believed to be as follows. We 'know'
that a cloud that is overhead will be larger than when
it moves towards the horizon. And an airplane that is
a mere speck on the horizon becomes large when it is
overhead. And we are all familiar with standing under
a tree which seems enormous, yet at a couple of hundred
paces seems insignificant. It would seem that so much
of our world is interpreted this way that we are ill-equipped
to cope with an object like the moon, that subtends
the same angle at the eye, whatever position it occupies
in the sky. And so our brain 'interprets' the image
that it 'sees', and tells us that the moon is larger
than it really is.
出典: Hendrik Ball, "The Moon