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The Moon Illusion
The moon illusion is one of the most
intriguing. If you are a 'boatie'
still in awe of the stunning moonrises commonly seen from the water, you may
want to skip this page.
Basically, the illusion is this. A full moon just above the horizon seems
larger than when overhead. Yet the moon is a fairly consistent quarter of a
million miles away from the earth, and should therefore always subtend the
same angle (approx. 0.5 degrees) wherever it appears in the sky. When the
moon is near the horizon, it is actually further from you (about 5000 miles
due to the size of the earth) than when directly above, and should in fact
appear smaller.
The main factor involved here is interpretation of
perspective. In a similar way to the same length lines looking different
across the rails (see distortion illusions) the moon appears larger than it
really is because of it's relationship with objects around it. A distant
tree or other earthly object physically looks smaller in the distance
compared to nearby. There is a large percentage change in appearance from
10m away to 100m away. An aircraft flying directly overhead may subtend a 5
degree angle, but as it moves towards the horizon, this decreases, and it
quickly reduces to less than 0.1 degrees. The moon, on the other hand, has a
much smaller percentage change when a few thousand kilometres further away,
and still subtends a similar angle. Our mind _expects_ the object to get
smaller as it moves towards the horizon, and when it doesn't, our feeble
minds interpret that as the object getting larger.
This relativity effect can be demonstrated by
trying the following experiment. When there is next a low full moon, view
the moon through a piece of paper with a hole on it, such that you can see
only the moon and not the horizon. The moon will look smaller, as the only
reference you have is the image size on your retina. If look past the paper
with your other eye, viewing both the moon and the horizon, the each eye
will see a different size moon!
This is why photos of full moons are invariably
disappointing - no matter how large it seems to the naked eye, the moon will
only be 100th of the width of a photo taken with a standard 50mm lens
Something to try
A way to mimic the moon illusion is to use afterimages to demonstrate this
effect. Stare at a small (2 cm dia) circle of colored paper, held at arm's
length, for 20-30 seconds - the color doesn't matter, but if you use bright
purple, you get a nice yellow moon afterimage. It also helps to have a small
dot in the circle's center to focus on -- otherwise the eye tends to wander
across the circle and make the afterimage a little blurry. After staring at
the circle, stare straight up at the ceiling (some 2 m above your head) and
observe the small moon-like after image. Then repeat the procedure, but this
time look over at a distant wall (some 5-10 m away). The "horizon
moon" is huge, even though it is still the same size image burnt into
our retina as the overhead moon. Since we mentally project that object to be
a greater distance away, when it's image does not get smaller, we
"see" it as much bigger. |
Micropause
