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June
27, 2007: Sometimes you can't believe your eyes.
This weekend is one of those times.
On
Saturday night, June 30th, step outside at sunset and look
around. You'll see a giant moon rising in the east. It looks
like Earth's moon with the usual craters and seas, but something's
wrong. This full moon is strangely inflated. It's huge!
You've
just experienced the Moon Illusion.
Sky
watchers have known for thousands of years that low-hanging
moons look unnaturally big. Cameras don't see it, but human
eyes do; it's a genuine illusion.
Above:
A time-lapse sequence of the moon rising over Seattle. To
the camera, the moon appears to be the same size no matter
what its location on the sky. Credit and copyright: Shay Stephens.
[More]
This
weekend's full moon hangs lower in the sky than any other
full moon of 2007, so the Moon Illusion is going to be strong.
What makes the moon so low? Consider the following: The sun
and full moon lie on opposite sides of the sky. They are like
a see-saw: when one is high, the other is low. Because the
summer solstice was just last week (June 21st), the sun is
near its highest point in northern skies. The full moon is
correspondingly low.
When
you look at the moon, rays of moonlight converge and form
an image about 0.15 mm wide in the back of your eye. High
moons and low moons make the same sized spot. So why does
your brain think one is bigger than the other? After all these
years, scientists still aren't sure of the answer.
 A
similar illusion was discovered in 1913 by Mario Ponzo, who
drew two identical bars across a pair of converging lines,
like the railroad tracks pictured right. The upper yellow
bar looks wider because it spans a greater apparent distance
between the rails. This is the "Ponzo Illusion."
Right:
The Ponzo Illusion. Image credit: Dr. Tony Phillips. [More]
Some
researchers believe that the Moon Illusion is Ponzo's Illusion,
with trees and houses playing the role of Ponzo's converging
lines. Foreground objects trick your brain into thinking the
moon is bigger than it really is.
But
there's a problem: Airline pilots flying at very high altitudes
sometimes experience the Moon Illusion without any objects
in the foreground. What tricks their eyes?
Maybe
it's the shape of the sky. Humans perceive the sky as a flattened
dome, with the zenith nearby and the horizon far away. It
makes sense; birds flying overhead are closer than birds on
the horizon. When the moon is near the horizon, your brain,
trained by watching birds (and clouds and airplanes), miscalculates
the moon's true distance and size.
Below:
The "flattened sky" model for the Moon Illusion.
[More]
There
are other explanations,
too. It doesn't matter which is correct, though, if all you
want to do is see a big beautiful moon. The best time to look
is around moonrise, when the moon is peeking through trees
and houses or over mountain ridges, doing its best to trick
you. The table below (scroll down) lists moonrise times for
selected US cities.
A
fun activity: Look at the moon directly and then through a
narrow opening of some kind. For example, 'pinch' the moon
between your thumb and forefinger or view it through a cardboard
tube, which hides the foreground terrain. Can you make the
optical illusion vanish?
Stop
that! You won't want to miss the Moon Illusion.
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Moonrise
over Selected US Cities
If your city does not appear
in the list, click
here for more data
from the US Naval Observatory.
Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
|
