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June
16 , 2008: Sometimes you just can't believe your
eyes. This week is one of those times.
On
Wednesday night, June 18th, step outside at sunset and look
around. You'll see a giant form rising in the east. At first
glance it looks like the full Moon. It has craters and seas
and the face of a man, but this "moon" is strangely
inflated. It's huge!
You've
just experienced the Moon Illusion.
Above:
The full Moon beams through trees in Manchester, Maryland.
Credit: Edmund E. Kasaitis. Copyright 2008; all rights reserved.
There's
no better time to see it. The full Moon of June 18th is a
"solstice moon", coming only two days before the
beginning of northern summer. This is significant because
the sun and full Moon are like kids on a see-saw; when one
is high, the other is low. This week's high solstice sun gives
us a low, horizon-hugging Moon and a strong Moon Illusion.
Sky
watchers have known for thousands of years that low-hanging
moons look unnaturally big. At first, astronomers thought
the atmosphere must be magnifying the Moon near the horizon,
but cameras showed that is not the case. Moons on film are
the same size regardless of elevation: example.
Apparently, only human beings see giant moons.
Are
we crazy?
After
all these years, scientists still aren't sure. When you look
at the Moon, rays of moonlight converge and form an image
about 0.15 mm wide on the retina in the back of your eye.
High moons and low moons make the same sized spot, yet the
brain insists one is bigger than the other. Go figure.
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.
Source: Explaining
the Moon Illusion by Lloyd Kaufman and James H. Kaufman.
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. The table below (scroll
down) lists rise 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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Author: Dr.
Tony Phillips | Credit: Science@NASA
Moonrise
over Selected US Cities
If your city does not appear
in the list, click
here for more data
from the US Naval Observatory.
City |
Time
Zone |
June
17 |
June
18 |
June
19 |
New
York, NY |
EDT |
8:07 p.m.
|
8:58 p.m.
|
9:41 p.m.
|
San
Diego, CA |
PDT |
7:37 p.m.
|
8:28 p.m.
|
9:13 p.m.
|
Washington,
DC |
EDT |
8:13 p.m.
|
9:03 p.m.
|
9:47 p.m.
|
Honolulu,
HI |
HST
|
6:53 p.m.
|
7:44 p.m.
|
8:31 p.m.
|
Chicago,
IL |
CDT |
8:09 p.m.
|
8:59 p.m.
|
9:42 p.m.
|
Houston,
TX |
CDT |
7:58 p.m.
|
8:49 p.m.
|
9:35 p.m.
|
Denver,
CO |
MDT |
8:12 p.m.
|
9:02 p.m.
|
9:45 p.m.
|
Miami,
FL |
EDT |
7:43 p.m.
|
8:35 p.m.
|
9:22 p.m.
|
Seattle,
WA |
PDT
|
9:02 p.m.
|
9:51 p.m.
|
10:30 p.m.
|
Anchorage,
AK |
ADT |
12:30 a.m.
|
1:04 a.m.
|
1:15 a.m.
|
Augusta,
ME |
EDT
|
8:06 p.m.
|
8:57 p.m.
|
9:39 p.m.
|
|
|