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From: Andrew Yee <ayee@No-Spam>
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Subject: Father-Son Scientists Confirm Why Horizon Moon Appears Larger
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Date: Wed, 5 Jan 1999 11:10:16 -0500
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IBM Almaden Research Center
San Jose, California
Father-Son Scientists Confirm Why Horizon Moon Appears Larger
Monday, January 3, 1999 -- Last month's winter-solstice full moon may have
been the biggest and brightest in decades, but throughout human history every
rising or setting moon near the horizon has appeared to be much larger than
the same moon when it is higher in the sky.
This "moon illusion" is a genuinely mind-baffling illusion, since the horizon
and elevated moons are actually the same size and distance away from earthly
viewers. Possible explanations have been discussed and debated for centuries.
Now a father-son pair of scientists have teamed up to show decisively that
the moon illusion occurs because the brain interprets the horizon moon as
being much farther away than the elevated moon. Thus, the "apparent distance"
to the moon -- rather than the real distance -- determines its perceived size.
Dr. Lloyd Kaufman and his son, Dr. James H. Kaufman, presented their findings
in cover story of the Jan. 4, 1999 issue of the Proceedings of the National
Academy of Sciences. The elder Kaufman is Professor Emeritus at New York
University, where for many years he was Professor of Psychology and Neural
Science. He is now Senior Research Scientist at the C.W. Post Campus of Long
Island University. James Kaufman is a physicist and Manager of Advanced
Materials for Technology and Storage at IBM's Almaden Research Center in
San Jose, California.
"Understanding such a pervasive and historic phenomenon as the moon illusion
is central to scientists' quest to understand how our brains perceive space
and distance." said Professor Kaufman. "Our latest results leave no doubt
that perceived distance information plays a primary role in creating the
moon illusion."
In 1960 as a young graduate student, the elder Kaufman and his mentor, Irvin
Rock, first presented experimental results supporting the apparent-distance
theory to explain the moon illusion. This theory says that the brain "computes"
perceived distances to objects. When the moon is just above the horizon, the
information presented by the intervening terrain affects the "computation"
by indicating that the moon is at a vast distance. By contrast, a view of the
elevated moon contains weaker cues to distance, so the brain responds as
if the moon were closer. Since information regarding an object's apparent
distance determines its perceived size, the more distant horizon moon is
perceived as being up to twice as large as the elevated moon. This is similar
to the classic Ponzo perspective illusion
[http://www.sandlotscience.com/Distortions/Ponzo_java.htm] dating from
1913 in which two same-length lines are drawn between or across a pair of
converging lines resembling railroad tracks going off into the distance. The
upper line appears much larger because it spans a greater apparent distance
between the rails, which our mind assumes are parallel.
A class of alternative explanations for the moon illusion based on an
"apparent-size" theory was first described in 1965. According to this
theory, since the elevated moon is perceived as being smaller, it must also
be perceived as being farther away than the apparently larger horizon moon.
In general, this theory holds that the apparent size of an object governs its
apparent distance -- diametrically opposed to the apparent-distance theory.
In the case of the moon, most apparent-size proponents believe that other
cues to distance, such as terrain, are irrelevant. Some proponents also hold
that when we view the elevated moon our eyes focus and converge to a
different distance than when we view the horizon moon, and that this
difference leads directly to the reduced apparent size of the elevated moon.
"For many years, my father and I discussed the causes of the moon illusion,
and we looked at many moons," recalled James Kaufman. "While considering
the details of both theories a few years ago, we realized that one reason
for the continued controversy was that virtually all experimental studies
measured or manipulated the perceived size of the moon but made only
inferences about its perceived distance from the viewer. I asked why we
couldn't measure the apparent distance directly rather than just deduce it."
The Kaufmans then designed two experiments to measure directly the
perceived distance to the moon. Both tests used an apparatus built at IBM
Research to project stereoscopic images of artificial moons from an IBM
ThinkPad computer display to optical infinity so viewers could see them
against an actual sky. Professor Kaufman then took people to a Long Island
hilltop, where he made hundreds of measurements of their perceptions of
the distance to the moon.
Each person was first asked to position an artificial moon so it appeared
to be halfway between themselves and a fixed moon that was either near
the horizon or elevated. In every case, the viewers placed the halfway point
to the horizon moon as being much farther away -- on average four times
more distant -- than the halfway point to the elevated moon. This is
entirely consistent with the apparent-distance theory.
In their second test, the viewer used the ThinkPad to adjust the apparent
distance to a moon projected either on the horizon or elevated sky. In all
cases, as the subjects moved the projected moon closer, they reported that
it appeared to become smaller, not larger -- a direct contradiction of the
apparent-size theory. An animated simulation of this surprising but
convincing second experiment can be viewed on the Web
[http://www.research.ibm.com/news/detail/newmoon.html
or
http://www.pnas.org/cgi/content/full/97/1/500/DC1] .
Before the tests, each of Kaufman's subjects said they thought that the
apparently larger moon would appear closer. But the opposite occurred.
"A key element of a true illusion is that our conscious deductions and
preconceptions do not necessarily reflect how our brains actually respond
to the outside world," Professor Kaufman said.
"Humans can accurately perceive an object's size regardless of its distance,"
Professor Kaufman says. "This effect -- known as size constancy -- is why
we can discern the real size of a distant automobile, tree or building despite
its small image size. Our brain automatically takes the apparent distance
into account and compensates for the geometrical reality that the image
we see of a distant object is smaller than the image of that same object
nearby."
In most cases, the terrain provides a rich set of cues that enable us to
accurately perceive the sizes of objects at different distances, Professor
Kaufman added. But apparently such large distances as those to the moon
are beyond our brain's capability. As an example, Professor Kaufman
recommends viewing the moon through an aperture, such as pinching it
between your thumb and forefinger or viewing it through a tube, which
hides the the terrain leading up to the moon. The moon suddenly appears to
be small because our brain locates it at the nearby distance of the edges of
the aperture. Removing the aperture restores the terrain's distance cues and
the moon springs back to its large, illusory size.
The Moon Illusion is Ancient History
The moon illusion has been known since antiquity.In the second century A.D.,
the Greek-Egyptian astronomer Ptolemy was essentially correct in suggesting
that any object viewed across "filled space" -- such as the horizon moon --
would seem to be more distant than objects the same distance away but
viewed over empty space, such as the moon at its zenith. An 11th century
Arab astronomer (Al-Hazan) seems to have been the first to develop the
"apparent distance" theory in some detail.
Others who have written about the moon illusion over the years include such
notable scientists as: Aristotle, Roger Bacon, Leonardo da Vinci, Johann
Kepler, Rene Descartes, Marin Mersenne, Christiaan Huygens, Leonard Euler,
Alexander von Humboldt, Hermann von Helmholtz and Thomas Huxley II.
--- Andrew Yee ayee@No-Spam
-- Mark Wagner Visit the S.F. bay area's spot for active observational astronomers - The Astronomy Connection http://www.seds.org/TAC