Why does
the moon look so big now?
For the past few nights the moon has appeared larger than many
people have seen it for almost 20 years. It is the world's largest optical illusion,
and one of its most enduring mysteries.
It can put a man in
space, land a probe on Mars, but Nasa can't explain why the moon appears bigger
when it's on the horizon than when it's high in the night sky.
The mystery of the Moon
Illusion, witnessed by millions of people this week, has puzzled great thinkers
for centuries. There have even been books devoted to the matter.
Not since June 1987 has
the moon been this low in the sky, accentuating the illusion even further.
But opinion differs on
why there is such an apparent discrepancy in size between a moon on the horizon
and one in the distant sky.
Two
main theories dominate. The first, known as the Ponzo Illusion - named after
Mario Ponzo who demonstrated it in 1913 - suggests that the mind judges the
size of an object based on its background.
Ponzo drew two
identical bars across a picture of railway tracks which converge as they recede
into the distance (see pop-up, right). The upper bar looks wider because it
appears to span the rails, as opposed to the lower bar, which sits between the
rails.
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The Ponzo Illusion Two yellow bars are
drawn across railway tracks which converge towards the distant horizon. To the eye, the top
bar appears to be wider than the bottom bar. |
|
|
The Ponzo Illusion But the bars are the
same width. The upper bar seems wider because the brain judges it against the
tracks and, unlike the lower bar, it straddles both rails. In the same way, some
believe familiar foreground objects make a low-lying moon in the distance
appear bigger. |
|
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The Flattened Sky Alternatively, there
is the flattened sky theory, although some believe this works in conjunction
with Ponzo's illusion. The true size of the
moon remains the same because it stays a constant distance from the eye as it
rises into the sky. |
|
|
The Flattened Sky But, it's suggested
that the human brain perceives the sky to be a flattened dome rather than a
true hemisphere. In doing so, it
compensates for the supposed discrepancy, making the moon seem smaller than
it is. |
|
In the same way, with a
low-lying moon the trees and houses, which are familiar foreground reference
points, appear smaller against the moon, which appears bigger than it really
is.
Sceptics of this theory
point to airline pilots who also see the illusion, although they have no ground
reference points.
Alternatively, there's
the theory that the brain perceives the sky as a flattened dome rather than the
true hemisphere it really is.
Try for yourself
The theory runs that we
believe things immediately overhead, flying birds for example, are closer than
birds on the horizon. When the moon is on the horizon, the brain therefore
miscalculates its true size and distance.
Then there are those
who scoff that this is an illusion at all. They, at least, can be proved wrong.
Hold a coin up to a low-lying moon to and compare differences in size. Any
difference will remain exactly the same, as one traces the trajectory of the
moon through the night.
Indeed, it's said that
by viewing a low moon though a rolled up piece of paper, to block out the
surroundings, the illusion immediately vanishes.
But experts have yet to
agree on either or, indeed, any explanation. For the moment at least, the real
reason for the Moon Illusion remains up in the air.
Story
from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/uk_news/magazine/4619063.stm
Published: 2005/06/24 12:45:50 GMT
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