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Dear Cecil:
I can hardly believe my Genus Edition ears. My boyfriend has
the audacity to claim that his geography professor knows more than
I do. In a game of Trivial Pursuit not long ago I correctly
answered that the only man-made structure on earth visible from
outer space was the Great Wall of China. He has the nerve to tell
me that his teacher said this is not true, and worse, he is taking
the prof's word over mine. Who's right, the geek from college or
the trivia buff? --M.K.M., Los Angeles
Dear M.:
Prepare to eat crow,
babycakes--those wankers at Trivial Pursuit have screwed up again.
Any number of man-made structures can be seen from space, provided
we construe "structure" to mean "anything built." Many of these are
things that look like long, straight lines when seen from afar,
such as highways, railroads, canals, and of course walls. If the
orbit is low enough you can see even more. I have here a photo of
Cape Canaveral taken during the Gemini V flight in which the big
Launch Complex 39, used for the Apollo missions, is clearly
visible. Another photo of the Nile delta, taken from a height of
100 miles, shows an extensive road network. Gemini V astronauts
Gordon Cooper and Charles Conrad were able to spot, among other
things, a special checkerboard pattern that had been laid out in
Texas, a rocket-sled test in New Mexico, and the aircraft carrier
that would later pick them up in the Atlantic, along with a
destroyer trailing in its wake. Take it from me, honey--Trivial
Pursuit is one game I never lose.
THE TEEMING MILLIONS HIT THE
WALL, PART ONE
Dear Cecil:
Recently a reader incorrectly stated that
the only man-made object visible from Earth orbit was the Great
Wall of China. You set him straight--but you may be interested to
know the Wall is the only man-made object visible with the unaided
human eye from the surface of the moon. The quarter-million miles
does make a perceptible difference. --Kenneth L.,
Chicago
Cecil replies:
Nice try, Ken, but you whiffed too. According to
NASA, the earth as seen from the moon takes up less than one degree
of arc in the sky. Basically it looks like a big blue marble. No
man-made detail can be seen at all; sometimes even the continents
are barely distinguishable.
The NASA folks, I gather, are getting
a little tired of hearing about the Great Wall of China. Nobody
knows exactly where the story got started, although some think it
was speculation by some bigshot during an after-dinner speech in
the early days of the space program. The Teeming Millions are
humbly requested to give it a rest.
THE TEEMING MILLIONS HIT
THE WALL, PART TWO
Dear Cecil:
I was greatly disturbed by your blindly
taking the word of some NASA goofball who is unable to perform even
simple math correctly. Given an earth-moon distance of 239,000
miles and the diameter of the earth as 7,920 miles, the angle
subtended by the earth from the surface of the moon is almost two
degrees, not "less than one degree." Furthermore, how can you even
consider that the earth looks "basically ... like
a big blue marble"? The earth is 3.7 times as large in the lunar
sky as the moon is from earth, and I can easily see a large amount
of detail on the lunar surface, even through earth's polluted
atmosphere. The moon, of course, has no atmosphere or city lights
to obscure the view.
In fact, the data on visual acuity do not
seem to indicate that "no man-made detail can be seen at all." Hugh
Davson, in Physiology of the Eye, 4th edition, states
that in the common eyechart-type measure, a monocular resolving
power of approximately 20 seconds of arc is observed, and when
visual acuity is measured by the power to detect a single line on
a uniform background, the normal eye can resolve 0.5 seconds of
arc. Davson gives an increase in acuity of the square root of two
for binocular vision, thus indicating a potential resolution from
the moon of objects 0.4 miles across. While the earth may not be a
perfect test surface, neither are all eyes "normal." And I can name
several man-made details many times larger than 0.4 miles
across--the average city, for one. Admittedly, the Great Wall at
12-40 feet in width is much smaller than 0.4 miles, but I think in
magnitude of error, it was you, not Mr. L., who really "whiffed on
this one." --Rick A., Chicago
Cecil replies:
Think so? Tom Burnam, author of More
Misinformation (1980), quotes a letter from astronaut Alan
Bean on the subject:
"The only thing you can see from the moon is
a beautiful sphere, mostly white (clouds), some blue (ocean),
patches of yellow (deserts), and every once in a while some green
vegetation. No man-made object is visible on this scale. In fact,
when first leaving earth's orbit and only a few thousand miles
away, no man-made object is visible at that point either."
You're
right about one thing, though. The earth takes up two degrees of
arc in the lunar sky, not one.
--CECIL ADAMS
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