Today, we measure the earth. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
The Discoverers,
Daniel Boorstin's monumental book on human
curiosity, tells about the complex task of figuring
out the form of our planet. His chapter The
Geography of the Imagination helps us
understand how slowly Earth revealed itself
to us.
Two complex ideas, the wheel and the globe, are
grooved into our minds from infancy. It was only
5500 years ago that we finally saw how a rotating
wheel could produce forward motion. Recognizing
that Earth's apparently flat surface bends into the
shape of a sphere was even more recent. Some
cultures imagined Earth as a disc, some,
box-shaped. The Egyptians said it was an egg,
guarded at night by the moon. Only 2500 years ago,
the Greeks finally decided Earth was a sphere.
Plato argued that, since the sphere is a perfect
shape, Earth must be spherical. Aristotle used
observation. He pointed to the circular shadow
Earth casts on the moon during an eclipse.
The poet John Donne looked back on all that
difficulty a century after Magellan finally
circumnavigated the Earth, and he wrote, "At the
round earths imagined corners, blow Your trumpets,
Angells."
The Greeks had no way of knowing how large the
globe might be. The most daring travelers saw Earth
reaching farther still beyond the fringe of their
journeys. Then, in 200 BC, travelers told the head
of the Alexandria Library, Eratosthenes, about a
well near present-day Aswan. The bottom of the well
was lit by the sun at noon during the summer
solstice. At that moment the sun was straight
overhead. Eratosthenes realized he could measure
the shadow cast by a tower in Alexandria while no
shadow was being cast in Aswan. Then, knowing the
distance to Aswan, it'd be simple to calculate
Earth's radius. (You geometry students, try that
one.)
There was no accurate timekeeping back then. For
Eratosthenes to make his observation, it had to be
precisely noon in both cities. And he needed an accurate
north-south distance from Alexandria to Aswan.
Actually, Aswan lay south by southeast instead of due south,
but the error wasn't great. His calculated size of Earth was high
by only fifteen percent.
Three centuries later, the astronomer Ptolemy
created many methods of modern geography. It was he
who abandoned the idea that we're girdled by a
great unsailable ocean. Ptolemy believed that other
lands lay out in the terra incognita. He
built upon Eratosthenes; but he also criticized
him. When Ptolemy made his own estimate of size, he
came out twenty-eight percent low.
Ptolemy's thinking suited Columbus, for it shrank
Earth to fit his ships. He was plain dumb lucky
that the West Indies intervened. And we need to
remember what a mental leap the ancients had to
make. The imagined four corners of the earth
thwarted our understanding until a scant 2500 years
ago. Only then did we finally fold our minds around
the idea of a round earth. And it was only four
hundred years ago that we actually managed the
mind-numbing trick of traveling to the east by
sailing west.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
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