54 PART 1^ |^ EXPLORING THE SKY
Aristotle and the Nature of Earth
Aristotle (384–322 bc), another of Plato’s students, made his
own unique contributions to philosophy, history, politics, ethics,
poetry, drama, and other subjects (■ Figure 4-6). Because of his
sensitivity and insight, he became the greatest authority of antiq-
uity, and his astronomical model was accepted with minor varia-
tions for almost 2000 years.
Much of what Aristotle wrote about scientifi c subjects was
wrong, but that is not surprising. Th e scientifi c method, with
its insistence on evidence and hypothesis, had not yet been
invented. Aristotle, like other philosophers of his time,
attempted to understand his world by reasoning logically and
carefully from fi rst principles. A fi rst principle is something
that is held to be obviously true. Th e perfection of the heavens,
meaning that the heavens had to be composed of rotating per-
fect spheres, was, for Aristotle, a fi rst principle. Once a prin-
ciple is recognized as true, whatever can be logically derived
from it must also be true.
Aristotle believed that the universe was divided into two
parts: Earth, imperfect and changeable; and the heavens, perfect
and unchanging. Like most of his predecessors, he believed that
Earth was the center of the universe, so his model is called a
geocentric universe. Th e heavens surrounded Earth, and he
added to the model proposed by Eudoxus to bring the total to 55
crystalline spheres turning at diff erent rates and at diff erent
angles to carry the sun, moon, and planets across the sky. Th e
lowest sphere, that of the moon, marked the boundary between
the changeable imperfect region of Earth and the unchanging
perfection of the celestial realm above the moon.
Because he believed
Earth to be immobile,
Aristotle had to make
this entire nest of sph-
eres whirl westward aro-
und Earth every 24
hours to produce day
and night. Th e spheres
also had to move more
slowly with respect to
one another to pro-
duce the motions of the
sun, moon, and planets
against the background
of the stars. Because his
model was geocentric,
he taught that Earth
could be the only center of motion, meaning that all of the whirling
spheres had to be centered on Earth.
About a century after Aristotle, the Alexandrian philosopher
Aristarchus proposed that Earth rotated on its axis and revolved
around the sun. Th is theory is, of course, generally correct, but
most of the writings of Aristarchus were lost, and his theory was
not well known. Later astronomers rejected any suggestion that
Earth could move, because it confl icted with the teachings of the
great philosopher Aristotle.
Aristotle had taught that Earth had to be a sphere because
it always casts a round shadow during lunar eclipses, but he
could only estimate its size. About 200 bc, Eratosthenes, work-
ing in the great library in the Egyptian city of Alexandria,
found a way to measure Earth’s radius. He learned from travel-
ers that the city of Syene (Aswan) in southern Egypt contained
a well into which sunlight shone vertically on the day of the
summer solstice. Th is told him that the sun was at the zenith at
Syene; but, on that same day in Alexandria, he noted that the
sun was 1/50 of the circumference of the sky (about 7°) south
of the zenith.
Because sunlight comes from such a great distance, its rays
arrive at Earth traveling almost parallel. Th at allowed Eratosthenes
to use simple geometry to conclude that the distance from Alexandria
to Syene was 1/50 of Earth’s circumference (■ Figure 4-7).
To fi nd Earth’s circumference, Eratosthenes had to learn the
distance from Alexandria to Syene. Travelers told him it took
50 days to cover the distance, and he knew that a camel can
travel about 100 stadia per day. Th at meant the total distance was
about 5000 stadia. If 5000 stadia is 1/50 of Earth’s circumfer-
ence, then Earth must be 250,000 stadia in circumference, and,
dividing by 2π, Eratosthenes found Earth’s radius to be 40,000
stadia.
How accurate was Eratosthenes? Th e stadium (singular of
stadia) had diff erent lengths in ancient times. If you assume
6 stadia to the kilometer, then Eratosthenes’s result was too big
by only 4 percent. If he used the Olympic stadium, his result was
14 percent too big. In any case, this was a much better measure-
ment of Earth’s radius than Aristotle’s estimate, which was much
too small, about 40 percent of the true radius.
You might think this is just a disagreement between two
ancient philosophers, but it is related to a Common
Misconception. Christopher Columbus did not have to
convince Queen Isabella that the world was round. At the time
of Columbus, all educated people knew that the world was
round and not fl at, but they weren’t sure how big it was.
Columbus, like many others, adopted Aristotle’s diameter for
Earth, so he thought Earth was small enough that he could sail
west and reach Japan and the Spice Islands of the East Indies. If
he had accepted Eratosthenes’ diameter, Columbus would never
have risked the voyage. He and his crew were lucky that America
was in the way; if there had been open ocean all the way to Japan,
they would have starved to death long before they reached land.■ Figure 4-6
Aristotle, honored on this Greek stamp, wrote on such a wide variety of
subjects and with such deep insight that he became the great authority on
all matters of learning. His opinions on the nature of Earth and the sky were
widely accepted for almost two millennia.