CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 53
astronomical thought from the classical Greek era through the
Renaissance, you should remember the point made in Chapter
1 that the terms solar system, galaxy, and universe have very dif-
ferent meanings. You know now that our solar system, consist-
ing of Earth, the moon, the sun, Earth’s sibling planets, their
moons, and so on, is your very local neighborhood, much
smaller than the Milky Way Galaxy, which in turn is tiny com-
pared with the observable universe. However, from ancient
times up through Copernicus’s day, it was thought that the
whole universe, everything that exists, did not extend much
beyond the farthest planet of our solar system. Asking back then
whether Earth or the sun is the center of the solar system was
the same question as asking whether Earth or the sun is the
center of the universe. As you read this chapter, but only this
chapter, you can pretend to have the old-fashioned view in
which “solar system” and “universe” meant much the same
thing.
Th is new Greek attitude toward studying the heavens with
logic and reason was a fi rst step toward modern science, and it
was made possible by two early Greek philosophers. Th ales of
Miletus (c. 624–547 bc) lived and worked in what is now
Turkey. He taught that the universe is rational and that the
human mind can understand why the universe works the way it
does. Th is view contrasts sharply with that of earlier cultures,
which believed that the ultimate causes of things are mysteries
beyond human understanding. To Th ales and his followers, the
mysteries of the universe were mysteries only because they were
unknown, not because they were unknowable.
Th e other philosopher who made the new scientifi c attitude
possible was Pythagoras (c. 570–500 bc). He and his students
noticed that many things in nature seem to be governed by geo-
metrical or mathematical relations. Musical pitch, for example, is
related in a regular way to the lengths of plucked strings. Th is led
Pythagoras to propose that all nature was underlain by musical
principles, by which he meant mathematics. One result of this
philosophy was the later belief that the harmony of the celestial
movements produced actual music, the music of the spheres.
But, at a deeper level, the teachings of Pythagoras made Greek
astronomers look at the universe in a new way. Th ales said that
the universe could be understood, and Pythagoras said that the
underlying rules were mathematical.
In trying to understand the universe, Greek astronomers
did something that Babylonian astronomers had never done—
they tried to describe the universe using geometrical forms.
Anaximander (c. 611–546 bc) described a universe made up
of wheels fi lled with fi re: Th e sun and moon were holes in the
wheels through which the fl ames could be seen. Philolaus
(fi fth century bc) argued that Earth moved in a circular path
around a central fi re (not the sun), which was always hidden
behind a counter-Earth located between the fi re and Earth.
Th is, by the way, was the fi rst theory to suppose that Earth is
in motion.
■ Figure 4-5
The spheres of Eudoxus explain the motions in the heavens by means of
nested spheres rotating about various axes at different rates. Earth is located
at the center. In this illustration, only four of the 27 spheres are shown.Th e great philosopher Plato (428–347 bc) was not an
astronomer, but his teachings infl uenced astronomy for 2000
years. Plato argued that the reality humans see is only a distorted
shadow of a perfect, ideal form. If human observations are dis-
torted, then observation can be misleading, and the best path to
truth, said Plato, is through pure thought on the ideal forms that
underlie nature.
Plato argued that the most perfect geometrical form was the
sphere, and therefore, he said, the perfect heavens must be made
up of spheres rotating at constant rates and carrying objects
around in circles. Consequently, later astronomers tried to
describe the motions of the heavens by imagining multiple rotat-
ing spheres. Th is became known as the principle of uniform
circular motion.
Eudoxus of Cnidus (409–356 bc), a student of Plato, applied
this principle when he devised a system of 27 nested spheres that
rotated at diff erent rates about diff erent axes to produce a mathe-
matical description of the motions of the universe (■ Figure 4-5).
At the time of the Greek philosophers, it was common to
refer to systems such as that of Eudoxus as descriptions of the
world, where the word world included not only Earth but all of
the heavenly spheres. Th e reality of these spheres was open to
debate. Some thought of the spheres as nothing more than math-
ematical ideas that described motion in the world model, while
others began to think of the spheres as real objects made of per-
fect celestial material. Aristotle, for example, seems to have
thought of the spheres as real.