The Solar System

CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 59

motion. In fact, Copernicus objected strongly to Ptolemy’s use of

the equant. It seemed arbitrary to Copernicus, an obvious viola-

tion of the elegance of Aristotle’s philosophy of the heavens.

Copernicus called equants “monstrous” because they under-

mined both geocentrism and uniform circular motion. In devis-

ing his model, Copernicus demonstrated a strong belief in

uniform circular motion.

Although he did not need epicycles to explain retrograde

motion, Copernicus quickly discovered that the sun, moon, and

planets suff ered other small variations in their motions that he

could not explain using uniform circular motion centered on the

sun. Today astronomers recognize those variations as the result of

planets following elliptical orbits, but because Copernicus held

fi rmly to uniform circular motion, he had to introduce small

epicycles to try to reproduce these minor variations in the

motions of the sun, moon, and planets.

was speaking harshly about fundamental Church teachings, and
others, both scholars and scoundrels, were questioning the
Church’s authority. Even matters as abstract as astronomy could
stir controversy. Remember, too, that Earth’s place in astronomi-
cal theory was linked to the geometry of heaven and hell, so
moving Earth from its central place was a controversial and per-
haps heretical idea.
Another reason Copernicus may have hesitated to publish
was that his work was incomplete. His model could not accu-
rately predict planetary positions, so he continued to refi ne it.
Finally in 1540 he allowed the visiting astronomer Joachim
Rheticus (1514–1576) to publish an account of the Copernican
universe in Rheticus’s book Prima Narratio (First Narrative). In
1542, Copernicus sent the manuscript for De Revolutionibus off
to be printed. He died in the spring of 1543 before the printing
was completed.
Th e most important idea in the book was the location of the
sun at the center of the universe. Th at single innovation had an
astonishing consequence—the retrograde motion of the planets
was immediately explained in a straightforward way without the
large epicycles that Ptolemy had used.
In the Copernican system, Earth moves faster along its orbit
than the planets that lie farther from the sun. Consequently,
Earth periodically overtakes and passes these planets. To visualize
this, imagine that you are in a race car, driving rapidly along the
inside lane of a circular racetrack. As you pass slower cars driving
in the outer lanes, they fall behind, and if you did not realize you
were moving, it would look as if the cars in the outer lanes occa-
sionally slowed to a stop and then backed up for a short interval.
■ Figure 4-9 shows how the same thing happens as Earth passes
a planet such as Mars. Although Mars moves steadily along its
orbit, as seen from Earth it appears to slow to a stop and move
westward (retrograde) as Earth passes it. Th is happens to any
planet whose orbit lies outside Earth’s orbit, so the ancient
astronomers saw Mars, Jupiter, and Saturn occasionally move
retrograde along the ecliptic. Because the planetary orbits do not
lie in precisely the same plane, a planet does not resume its east-
ward motion in precisely the same path it followed earlier.
Consequently, it describes a loop whose shape depends on the
angle between the orbital planes.
Copernicus could explain retrograde motion without epicy-
cles, and that was impressive. Th e Copernican system was elegant
and simple compared with the whirling epicycles and off -center
equants of the Ptolemaic system. You can see Copernicus’s own
diagram for his heliocentric system in ■ Figure 4-10a. However,
De Revolutionibus failed in one critical way—the Copernican
model could not predict the positions of the planets any more
accurately than the Ptolemaic system could. To understand why
it failed, you must understand Copernicus and his world.
Copernicus proposed a revolutionary idea when he made the
planetary system heliocentric, but he was a classical astronomer
with tremendous respect for the old concept of uniform circular

■ Figure 4-9

The Copernican explanation of retrograde motion. As Earth overtakes Mars

(a–c), Mars appears to slow its eastward motion. As Earth passes Mars (d),

Mars appears to move westward. As Earth draws ahead of Mars (e–g), Mars

resumes its eastward motion against the background stars. The positions of

Earth and Mars are shown at equal intervals of one month.

East West

Mars

Sun a Earth

b

e d c

f

g

Apparent path of Mars

as seen from Earth