The Astronomy Book

35

concern related to predicting
the movements and appearances
of the planets. According to the
Aristotelian version of geocentrism,
the planets—like all other celestial
bodies—were embedded in
invisible concentric spheres
that revolved around Earth, each
rotating at its own steady speed.
But if this were true, each planet
should move across the sky at
a constant pace and with an
unvarying brightness—and this
wasn’t what was observed.

Ptolemy’s fixes
The most glaring anomaly was
Mars, which had been carefully
observed in ancient times by both
the Babylonians and the Chinese.
It appeared to speed up and slow
down from time to time. If its
movements were compared to
those of the rapidly rotating outer
sphere of fixed stars, Mars usually
moved in a particular direction, but
occasionally it reversed direction—
a strange behavior described as
“retrograde motion.” In addition, its
brightness varied greatly over the
course of a year. Similar, but less
dramatic, irregularities were also
observed in the other planets. To

address these problems, Ptolemy

modified the original Aristotelian

geocentric model. In his revised

model, the planets were attached

not to the concentric spheres

themselves, but to circles attached

to the concentric spheres. He

called these circles “epicycles.”

These were suborbits around

which the planets circled while

the central pivot points of these

suborbits were carried around the

sun. These modifications, Ptolemy

thought, sufficed to explain the

anomalies observed and matched

observational data. However, his

model became hugely complicated,

See also: The geocentric model 20 ■ Early heliocentric model 21 ■ Consolidating knowledge 24–25 ■ The Tychonic
model 44–47 ■ Elliptical orbits 50–55 ■ Galileo’s telescope 56–63 ■ Stellar aberration 78 ■ Al-Battani (Directory) 334

FROM MYTH TO SCIENCE

Ptolemy tried to fix some of the anomalies in Aristotle’s

geocentric model by proposing that each planet moved in a

small circle called an epicycle. Each epicycle was embedded

in a sphere called a deferent. Each planet’s deferent rotated

around a point slightly displaced from Earth’s position in

space. This point, in turn, continuously rotated around another

point called an equant. Each planet had its own equant.

In so many and such important

ways, then, do the planets bear

witness to the Earth’s mobility.

Nicolaus Copernicus

as further epicycles needed to

be added to keep prediction in

line with observation.

Alternative views

From about the 4th century bce,

a number of astronomers had

suggested theories refuting the

geocentric model. One of these

ideas was that Earth spins on its

own axis, which would account

for a large proportion of the daily

movements of celestial objects.

The concept of a rotating Earth

had initially been put forward by

a Greek, Heraclides Ponticus, in

about 350 bce and later by various ❯❯

Earth

Planet

Center of

deferent

Equant

Center of

epicycle

Ep

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