70 Chapter 1 Is Earth the Center ofthe Universe?
What Kind of Orbit Did
a Planet Trace in the
Geocentric Theory?
The most likely reason that the geocentric the-
ory was the mainstream model of the universe
for so long was that every time someone made
an observation of the heavenly bodies that
might overturn it, someone else made an argu-
ment that explained why the new observation
was still consistent with the geocentric theory.
Ptolemy’s map of space, which explained the
motion of the planets in a way that accounted
for their apparent position or brightness vary-
ing subtly, is certainly a representative example
of this.
A part of this map is given here. If this
is the only theory you look at, it can be per-
suasive. However, if we continue to develop a
theory of planetary motion based on this dia-
gram, it is apparent that the planet will move
in an orbit shaped like a stretched-out spring,
as shown in the next diagram. Why would the
planets move in a spiral, while the Moon has a
circular orbit? Upon careful consideration, this
seems like an awfully contrived and complicated
explanation.
Even so, people seemed to want to stick
to the geocentric theory, and even though new
observation results continued to emerge, people
did not readily accept the heliocentric theory.
The Tychonic System that
Embellished the Geocentric
Theory
What might be called the last gasp of the geocentric theory was the astronomy diagram of
Tycho Brahe (1549–1601).
Tycho Brahe was a Danish astronomer who lived a little before Galileo Galilei (who lived
1564–1642). Tycho proposed a model of the universe that was a compromise between the
geocentric and heliocentric theories. The outer edges of the model looked almost exactly like
a diagram drawn in accordance with the heliocentric theory, but the model was drawn cen-
tered on Earth, since Tycho firmly believed that Earth was at rest. The result was a rather
attractive astronomy diagram.
Earth
Planet
Planetary motion according to the
geocentric theory (Ptolemy)
Earth
Planet
According to this theory, planets swirled
around Earth.

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