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Physics 202: Introduction to
Astronomy – Lecture 3
Carsten Denker
Physics Department
Center for Solar–Terrestrial Research
Chapter 1.2 – 1.3
Planetary motion
 Retrograde motion
 Geocentric model
 Aristotle
 Epicycle
 Deferent
 Ptolemaic model
 Aristarchus of Samos
 Heliocentric model
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Copernican revolution
 Modern Astronomy
 The Scientific Method
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Theory
Predictions
Observations
Galilean moons
 Venus phases
 Sunspots
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Center for Solar-Terrestrial Research
Ancient Greek
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Physical (geocentric) model of
the cosmos by Aristotle (384 –
322 BC)
Mathematical model of
planetary motion by Ptolemy
(100 – 170), terrestrial/celestial
sphere, basic elements: earth,
water air, and fire/quintessence
The Aristotelian cosmos. The
Earth sits motionless at the
center of the universe, and the
outer sphere, the Primum
Mobile, is assumed to undergo
a full revolution in 24 hours.
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Epicycle and Deferent
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Mercury transit on 15 November 1999
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Venus Transit 8 June 2004
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The Phases of Venus
Why did Plato
propose that all
heavenly motion
was uniform and
circular?
 How do the
epicycles of Mercury
and Venus differ
from those of Mars,
Jupiter, and Saturn?
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Phases of Venus in 2004
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Nicholas Copernicus
(1473–1543)

De Revolutionibus Orbium
Coelestum in 1543

Heliocentric planetary model:
The Sun is at the center of all planetary
motions, except for the Moon which
orbits Earth. Under this arrangement the
orbital speed of planets decreases
steadily outwards, and the outer sphere
of fixed stars is truly motionless. In
Copernicus' original model the Earth has
three motions: a daily 24-hr axial
rotation, a yearly orbital motion about
the Sun, and a third motion, somewhat
related to precession which Copernicus
thought necessary to properly reproduce
ancient observations.
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Galileo Galilei
(1564 – 1642)
First telescopic
observations
of the Sun!
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Retrograde Motion of Planets
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Retrograde Motion of Mars in 2003
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January 25, 2006
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