Geocentric vs.
Heliocentric
A Battle for the Ages
The Beginnings of the
Geocentric Model
of the Universe
Early Greek Astronomy
• Aristotle
(384 – 322 BCE):
States that the
Universe is a
perfect sphere
with Earth at
the center.
• Objects in space
also move in
perfect circles.
Early Greek Astronomy
• Ptolomy
(140 AD): Still believed that
Earth was center of Universe
(geocentric theory). Noticed that
Aristotle’s model did not explain
planetary movements.
• Planets and stars moved in
EPICYCLES. In other words planets
move in perfect CIRCLES WITHIN
CIRCLES!
Ptolemaic
System
Mars
Jupiter
Ptolemy’s system provided
the intellectual framework
for all discussion of the
universe for nearly 1600
years!! So in a very true
sense,
this
idea
was
stupendously
successful
even though we now know
that it was incorrect.
Saturn
Earth & Moon
Mercury
Sun
Venus
The Revolution of Ideas
The Beginnings of the
Heliocentric Model
of the Universe
Copernicus
(1473-1543)
A Polish Astronomer
The Copernican Revolution!
Copernicus
1. Earth is not the center of “everything.”
2. All the planets revolve around the Sun!
(heliocentric theory)
3. Stars are very much farther away than
the Sun
4. Any motion of the stars is a result of
the Earth’s rotation
5. Still used “perfect” circles for the orbits.
Changing Ideas is Not Easy.
Copernicus was no fool. He waited until his own
death before he published his new ideas.
On
March
5,
1616,
Copernicus'
work
was
banned from being taught
and discussed by the
Congregation of the Index
"until corrected." It stayed
on this list of prohibited
books and teachings until
1822!!!!
Italian scientist
• Galileo Galilei
1600s):
(late 1500s - early
First to point the refracting
TELESCOPE towards the heavens.
He studied the movement and orbits
of many nearby planets in our solar
system.
• Offered PROOF to Copernicus’s
Heliocentric Model of our Universe.
• Led to the discovery of the moons of
Jupiter
Galileo Galilei
The Great
Compromise
Tycho Brahe
(1546-1601)
Danish Nobleman
Experimentalist &
Observer.
Made very careful
measurements of star’s
positions. Earth must be
stationary.
Tychonic Model
Tycho developed a system that
combined the best of both worlds.
He kept the Earth in the center of
the universe, so that he could
retain Aristotelian physics and
Ptolemy’s geocentric idea.
The Moon and Sun revolved about the Earth, and
the shell of the fixed stars was centered on the
Earth.
But Mercury, Venus, Mars, Jupiter, and Saturn
revolved about the Sun.
Tychonic Model
The German Student
• Johannes Kepler
(early 1600’s):
Followed Tycho’s work
BUT followed the
Heliocentric Model.
• Used Math to calculate
that planets moved in
ORBITS and along
ELLIPSES.
Newton’s Idea of
Law of Universal Gravitation
Supported Kepler’s
Laws of Planetary Motion
Kepler’s Planetary Diagram
Issac Newton
•English scientist
(mid 1660’s)
•3 laws of motion
explain how the
inertia of a planet
with gravity causes
orbital motion.
•Also, developed the
reflecting telescope.
Today!