Download Lecture 3 Geocentrism vs.Heliocentrism

Lecture 3
Geocentrism vs. Heliocentrism
Dennis Papadopoulos
ASTR 340
Fall 2006
Aristotle’s Universe
Planets on physical spheres – each moving the other – earth and sphere
of prime mover stationary
Aristotle-Ptolemy
and the Church
Aristotle’s influence
Science roots of cosmology
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Pythagoras and Plato described the world through
mathematical relationships. Many common ideas – Aristotle
started from these with focus on
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Universe is dynamic, living, organic structure perfectly ordered on
musically mathematical proportions
Earth is center of the universe and planets revolve in perfect circles
Aristotle used Mathematics and Observations for his cosmos
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Cosmology based on the fact that there are natural laws that
describe how objects behave in the Universe
Using these laws and sufficient observations we can understand the
universe
Aristotle’s laws of motion
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Motion – Kinematics-On Earth
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Why do objects move the way the do on earth?
Why do objects fall when dropped?
Why stones fall while bubbles rise on water
Answer: They move till they found their natural state- rest
Different elements move differently
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Earth towards the center of the earth
Fire away from center of the earth
Water and air occupy the in between space
Motion linear
Concept of force Force=(Resistance)x(speed)
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Continuous motion required application of force - inertia
Laws of motion
The heavens - Ether
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Motion – kinematics – heavens
Fifth element ether
 Objects in heavens (above moon) move in circles
 Why? Universe finite – linear motion would force
them to fall off the end of universe
 Why earthly objects cannot reach the end of
universe? – they move in circles as soon they
approach heavens
Aristotle’s cosmology inextricably linked to laws of
motion (physics) as today’s cosmology
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Aristotle vs. Aristarchos
Geocentric vs. heliocentric
Pros and cons
Medieval Universe
Enter Copernicus - 1543
Copernican Principle:
THE EARTH IS NOT
LOCATED IN A
PRIVILEDGED POSITION
Tycho Brahe
Johannes Keppler
Tycho Brahe aristocrat/nobleman
observer – naked eye
Observations from island observatory
Amassed data with extreme precision
In 1572 observed first supernova in the
constellation of Cassiopeia. Could not
measure parallax but he realized that it
was not an atmospheric brightening but
it belonged to the realm of fixed stars
-> Heavens not immutable
Used parallax to show that comet orbits
lay outside the orbit of the moon.
Another strike against Aristotle
Brahe on observing
Supernova
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Amazed, and as if astonished and stupefied, I
stood still, gazing for a certain length of time
with my eyes fixed intently upon it and
noticing that same star placed close to the
stars which antiquity attributed to Cassiopeia.
When I had satisfied myself that no star of
that kind had ever shone forth before, I was
led into such perplexity by the unbelievability
of the thing that I began to doubt the faith of
my own eyes.
Brahe’s Universe
Earth at the center
orbited by moon
and Sun
Planets orbit Sun
Failed to confirm
Copernicus by
testing parallax
change of Mars,
Jupiter due to
inaccurate
knowledge of earth’s
orbit (by a factor of
20)
Keppler
Elliptical paths
Conic section
Foci
Major axis
Minor axis
Circular vs. elliptical motion
KEPLER’S LAWS
The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.
R is semi-major axis
(P/yr)2=(R/1 AU)3
1 AU=150 million km
The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.
From ellipse to parabola
Keppler vs. Copernicus
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Wrong assumptions
of Copernicus
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Planets move in
perfect circles
Planets move in
constant speed
Sun at the center of
these orbits
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Keppler showed
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Planets move in
ellipses
Planetary speed
varies constantly
Sun is not quite at
the center of the
planetary orbits
Galileo Galilei
Galileo
Looking at the sky
PHASES
OF
VENUS
MOON
Geocentric vs. heliocentric
Post Galileo
Galileo’s
drawing
Huyghen’s
drawing
Hubble
image
Zeno
Achilles and the turtle
Atomists – Zeno-Democritus-Lefkippos
Universe made up of little particles –
atoms- indivisible and eternals –quanta
They move in vacuum –
Aristotle -> no vacuum
Stoic Universe