Heliocentric vs Geocentric view points/history

1. The Copernican
Revolution
The Birth of Modern Science
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2. What do we see in the sky?
• The stars move in the sky but
not with respect to each other
• The planets (or “wanderers”)
move differently from stars
– They move with respect to the
stars
– They exhibit strange
retrograde motion
• What does all this mean?
• How can we explain these
movements? 2
• What does the universe
look like?

3. Geocentric vs. Heliocentric
• How do we decide
between two theories?
• Use the Scientific method:
– These are both explanations
based on the observation of
retrograde motion
– What predictions do the
models make?
– How can these predictions 3
be tested?

4. Geocentric vs. Heliocentric Models of the
Solar System
Ancient Greeks knew of Sun, Moon, Mercury, Venus, Mars, Jupiter
and Saturn.
Aristotle vs. Aristarchus (3rd century B.C.)
Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.
Aristarchus: 1st Heliocentric model
Aristotle: But there's no wind - Doesn’t “feel” like
we are moving. (Actually orbiting sun at 70,000 mph!)
Difficulties with "Geocentric" model
- Retrograde motion of planets
- Phases of Venus 4

5. Geocentric Model of Solar System
(Earth Centered)
What are some reasons that the
geocentric model of the universe
seems to make intuitive sense?
It doesn't feel like we are
moving – wouldn't there be a
wind or something?
Why would things fall down and
not towards the center of the
universe?
Why don't we see stellar
parallax?
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6. GeoCentric
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7. Geocentric Theory
Ancient Greece
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8. Geocentric Model
(Earth Centered)
• Fairly good agreement with
retrograde motion of
planets
• Some predictive power
• More precise
measurements showed
errors
• To account for unusual
planetary motion epicycles
were introduced
• Fit the Greek model of
heavenly perfection – spheres
are the perfect shape, circular
the perfect motion 8

9. Ptolemy’s Prediction: Future planetary positions
Observation: retrograde motion of planets
Refine: epicycles
Success! For 1500 years
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cfa-www.harvard.edu/seuforum/mtu/MTUcosmology.ppt

10. Motion of the other planets
Mars Retrograde Motion
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12. 12
Ptolemy's geocentric model (A.D. 140)
(VIDEO CLIP)

13. Ptolemy’s system provided the first
framework for all discussion of the
universe for nearly 1500 years!!
But… there was a problem
Problems with Geocentric Theory
1. This model does not explain all
apparent motions of celestial (space)
objects
2. Cannot explain Foucault’s Pendulum or
the Coriolis Effect 13

14. Geocentric model fails to
account for phases of Venus
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15. Aside: Aristarchus
- Written in the second century BC he
calculated the ratio of the distance
between the Earth and Sun to that
between the Earth and the Moon. (His
estimate was more than an order of
magnitude too small, but the fault was in
his lack of accurate instruments rather
than in his method of reasoning.)
-This image compares the line
subtending the arc dividing the light and
dark portions of the Moon in a lunar
eclipse with the relative diameters of the
Moon and Sun.
-Aristarchus also found an improved
value for the length of the solar year.
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16. Geocentric vs. Heliocentric
• Against heliocentric
– It predicted planetary motions and events no better
than the Geocentric system
– The earth does not move (things do not fly off)
– The earth is different from the heavens (from Aristotle –
the heavens are perfect and unchanging) and cannot
be part of the heavens
• For heliocentric
– Simplified retrograde motion, but epicycles were
necessary to account for the planets’ changing speed
– The distances to the planets could be measured.
These distances were ordered, and therefore
aesthetically pleasing to the philosophy of the day16

17. Heliocentric (Copernican) System
• Sun at center (heliocentric)
• Uniform, circular motion
– No epicycles (almost)
• Moon orbited the earth, the
earth orbited the sun as another
planet
• Planets and stars still on fixed
spheres, stars don’t move
• The daily motion of the stars
results from the Earth’s spin
• The annual motion of the stars
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results from the Earth’s orbit

18. Heliocentric model easily
accounts for phases of Venus
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19. Timeline
Tycho Galileo
1473-1543 1546-1601 1571-1630 1564-1642 1642-1727
Copernicus Kepler Newton

21. Heliocentric Model
“Rediscovered” by Copernicus in 16th century.
Copernicus 1473-1543
Much simpler was the main attraction for
Copernicus. Simply explains retrograde motion.
Put Sun at center of everything, but
still insisted on circles, thereby
retainig unnecessary complexity.
Opposed by Catholic Church
Copernican revolution – critical
realization that Earth is not at the
center of the universe, only accepted
after his death.

22. Heliocentric Theory

23. • In the heliocentric model, apparent
retrograde motion of the planets is a
direct consequence of the Earth’s
motion

24. Retrograde Motion of Planets
Earth overtakes slow outer planet so the outer planet appears to
slow down, move in reverse, and then move forward again with
respect to the fixed stars
Planets sometimes appear to loop
back - retrograde motion
Loops are called "epicycles"
July
Mars 7
Earth *
7 6
6
*
5
Apparent motion of 3 *
Mars against 4 4 *
*
"fixed" stars 3
5
2
2 24
1
*
1
January

25. Stellar Parallax
• Parallax caused by the motion of
the earth orbiting the Sun
• Not observed with the naked eye
• The heliocentric model predicts
stellar parallax, but Copernicus
hypothesizes that the stars are
too far away (much farther than
the earth from the Sun) for the
parallax to be measurable
with the naked eye

26. Parallax
• Triangulation - Measure
angles at points A and B
• Parallax - Know
Baseline. Measure third
angle in triangle made
by A, B, and object in
space
– Baseline problem
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The apparent displacement (shift) of a foreground object relative to the background as the observer’s
location changes is known as parallax.

27. Misconceptions
1. The Copernican model has a force between the
sun and the planets. Actually, the natural motion
of the celestial spheres drove the planetary
motions.
2. The Copernican model was simpler than the
Ptolemaic one. In fact, though Copernicus
eliminated circles to explain retrograde motion, he
added more smaller ones to account for
nonuniformities of planetary motions.
3. The Copernican model predicted the planetary
motions better. Because both models demanded
uniform motion around the centers of circles, both
worked just about as well – with errors as large
as a few degrees at times.

28. 29

29. Tycho Brahe (1546-1601): the
greatest of the pre-telescope
observers in Europe.
20+ years measuring the positions
of the Sun, Moon and planets with
great accuracy.

30. Tycho Brahe
• Had two sets of astronomical
tables: one based on Ptolemy’s
theory and one based on
Copernicus’.
• He found that both tables’
predictions were off by days
to a month.
• He believed that much better
tables could be constructed
just by more accurate observations.
• Tycho’s homemade instruments improved
measurement precision from ten minutes of
arc (which had held since Ptolemy) to less
than one

31. The skies change
Tycho observed 2 phenomena that
showed the heavens DO change:
– In November 1572, Tycho noticed
a new star in the constellation
Cassiopeia
– Comet of 1577
• Prior to this sighting,
comets were thought to be atmospheric
phenomena because of the immutability
of the heavens
• But neither the star nor the comet changed
position as the observer moved, as expected
for atmospheric phenomena

32. • Johannes Kepler (1609)
• Johannes Kepler was an assistant to
Tycho Brahe. He used Brahe’s
observations to study the orbit of Mars
• Discovered three laws of planetary
motion: predict with accuracy the
motions of the planets
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33. Johannes Kepler
• Kepler succeeded Tycho as the Imperial mathematician
(but at only 1/3 the salary of the nobleman)
• Kepler worked for four years trying to derive the
motions of Mars from Brahe’s observations
• In the process, he discovered that the plane of the
earth’s orbit and the plane of Mars’ (and eventually the
other planets) passed through the sun
• Suspecting the sun had a force over the planets, he
investigated magnetism
• While this is not true, it did lead him to the idea of
elliptical orbits
• “With reasoning derived from physical principles 37
agreeing with experience, there is no figure left for
the orbit of the planet except a perfect ellipse.”

34. Galileo (1564-1642),
Experimentalist
Built his own telescope.
Discovered four moons orbiting
Jupiter. What does this suggest?
Discovered sunspots. What might
we infer about the Sun from these
observations?
Observed phases of Venus.
Was imprisoned for the last 9 years of his life for his scientific
discoveries.

35. Galileo Galilei
• Turned a telescope toward the heavens
• Made observations that:
– contradicted the perfection of the heavens
• Mountains, valleys, and craters on the Moon
• Imperfections on the Sun (sunspots)
– Supported the heliocentric universe
• Moons of Jupiter
• Phases of Venus – shows a full phase

36. • Galileo (1564-1642) first scientist to
use a telescope to observe the sky. He
observed the phases of Venus and the
moons of Jupiter. Both observations
supported the heliocentric model.
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38. Modern Universe
• Is the Sun the center of the Universe?
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