Planetary Motions

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Planetary Motions and Lessons in Science
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Can One Prove that the Earth is Round?

• Shadow of the Earth during a lunar eclipse

• Height of Polaris above the horizon

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Can One Prove that the Earth is Round?

• Shadow of the Earth during a lunar eclipse

• Eratosthenes experiment

• Height of Polaris above the horizon

Inferred size of Earth 250,000 stadia This is
either 20 off, or good to about 1, depending on
the definition of stadia.
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Can One Prove that the Earth goes Around the Sun?

• Proof of motion is through parallax

An objects position will appear to shift due to
change in the observers position. This MUST
occur!!!
Since parallax was not seen, the Earth must not
be moving!
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Geocentric Properties of the Original
Planets(Mercury, Venus, Mars, Jupiter, Saturn)

• The word planet means wanderer.
• The planets always stay close to the ecliptic
  plane, i.e., they move through the zodiac
  constellations.
• Mercury and Venus are inferior planets they are
  never seen very far from the Sun (Mercury never
  more 23, Venus never more than 46).
• Mars, Jupiter, and Saturn are superior planets,
  and can be found anywhere in the zodiac.
• Planets usually move west-to-east against the
  fixed stars. But sometimes the planets move
  backwards (east-to-west). This is called
  retrograde motion.

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Retrograde Motion
Path of a planet with respect to the background
stars
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Retrograde Motion
Path of a planet with respect to the background
stars
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Retrograde Motion
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The Science of Aristotle

• Aristotles ideas
• Heavy objects fall faster than light objects
• Objects have inertia all objects prefer to be
  at rest
• The heavens are perfect and immutable
• All heavenly objects travel about the Earth at a
  constant speed in a perfect circle

So how did Aristotle explain retrograde motion?
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Explaining Retrograde MotionAristotles Model
(350 B.C.)

• Earth at the center (since it is not moving).
• Sun and Moon orbit the Earth (west to east).
• Planets move at a constant speed around small
  circles called epicycles.
• Epicycles orbit around Earth (west-to-east) at a
  constant speed in a circle called a deferent.

Combination of orbital and epicyclic motion
creates retrograde motion.
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Explaining Retrograde MotionAristotles Model
(350 B.C.)

• Earth at the center (since it is not moving).
• Sun and Moon orbit the Earth (west to east).
• Planets move at a constant speed around small
  circles called epicycles.
• Epicycles orbit around Earth (west-to-east) at a
  constant speed in a circle called a deferent.

Combination of orbital and epicyclic motion
creates retrograde motion.
Trouble is, it doesnt do a very good job of
predicting exact positions.
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Explaining Retrograde MotionPtolemys
Refinement (140 A.D.)

• Put the Earth slightly off center at a point
  called the eccentric
• State that epicycles only move at a constant
  speed about the deferent when viewed from a
  special place called the equant

Model is more complicated, and, though it does
better, it still doesnt predict the exact
positions of the planets.
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Explaining Retrograde MotionThe Copernican
Model (1530 A.D.)

• Since the planets are in the heavens, they must
  move in perfect circles at a constant speed. But
  
• The heavenly bodies do not all move around the
  same center.
• The Earth is not at the center of the planetary
  system (i.e., the universe). Only the Moon goes
  around the Earth.
• The Sun is at the center of the planetary system.
• Compared to the distance of the fixed stars, the
  distance from the Earth to the Sun is negligible.
• The daily revolution of the sky is due to the
  Earths rotation.
• The Suns annual motion is due to the Earths
  orbit around the Sun.
• Retrograde motion is due to the Earths orbit
  around the Sun.

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Explaining Retrograde MotionThe Copernican
Model (1530 A.D.)

• Retrograde motion is explained by the Earth
  passing (or being passed by) another planet in
  its orbit.

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Explaining Retrograde MotionThe Copernican
Model (1530 A.D.)

• Retrograde motion is explained by the Earth
  passing (or being passed by) another planet in
  its orbit.

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The Heliocentric Model

• The Heliocentric model also naturally explains
  the difference between inferior and superior
  planets.

But the model is no better at predicting the
positions of the planets than Aristotles model.
(And are the stars really so far away that we
cant see parallax???)
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Galileos Experiments

• Galileo tried something new doing experiments!
• Dropping balls to measure gravity
• Rolling balls to examine inertia
• Observing the sky through a telescope!

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What Galileo Saw

• An imperfect Sun (sunspots)

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What Galileo Saw

• An imperfect Sun (sunspots)
• A Moon with mountains and craters

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What Galileo Saw

• An imperfect Sun (sunspots)
• A Moon with mountains and craters
• The ears of Saturn

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What Galileo Saw

• An imperfect Sun (sunspots)
• A Moon with mountains and craters
• The ears of Saturn
• Four moons orbiting Jupiter

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What Galileo Saw

• An imperfect Sun (sunspots)
• A Moon with mountains and craters
• The ears of Saturn
• Four moons orbiting Jupiter
• The Milky Ways stars

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What Galileo Saw

• An imperfect Sun (sunspots)
• A Moon with mountains and craters
• The ears of Saturn
• Four moons orbiting Jupiter
• The Milky Ways stars
• The Phases of Venus

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Next time -- Gravity