Understanding Planetary Motion

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Understanding Planetary Motion

• Seven Key Players
• NOTE none of the links in this presentation are
  LIVE you must copy the link into your
  address-bar on the web.

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1. Aristotle (384 ? 322? BCE)

• Greek Philosopher and Scientist
• Believed that motion was caused by a body's
  desire to move and that in order to change the
  motion of an object some violent outside cause
  was required.
• For example He would observe a tree that
  remained at rest for years the tree was at rest
  because it didnt want to move. A strong storm
  however, was a violent cause that could make the
  tree move (i.e fall down)
• Devised the theory of a Geocentric Solar System
• Geo (Earth) Centric (Centered) Earth
  Centered System
• Meaning that the Earth is the center of the
  universe and all bodies revolve about the Earth.

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2. Ptolemy (125 ? 160? CE)

• Greek astronomer and geographer
• Supported Aristotles Geocentric Theory and
  offered proof by acknowledging the phenomena of
  retrograde motion.
• Retrograde motion is the apparent reversing of an
  objects motion. This is apparent when charting
  the motion of planetary bodies.
• Although retrograde motion is an actual
  phenomena, Ptolemy explained it incorrectly (in a
  way that supports a Geocentric theory)

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Ptolemys Retrograde Motion

• Ptolemys explanation for retrograde motion was
  that a planet will move forward in its orbit
  then suddenly circle back in the opposite
  direction.
• He explained that planets move in two paths in
  their orbits. The deferent (the larger orbit)
  and smaller epicycles (in which they circle back)
• Visit the following link to view an animation of
  Ptolemys retrograde motion.
• http//abansil.physics.neu.edu/gross/Animations/P
  tolemy/epicycle.html

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3. Copernicus (1473 ? 1543)

• Polish Monk and astronomer
• Challenged the Ptolemaic (Geocentric) Theory by
  devising the Copernican Heliocentric System
• Helios (Sun) Centric (Centered) Sun-Centered
  System
• Copernicus has four proofs for his new
  (controversial) theory
• Retrograde Motion (new explanation)
• Parallax of Stars
• Seasons
• Aberration of Starlight

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Copernicuss Retrograde Motion

• Copernicus agreed that retrograde motion was a
  real phenomena, but he said that it occurred when
  planets passed each other in their orbits.
• If Earth passes a planet the other planet seems
  to move backward
• This confirmed that the planets must be moving in
  the same direction just at different speeds. It
  is just like passing a slow moving car on the
  highway.

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Visit the following link to view an animation of
retrograde motion. Follow the instructions on
the page. http//www.astro.utoronto.ca/zhu/ast210
/helicentric.html
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Parallax of Stars

• Parallax is the apparent shifting of position of
  an object when viewed from different locations.
• If you watch any object it will seem to zig-zag
  back and forth across the background as you move
  relative to it.
• For star as we move in our orbit, the stars seem
  to shift their position with respect to more
  distant stars.

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• As the Earth revolves around the Sun a near star
  will seem to shift back and forth across a
  backdrop of distant stars.
• For example in July the near star seems to align
  with the right-most distant star while in
  December it is aligned with the left-most star.

July Dec
Visit the following link to view an animation of
parallax of stars. http//www.astro.washington.edu
/labs/parallax/solar.html
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The Seasons

• Copernicus knew that the Earth was tilted on its
  axis at 23.5-degrees. If the Earth was
  stationary, then the amount of light that a given
  part of the Earth receives would not vary.
• We know that this does not occur as the amount of
  daylight varies according to location and time of
  year.
• How did they know 23.5-degrees? We owe this to
  Eratosthenes who first determined it around 200
  BCE. There is more to come about his discoveries
  later.

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Aberration of Starlight

• The true path of light from a star to a planet is
  in a straight line, however, due to the motion of
  a planet it appears as if starlight bends into a
  planet. This makes it seem as if the light is
  originating from a different location.
• This is similar to driving in the snow. The snow
  may actually be falling straight down but as we
  drive we crash into the snow. It seems as if
  the snow if coming toward the car diagonally from
  a point ahead of the vehicle.

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4. Tycho Brahe (1546 ? 1601)

• Danish Mathematician and astronomer
• Tycho kept meticulous records of the motion of
  planetary bodies.
• He tried to mathematically analyze the motion to
  derive patterns, but was unsuccessful.
• Reluctantly provided Johannes Kepler with his
  records of planetary motion.
• Interestingly, Tycho was NOT a believer of the
  heliocentric theory. His planetary data however,
  helped to solidify this theory.

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5. Galileo Galilei (1564 ? 1642)

• Italian astronomer, mathematician and physicist.
• First to use a telescope to observe planetary
  bodies.
• He discovered the moons of Jupiter.
• Devised the theories of motion that are part of
  the foundation for classical mechanics
• 1. Neglecting air resistance, all objects
  accelerate toward Earth at the same rate.
• 2. Derived the relationships for uniform linear
  motion (i.e. the Big Five)
• 3. Defined Inertia

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The other players

• 6) Johannes Kepler (1571 ? 1630)
• 7) Isaac Newton (1642 ? 1727)
• We will discuss the contributions of these
  gentlemen in class.

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Honorable Mentions

• Eratosthenes His early discoveries about the
  geometry, tilt and mass of the Earth laid the
  foundation for future scientists. For more
  information visit
• www.plu.edu/dornerbc/teaching/math203/eratosthene
  s.pdf
• http//www.utc.edu/Faculty/Jonathan-Mies/wkshop/wk
  shop_prod/eratosthenes.pdfsearch'Eratosthenes20
  2C20tilt20of20earth
• Albert Einstein In trying to explain gravity,
  he developed theories that forever changed
  physics. For more information visit
• http//www.aip.org/history/einstein/