Planetary motion

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Planetary motion

• Watching the sky night after night, month after
  month, some stars appear to move
• Planetary motion is much slower than diurnal
  motion
• Five wandering stars known to ancient people
  were Mercury, Venus, Mars, Jupiter, and Saturn.

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Greek Astronomy(Aristotle Ptolemy)

• Believed that the earth was at the center of the
  universe
• All heavenly objects moved on perfect circles
• Models became very complicated to explain
  retrograde motion
• Adopted by Catholic theologians in the Middle Ages

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Nicolaus Copernicus (1473-1543)

• Born Nicolaus Kopernik in Poland
• Priest, mathematician, and physician
• Proposed that the earth is not at the center of
  the universe
• Explained retrograde motion of planets without
  epicyles

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Tycho Brahe(1546-1601)

• Born in Denmark to noble family
• Made the most accurate observations up to that
  time without a telescope
• Measured
• positions of stars planets
• length of year (to within 1 second)
• distance to supernova

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Parallax
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Galileo Galilei(1564-1642)

• Italian inventor and physicist
• First to use a telescope for astronomy
• Discovered
• craters mountains on the moon
• moons of Jupiter
• phases of Venus
• convicted by Inquisition at age 70 and forced to
  recant his theories

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Phases of Venus
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Johannes Kepler(1571-1630)

• Hired as assistant to Tycho Brahe a year before
  Tychos death
• Used Tychos data to work out first truly
  accurate model of solar system
• Obsessed by idea of harmony of the spheres

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Keplers Laws

• The orbit of a planet is an ellipse with the sun
  at one focus
• A line joining a planet and the sun sweeps out
  equal areas in equal intervals of time
• The square of a planets period is directly
  proportional to the cube of the orbits semimajor
  axis
• P2 a3

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Sir Isaac Newton(1643-1727)

• Lucasian Professor of Mathematics at Cambridge
  University, England
• Invented calculus (at the same time as Leibnitz
  in Germany)
• Studied the nature of light
• Combined physics and astronomy for the first time

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Newtons Laws of Motion

• Every object tends to remain in constant motion
  unless acted upon by an outside force
• Force equals mass times acceleration
• Fma
• For every action there is an equal and opposite
  reaction

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Newtons Law of Universal Gravitation

• All objects attract each other with a force
  that is directly proportional to the product of
  their masses and inversely proportional to the
  square of the distance between them.

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Newtons Laws explain planetary motion

• Physics can predict Keplers Laws
• Not all orbits are ellipses!
• Parabolas and and hyperbolas are also possible
  (one time orbits)

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