One Chapter 6

• The Greeks thought there was two types of motion, each considered to be independent of each other.
  • Terrestrial
  • Celestial
• Our present view has developed step-by-step with one idea depending upon past ideas.
• The Greeks understood the motion of stars, planets, the sun and moon as follows:
  • Stars appeared to be on the inside surface of a bowl.
  • Stars group into constellations
  • Stars and constellations moved as the bowl (celestial sphere) moved.
  • Five planets (star-like objects) moved in relation to the rest (Named after Greek gods).
  • Fixed stars (including the sun and moon), moved east to west.
  • The Moon moved east with respect to the stars.
  • Planets seemed to move east with respect to the stars, but motion was more irregular than the moon's motion. Periodically they moved westward with respect to the stars (retrogression).
• The Greeks only observed these things. They had no desire like us to explain them.
• Today, we build models to explain and predict.

• Thales and Pythagoras proposed the following model of the universe.
  • The heavens were eight concentric circles (transparent).
  • All spheres were centered about the Earth and all rotated with different speeds.
  • This is a geocentric view of the universe.
• Geocentric - Earth-centered view of the universe.
• Plato and Aristotle's view of celestial motion was slightly different than Thales and Pythagoras.
  • Celestial objects had to move in perfect circles.
  • To explain observations, and motion in perfect circles, their model consisted of 54 spheres for the various objects in the heavens.
• Ptolemy, from extensive observations from the observatory at Alexandria revised the geocentric model.
  • The planets, sun and moon moved in simple circles with other patterns superimposed on the circle.
  • Ptolemy's model survived until Copernicus (1473 - 1543)
• Helio centric - a sun-centered view of the universe.

• Copernicus realized that planetary predictions were easier to predict if the sun was the center of the universe and not the Earth. Therefore his model states that:
  • Earth was just another planet.
  • Earth rotates on its axis.
  • The moon orbits the Earth.
• The Copernican model was not easily accepted.
  • Opposition was wide spread and people were put on trial for supporting the idea (e.g. Galileo)
  • His theory, however, made better predictions and explained observations better (e.g. retrograde motion).
• A manuscript of Copernicus' ideas circulated among Europe's scholars.
• In 1543 his work was published and dedicated to the pope.
• His theory, (Though he was not the first to propose the idea) began the scientific revolution that dethroned Greek science and started the modern age of science.

• Tycho Brahe made the following contributions to our understanding of the universe.
  • He observed a nova and therefore proved that the heavens were not unchanging. It was a common belief of the time, that the heavens never changed.
  • He showed that the nova was further away than the sun.
  • He built a quadrant and used it to plot the locations of the stars and planets.
  • In 1577 he observed a comet and showed its path was not circular.
  • Brahe's measurements resulted in the establishment of the Gregorian Calendar (The one we use today).
• Johann Kepler was Brahe's assistant and inherited his data.
  • Kepler tried to see if the data fit the geocentric view of the universe and failed.
  • Kepler tried to see if the data fit other shapes besides perfect circles
  • He discovered the data fits if the planets have elliptical orbits about the sun.
• Kepler established three laws from this work.
  • The planets move in elliptical orbits with the sun at one of the foci.
  • The line connecting a planet to the sun sweeps out equal areas in equal times as the planet orbits.
  • The square of the orbital period is proportional to the cube of its mean distance from the sun.

• Kepler's second law implies that the closer a planet is to the sun, the faster it moves.
• Consequences of Kepler's work.
  • Elliptical orbits put an end to Greek astronomy.
  • Without the celestial sphere, an explanation was needed as to why the planets stayed in orbits. Newton found an answer 50 years later.

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November 5, 2013