Historical Perspectives of Cosmological Theories

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Historical Perspectives of Cosmological Theories

• From the Ancient Greeks to Kepler

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Greeks

• Observation was important for navigation
• Observations at night-time
• Stars were in fixed patterns i.e. had the same
  position relative to one another.
• These constellations rotated once a day.
• Stars rotated about a fixed point once a year.
• There were wandering stars (planets).
• Stars in the sky are at different angles to the
  ground when viewed from different positions on
  Earth.
• Observations during daytime
• Eclipses
• The sails of an approaching ship are seen before
  any other part.
• Shadows at the same time of day in two different
  places on Earth differ.

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Greek theory Ptolemys Geocentric Model

• Earth is round.
• Earth is at the centre of the universe.
• All the planets , stars and the sun rotate round
  it.
• All motion is in circles as the circle is the
  perfect shape, having neither beginning nor end.
• All bodies are carried by invisible crystal
  spheres.
• The heavens belonged to the gods, so were perfect.

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Ptolemys Geocentric ModelProblems

• Could not easily describe the retrograde motion
  of some planets.
• The solution was to refine the theory and
  introduce epicycles into the model.
• Some philosophers did not like this complication.
• But the theory did predict accurately the
  positions of the stars and the planets.

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New TheoryCopernicuss Heliocentric theory 1543

• Copernicuss instinct was that the Sun should be
  the centre of the universe. Ptolemys theory was
  too unwieldy.
• The Earth turned on its axis.
• Star sphere kept still.
• Problems with theory
• Could not accurately predict the position of the
  planets.
• Stones fell to Earth!
• No parallax was observed when looking at the
  stars at different times of the year.

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Other contributors to the Heliocentric theory

• Tycho Brahe (1546-1601)
• He made very accurate measurements
• He observed comets and showed that they came from
  outside the crystal sphere.
• No sphere at all??

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Other contributors to the Heliocentric theory

• Johann Kepler 1600
• Must be underlying harmony and perfection in the
  universe.
• Devised a model using five regular solids to
  provide a pattern in the distance of the planets
  from the sun. This model did not work.
• Introduced the idea that planets travelled in an
  ellipse.

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Keplers laws

• 1st Law
• Planets travel round the sun as an ellipse with
  the Sun at one of the foci. (The Copernican
  model then fits the observed data without the
  need for epicycles.)
• 2nd Law
• For a given period of time an imaginary line
  joining the Sun and a particular planet always
  sweeps out an equal area. (Kepler had realised
  that the planets could not be travelling at a
  constant speed.)
• 3rd Law
• - Orbital period2 ? distance from the sun3
• T2 ? d3
• (This law fitted in with Keplers idea of
  harmony)