Giants%20of%20Science

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Giants of Science

• Our understanding of the Universe developed
  slowly over centuries.
• Most of the breakthroughs came through careful
  study of the positions and motions of the Sun,
  Moon, planets and stars.
• The careful analysis of these large sets of
  measurements and the use of mathematics led to
  the creation of physical models that were used to
  make predictions on the future position of
  planets, Sun, Moon, etc.
• When these models failed to explain observed
  phenomena they were abandoned or modified.
  (eventually!)
• The following is a short summary of several
  notable philosopher/scientists and their
  contributions.

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Aristotle (c. 384 - 322 B.C.)

• Taught that the Earth was the unmoving center of
  the Universe.
• Taught that the Earth was spherical
• Lunar eclipses due to Earths shadow are always
  round.
• Different stars are visible the farther South you
  go.

Two examples Aristotle used to prove the Earth is
a sphere.
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Aristarchus (c. 310 - 230 B.C.)

• Used geometry to measure the size and distance of
  the Moon Sun. Moon results were good. Sun
  results were bad.
• Showed that the Sun was bigger than Earth and
  much farther than the Moon.
• Taught that the Earth went around the Sun. (Not
  much proof and not widely accepted).

4
Eratosthenes (c. 276 - 196 B.C.)

• Used shadows and geometry to measure the size of
  the Earth!
• On Summer Solstice Sun was directly overhead in
  Syene, Egypt.(no shadows at noon)
• On the same day there were shadows at noon in
  Alexandria, Egypt.
• Using shadow length and distance between Syene
  and Alexandria and geometry found Earths
  circumference.
• His value probably around 42,000 kilometers.
• Actual value about 40,000 km.

5
Ptolemy (100 - 170 A.D.)

• Last of the great Greek astronomers
• Used discoveries and ideas of others plus his own
  observations to develop a model of the Universe.
• His work survived from Arabic translation of
  Almagest (means the greatest compilation)
• His model of the Universe was undisputed for
  almost 1,400 years!

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Ptolemys Model of the Universe

• A geocentric (Earth-centric) model
• All the stars, planets, Sun and Moon moved around
  the Earth
• Motions were perfectly circular and perfectly
  uniform
• Fastest objects were closest to Earth.

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Retrograde Motion

• It was well known that the planets occasionally
  appear to stop and change direction in orbit, but
  why?
• It is an illusion based on the motion of the
  Earth and the planet.
• Ptolemy explained this effect without moving
  Earth. He used epicycles instead.

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Epicycles

• In a Ptolemys model these are small circles to
  which the planets are fixed.
• These epicycles are attached to the larger
  circles which move around Earth.
• Ptolemy adjusted the sizes of the circles and
  their rate of motion to replicate the motion of
  the planets including retrograde motion.
• Planets maintained perfect uniform, circular
  motion.

The large circle is called a deferent the smaller
circle is an epicycle.
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Nicholas Copernicus (1473 - 1543)

• Recognized that the predicted planetary positions
  using Ptolemys model were not always very
  accurate.
• Tried to fix Ptolemys model but eventually
  abandoned it.
• Returned to the idea of Aristarchus 1800 years
  before.
• Created a heliocentric (Sun-centric) model of the
  Universe.
• Made retrograde motion easier to explain.
• Distances of the planets from the Sun could be
  determined using geometry

Finding the distance of the planets from the Sun
using Copernicus heliocentric model.
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Problems with Copernicus Model

• Copernicus maintained that all planets move in
  perfectly circular orbits at constant rates (not
  correct).
• Didnt really do much better than Ptolemy at
  predicting planetary positions. He actually
  needed to keep epicycles to make the model work
  out right.
• Several objections
• We dont feel the Earth moving. Went against
  common-sense. Went against philosophical
  (Aristotle) and religious beliefs.
• Why dont we see stellar parallax?
• For these reasons it was not widely accepted at
  first

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Stellar Parallax

• Parallax - the shift in an objects position
  caused by the observers motion.
• Since the Earth moves we should see parallax but
  we dont.
• No one realized that stars are very far away so
  parallax is a very small effect.
• Only with powerful telescopes can they be
  observed. 300 years later the first parallax was
  observed.