Paradigm Shifts in Cosmology

1
Paradigm Shifts in Cosmology

• J. Mannion

2
Eratosthenes

• Used simple mathematical principles to establish
  the size of the earth to within 10 accuracy
• May have measured the earth sun distance to
  within 1
• May have measured the distance to the moon

3
Aristotle

• Pointed out that the earth casts a curved shadow
  on the moon
• Thought that the moon and the planets were rocky
  objects
• Suggested that the planets did not fall to earth
  because they were contained in crystalline spheres

4
Ptolemy

• His Almagest synthesised most of the Ancient
  Greek ideas on astronomy
• Coupled them with
• Accurate observations
• A workable mathematical model
• To produce a system that gave accurate
  predictions of planetary positions.
• His system survived for over 1800 years

5
The Ptolemaic system

• Ptolemy assumed that
• All orbits were circular
• The earth was at the centre of the universe
• This meant that he had to introduce epicycles to
  explain the retrograde motion of some planets.

6
A Ptolemaic Epicycle

• The outer planets take longer to orbit the sun
  that the earth does.
• As we 'overtake' them, they appear to go
  backwards across the sky. This is obvious if you
  assume that both the earth and the observed
  planet are orbiting the sun.
• If you assume the earth is fixed you have to
  introduce an epicycle to explain the apparent
  motion

7
Elaboration

• Logically if the crystal spheres moved they would
  make a sound
• This sound must be very beautiful the music of
  the spheres
• Something must move them
• In the middle ages the movers were thought to be
  angels
• Darkness is caused by the shadow of the earth
• The rest of space must be filled with light
• The concept of a vacuum was not well understood
• Logically space must be filled with the most
  'airy' air called 'ether'
• The only changing object in the sky was the moon
• The universe above the moon was unchanging
• Only the sub-lunar universe was changeable.

8
Imaginings
9
Shakespeare

• LORENZO
• How sweet the moonlight sleeps upon this
  bank!
• Here will we sit and let the sounds of music
• Creep in our ears soft stillness and the
  night
• Become the touches of sweet harmony.
• Sit, Jessica. Look how the floor of heaven
• Is thick inlaid with patines of bright gold
• There's not the smallest orb which thou
  behold'st
• But in his motion like an angel sings,
• Still quiring to the young-eyed cherubins
• Such harmony is in immortal souls
• But whilst this muddy vesture of decay
• Doth grossly close it in, we cannot hear it.
• Merchant of Venice Act 5 Sc1

10
Out of the Silent Planet

• ... all he ever remembered of his first meal in
  the spaceship was the tyranny of heat and light.
  Both were present in a degree which would have
  been intolerable on Earth, but each had a new
  quality. The light was paler than any light of
  comparable intensity that he had ever seen it
  was not pure white but the palest of all
  imaginable golds, and it cast shadows as sharp as
  a floodlight. The heat, utterly free from
  moisture, seemed to knead and stroke the skin
  like a gigantic masseur it produced no tendency
  to drowsiness rather, intense alacrity. His
  headache was gone he felt vigilant, courageous
  and magnanimous as he had seldom felt on Earth.
  Gradually he dared to raise his eyes to the
  skylight. Steel shutters were drawn across all
  but a chink of the glass, and that chink was
  covered with blinds of some heavy and dark
  material but still it was too bright to look at.
• 'I always thought space was dark and cold,' he
  remarked vaguely.
• 'Forgotten the sun?' said Weston contemptuously.
• Ransom went on eating for some time. Then he
  began, 'If it's like this in the early morning,'
  and stopped, warned by the expression on Weston's
  face. Awe fell upon him there were no mornings
  here, no evenings, and no night - nothing but the
  changeless noon which had filled for centuries
  beyond history so many millions of cubic miles.
• C.S. Lewis 1938

11
The system breaks down

• Ptolemy's system was preserved by Arabic
  astronomers and even reached India
• It was accurate enough for most purposes but the
  system of epicycles made it cumbersome to use
• By the sixteenth century new ideas began to emerge

12
The Copernican revolution

• Nicholas Copernicus, a Polish astronomer, was the
  first to suggest that the sun was at the centre
  of our solar system
• He foresaw that his views would be controversial
  and did not publish them until shortly before his
  death
• His system improved on Ptolemy's but still needed
  epicycles because he assumed that planets had
  circular orbits

13
And counter-revolution

• Copernicus was right to fear opposition from the
  church
• A helio-centric solar system did not fit in with
  the Biblical story of the creation
• Or with passages in the Bible in which the sun's
  progress across the sky was halted
• Scholars who taught the new system were brought
  before the Inquisition
• In January 1600 Giordano Bruno was burned at the
  stake in part for teaching Copernican views

14
Galileo

• Galileo Galilei, an Italian scientist, was the
  first to turn an efficient telescope on the sky.
  He discovered
• Mountains on the moon
• Satellites orbiting Jupiter
• The phases of Venus
• He was also an early advocate of the Copernican
  system
• None of these things fitted in with Church
  teachings and he came to the attention of the
  Holy Office.
• He was shown the instruments of torture and
  recanted his views, but on being released said
  'Eppur si muove'
• The Church admitted that the trial was mistake in
  1992.

15
Kepler

• Kepler finally established the true shape of
  planetary orbits ellipses.
• His mathematical model of the solar system was
  far more accurate than Ptolemy's and was quickly
  adopted by scientists and navigators
• His discoveries dealt the final blow to the idea
  of a geo-centric solar system.