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PHYSICS · Stage 7 · Chapter 5: The Earth in Space

Physics 5.7 Changing ideas 2: geocentric model — Ptolemy vs Copernicus

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Physics 5.7 Changing ideas 2: geocentric model — Ptolemy vs Copernicus — Alhussan International Academy — Physics (Stages 7–9) notes: Describe the geocentric model, the evidence for it, the observation it could not simply explain, and why the heliocentric model replaced it.Geocen…

  • Plato and Aristotle
  • Seven planets
  • Crystalline spheres
  • Everyday experience
  • Falling objects
  • Stars above and below the horizon

Part 1 of 5The Greek geocentric model

1Geocentric
= Earth-centred, since "geo" means Earth in Greek.
2Plato and Aristotle2 points
— Greek astronomers who lived about 2400 years ago and developed the model.
  • The Earth did not move; the stars and planets moved around it.
  • All objects, including the Earth, were spherical and moved at a steady speed.
3Seven planets
— Mercury, Venus, Mars, Jupiter, Saturn, the Sun and the Moon, with the stars beyond them.
4Crystalline spheres2 points
— Objects were carried round on invisible spheres that light could travel through.
  • The Moon was on the sphere closest to the Earth.
  • The stars were on the sphere furthest from the Earth.

Part 2 of 5Why the geocentric model was believed

  • Everyday experience — The ground does not feel as if it is moving, which supported a stationary Earth at the centre.
  • Falling objects — Greek philosophers thought all objects fall towards the centre of the Universe, and dropped objects fall to the Earth.
  • Stars above and below the horizon — Ptolemy saw that half the stars are above the horizon and half below at any time.
    • This fits an Earth at the centre; if it were not central the numbers would differ.
  • No apparent shift of the stars — If the Earth moved round the Sun, stars would seem to change position between January and July.
    • No shift could be seen without a telescope.
    • Stars are so far away that the shift is too small for ancient astronomers to detect.
  • Success — The model explained many observations and made predictions, and it fitted what people could see and feel.

Part 3 of 5The observation that was a problem

  • Retrograde motion — Planets seem to wander across the sky and sometimes move backwards against the stars before resuming their normal direction.
  • Ptolemy — A Greek astronomer who lived about 1800 years ago and changed the model to explain this motion.
    • He added small circles (epicycles) on top of the larger circles that were the planets' paths.
    • He worked out the size needed to explain the backward motion and predicted future planet positions.
    • His book describing the model was used for hundreds of years.
  • Drawback — The epicycles worked for predictions but made the model more and more complicated.

Part 4 of 5Copernicus and the heliocentric model

  • Heliocentric = Sun-centred, since "helio" means Sun.
  • Copernicus — In his model the planets orbit the Sun.
    • The backward motion is a natural result of the Earth overtaking an outer planet, like a faster car passing a slower one.
    • This explains the planets' motion more simply than epicycles.
  • Why scientific explanations change — They change when there is new evidence, often from things people cannot immediately see.

Part 5 of 5Geocentric and heliocentric models compared

FeatureGeocentric (Ptolemy)Heliocentric (Copernicus)
CentreEarthSun
Planets' backward motionNeeds epicyclesNatural result of orbits
Apparent shift of stars over a yearNo shift expectedShift expected but too small to see
SimplicityComplicatedSimpler

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