5.7Chapter 5: The Earth in Space

Changing ideas 2: geocentric model — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

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Lesson summary

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

Geocentric

Earth-centred, since "geo" means Earth in Greek.

Plato and Aristotle

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.

Seven planets

Mercury, Venus, Mars, Jupiter, Saturn, the Sun and the Moon, with the stars beyond them.

Crystalline spheres

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

Why 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

Copernicus 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

Feature	Geocentric (Ptolemy)	Heliocentric (Copernicus)
Centre	Earth	Sun
Planets' backward motion	Needs epicycles	Natural result of orbits
Apparent shift of stars over a year	No shift expected	Shift expected but too small to see
Simplicity	Complicated	Simpler

Key terms

Term	Meaning
Geocentric model	A model of the Universe with the Earth at the centre.
Heliocentric model	A model with the Sun at the centre of the Solar System.
Crystalline sphere	An invisible sphere thought to carry the Moon, Sun, a planet or the stars.
Retrograde motion	The apparent backward movement of a planet against the stars.
Epicycle	A small circle added to the geocentric model so that planets could move backwards.

Avoid these mistakes

Thinking people were foolish to believe the geocentric model — it fitted observations and made good predictions.

Thinking Copernicus proved the Earth moves — his model was simpler, but more evidence was needed.

Mixing up the prefixes — geo means Earth and helio means Sun.

Thinking stars do not shift because the Earth is still — they shift too little to see because they are very far away.

Last updated: 13 October 2026