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Theories of the Solar System

If you have read the file called 'Planetary motion' you will know the problems that faced the astronomers in ancient times when they tried to explain the movement of the planets.

The Ancients had to invent a theory that agreed with their three basic ideas about the Solar System.

1. All motion in the heavens is uniform circular motion
2. The objects in the heavens are made from perfect material, and cannot change their intrinsic properties (e.g., their brightness).
3. The Earth is at the centre of the Universe.

Of these three probably the one that caused most trouble was the need to have the Earth in the centre of the Solar System with all the planets circling round it. To explain this, scientists put forward some weird ideas.

Aristotle (384-322 BC), Eudoxus and others proposed that the heavens were made of up to 55 concentric, transparent spheres, each one carrying the Sun, a planet or some stars. Figure 1 shows the spheres to which the Sun and planets were fixed.

In 280 BC Aristarchus (310-230 BC) solved the problem by putting the Sun at the centre. However, Aristotle was more respected and so it was his theory that was believed.


However Aristotle's theory did not explain the real difficulty. This was what was known as "retrograde motion", the backwards movement in the sky of some of the planets at times during the year. The retrograde motion of Mars is shown in Figure 2.

Each sphere was supposed to have a constant angular velocity (it rotates through equal angles in equal times) within the outer sphere. This outer sphere was called the Prime mover the other spheres were fixed inside this outer sphere.

Planets on a steadily rotating crystal sphere would always move in the same direction through the sky. Also each planet did not change its distance from the Earth the brightness of the planet should stay the same it doesn't!

Ptolemy (85-165 AD) attempted to solve the retrograde motion by suggesting the idea of epicycles. He imagined a small circle (the epicycle) rotating on a large circle (the deferent). The planet was fixed to the epicycle. (Figure 3).

As the epicycle moved round the deferent there would be times (such as A to B) when the planet seemed to be moving backwards when seen from the Earth.

This fitted the observations until slightly more accurate measurements were made and this meant that Ptolemy had to add another epicycle on the first epicycle to get the correct planetary movement.


schoolphysics: Epicycles animation

To see an animation of the movement of epicycle motion click on the animation link.

A Mediaeval view of the Universe. The man is looking through a sphere to see all the machinery that makes it turn as day and night passes.

It all began to get really complicated. A pope of that time said "If I had been God I would have thought of something a lot more simple".

Fortunately Copernicus did.


Born in Torun, Poland. Copernicus is the Latin form of his name. Copernicus revived the model of the Solar System that had been held by the Greek, Aristarchus. However this idea was still not well received, this time by the church. Just as the Greeks insisted that the Earth must stay still and that all orbits must be circular as the circle was the 'perfect shape' so the church argued that the Earth must be the centre of the universe as it was on Earth that Christ was born. They used such quotations as: God has founded the Earth and it shall not be moved. (David in Psalm 89).

In 1514 he 'published' a small hand written book which he gave to some of his friends. In it he proposed the following:
There is no one centre in the universe.
The Earth's centre is not the centre of the universe.
The centre of the universe is near the sun.
The distance from the Earth to the sun is imperceptible compared with the distance to the stars.
The rotation of the Earth accounts for the apparent daily rotation of the stars.
The apparent annual cycle of movements of the sun is caused by the Earth revolving round it.
The apparent retrograde motion of the planets is caused by the motion of the Earth from which one observes.

In his book, De Revolutionibus, Copernicus proposed the heliocentric or Sun-centred theory of the Solar system. However, possibly because he was concerned about possible criticisms from some members of the scientific community and the church, he delayed its publication, only organising the final printing due to encouragement from a friend and pupil Rheticus. As a result he only saw the completed work on the day of his death .

Gradually however the heliocentric, idea of the Solar System was accepted and it has been proved by the space flights of this century.

It explained the backwards (retrograde) motion and also the change in the brightness of planets from year to year. His main book was published at his death in 1543.

Imagine the planets being like the runners on a circular running track, and the runners nearer the centre running faster than those in the outer lanes. Earth, in the third lane, moves faster than Mars, in the fourth lane and so overtakes it on the inside. This would give the appearance of Mars moving backwards against some distant street lamps (the stars) outside the track.


The looping motion of the planets when seen from the Earth can be explained fully as shown in Figure 6.

© Keith Gibbs 2013