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.
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.
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.
