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.
Saturn
Fixed stars
Sun
Jupiter
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.
Earth
Mars
Moon
Mercury
m
Venus
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.
Sphere of the Prime Mover
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.
Uranus
26th October 2003
th
28 July 2003
18th June 2003
MARS
Figure 2
26th September 2003
1
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!
apparent looping motion
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.
planet
A
P
D
epicycle
B
Earth
C
Q
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.
Figure 3
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.
Figure 4
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).
2
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.
Copernicus was forced to withdraw his theory on pain of death. Gradually however the
heliocentric, or Sun centred idea of the Solar System was accepted and it has been proved
by the space flights of the last thirty years.
It explained the backwards (retrograde) motion
Saturn
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.
Earth
Venus
Sun
Mars
Mercury
Jupiter
The looping motion of the planets when seen from the Earth can be explained fully as shown
in Figure 6.
Earth
Mars
7
6
Sun
5
4
3
2
1
Apparent looping motion of Mars
in its orbit against the background
of the stars
Figure 6
3