Islamic and Christian Cosmic Theories

Islamic Theories

As the ancient Greek civilisation and culture faded their focus on cosmic theories faded with it. Little or no progress was made for centuries. Eventually, the old Greek texts passed into the hands of Islamic philosophers and they became aware of the Almagest and similar works. They were very concerned about the use of equants and eccentres - believing that uniform motion and pe rfect circles must prevail. 

Ibn al-Haytham

Ibn al Haytham (~965-1040AD) tried to improve on Ptolemy's Almagest and proposed a set of concentric spheres (see right) such that each planet moved in an epicycle within its own space. However, it is difficult to make this model fit observational facts with much accuracy.

Ibn al-Shatir

To avoid eccentres and equants, Ibn al-Shatir (? - 1375) produced this epicycle-on-epicycle solution (see left) in the 14th century describing the motion of the Moon.

The Mean Sun moves round the Earth in a circle once a year. The Mean Moon (centre of large epicycle) moves round the same circle as the sun once a month. The centre of a smaller epicycle moves  clockwise round the larger epicycle. The Moon moves round the smaller epicycle anti-clockwise.

This approach also avoided the problem with Ptolemy's theory where the Moon's distance (and therefore apparent diameter) must vary by a factor of two (which it clearly does not).  This elaborate geometry may go some way towards predicting the Moon's position but provides no clue as to the physical nature of what is going on.

Latin - Christian Theories

For the most part, information about Babylonian, Greek and Islamic cosmologies did not reach Latin/Christian Europe until the end of the first millennium AD. That was when the Christians overtook Islamic cultural centres in Spain and Portugal, discovering the Arabic versions of the old  Greek texts.

There was much re-hashing of this material, but no real progress. Everyone was very unhappy about eccentres, epicycles and equants, and looking for alternatives.

The Church resisted rational debate by insisting on a cosmos based on  Aristotle's views (see right) suitably made to fit with an interpretation of Genesis. In the centre we have Earth/Water/Air/Fire, then we have each 'planet'  in it's sphere, then the firmament (fixed stars) then the "waters above the firmament". 

The fact that this did not  with observation did not seem to matter too much.

Copernicus

Copernicus (1473-1543) was looking for a more rational and integrated explanation of the cosmos than past theories could provide. He wrote the now famous 'De Revolutionibus', the basis of which is as follows:

The Sun is at the centre of the cosmos. Each of the planets revolves round the Sun. Their orbits are a combination of referent circles with small epicycles. The sequence of planets is, (as we now know them today): Mercury, Venus, Earth, Mars, Jupiter and Saturn. The Moon revolves round the Earth. Beyond Saturn are the "fixed stars".

Copernicus had taken that first major step to propose the Sun as the centre of the cosmos rather than the Earth. This did not go down well with The Church. Copernicus's theory was originally considered only a 'mathematical model' and not a picture of reality. Religious and political pressures continued to insist that Earth is at the centre. 

Tycho

 Tycho  Brahe (1546-1601) appreciated the benefits of Copernicus's theories, but could not accept that the Earth was moving. He devised the ' Tychonic' system shown here. In fact it is mathematically almost the same as the Copernican system, except that the Earth is considered to be static instead of the Sun.

In this scheme the Sun and Moon move round the Earth. All the other planets move round the Sun. 

Tycho made many valuable observations and designed and implemented new instruments. His observations of a comet convinced him that "the spheres" did not really exist and planets moved freely through space. This was a very important change of concept as it allowed scientists to think in terms of moving bodies.