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 perfect 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.
   
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