[4] See p. 6 for definition.



4. THE REIGN OF EPICYCLES--FROM PTOLEMY TO COPERNICUS.


After Ptolemy had published his book there seemed to be nothing more
to do for the solar system except to go on observing and finding more
and more accurate values for the constants involved--viz., the periods
of revolution, the diameter of the deferent,[1] and its ratio to that
of the epicycle,[2] the distance of the excentric[3] from the centre
of the deferent, and the position of the line of apses,[4] besides the
inclination and position of the plane of the planet's orbit. The only
object ever aimed at in those days was to prepare tables for
predicting the places of the planets. It was not a mechanical problem;
there was no notion of a governing law of forces.

From this time onwards all interest in astronomy seemed, in Europe at
least, to sink to a low ebb. When the Caliph Omar, in the middle of
the seventh century, burnt the library of Alexandria, which had been
the centre of intellectual progress, that centre migrated to Baghdad,
and the Arabs became the leaders of science and philosophy. In
astronomy they made careful observations. In the middle of the ninth
century Albategnius, a Syrian prince, improved the value of
excentricity of the sun's orbit, observed the motion of the moon's
apse, and thought he detected a smaller progression of the sun's
apse. His tables were much more accurate than Ptolemy's. Abul Wefa, in
the tenth century, seems to have discovered the moon's "variation."
Meanwhile the Moors were leaders of science in the west, and Arzachel