other at perigee; so that the centre of distance must be nearer the
earth. He found it best to assume the centre of distance half-way
between the centre of the earth and the excentric, thus "bisecting the
excentricity". Even this did not fit in the case of Mercury, and in
general the agreement between theory and observation was spoilt by the
necessity of making all the orbital planes pass through the centre of
the earth, instead of the sun, thus making a good accordance practically
impossible.

[Footnote 1: See Glossary for this and other technical terms.]

After Ptolemy's time very little was heard for many centuries of any
fresh planetary theory, though advances in some points of detail were
made, notably by some of the Arab philosophers, who obtained improved
values for some of the elements by using better instruments. From time
to time various modifications of Ptolemy's theory were suggested, but
none of any real value. The Moors in Spain did their share of the work
carried on by their Eastern co-religionists, and the first independent
star catalogue since the time of Hipparchus was made by another
Oriental, Tamerlane's grandson, Ulugh Begh, who built a fine observatory
at Samarcand in the fifteenth century. In Spain the work was not
monopolised by the Moors, for in the thirteenth century Alphonso of
Castile, with the assistance of Jewish and Christian computers, compiled
the Alphonsine tables, completed in 1252, in which year he ascended the
throne as Alphonso X. They were long circulated in MS. and were first
printed in 1483, not long before the end of the period of stagnation.

Copernicus was born in 1473 at Thorn in Polish Prussia. In the course of
his studies at Cracow and at several Italian universities, he learnt all
that was known of the Ptolemaic astronomy and determined to reform it.