AP8.2

Precesion of the equinoxes

Here we have three different tables. They describe a very slow celestial motion named the precesion of the equinoxes. This motion produces a slow shift of the equinoxe with respect to stars so that after a few centuries the zodiac seems to move forward or the equinoxe to move backward.

This very slow motion, first noticed by Hyparchus in antiquity, was very important for religious and calendrical reasons and for christian in particular with respect to the date of easter. It was one of the major concerns of astronomers well into the modern period.

Alfonsine astronomers had a specific theory for that phenomenon, expressed by their tables: a composition of two motions. There was first a regular motion (point 1) making a full revolution in 49 000 years. Then there is an irregular motion expressed by two tables (point 2 and 3) with a periodicity of 7000 years