motion 8° backwards and forwards in 672 years; and according to Al Batraki (Alpetragius in the twelfth century), the erroneous value of 36″ which Ptolemy had found for the constant of precession, gave rise to the whole mischief, as his successors could not believe that he had found an erroneous value. Al Battani was the only Arabian astronomer of note who was not an implicit believer in trepidation, but from the time of Al Zerkali of Cordova (about 1060) the theory of this wholly imaginary phenomenon was developed minutely. In the Alphonsine tables the period of the inequality of precession was assumed to be 7000 years, though King Alphonso personally seems to have believed precession to be uniform. From these tables and the Arabian authors the theory was spread to Europe, and was further investigated by Purbach and Regiomontanus, who assumed with Tabit that the apparent equinox moved in a small circle with a radius of 4° 18′ round the mean equinox, whereby the annual precession was sometimes accelerated and sometimes retarded. In the sixteenth century trepidation was made the subject of two treatises by Johannes Werner of Nürnberg, and in the third book of his great work Copernicus has also examined it in detail, and showed how annual precession had always varied from the time of Timocharis (300 B.C.) till his own time. It was a natural consequence of the belief in the motion of the equinox on a small circle that the obliquity of the ecliptic should also vary irregularly; and though it had been steadily diminishing since the days of Eratosthenes, even Copernicus considered such irregularities proved by the observations of the ancients and the Arabians. The first to see that the obliquity of the ecliptic had always diminished at a regular rate since the commencement of history seems to have been Fracastoro (1538), after whom the same was asserted by Egnazio Danti in 1578.[1]
- ↑ Primo volume dell' Uso e Fabrica dell' Astrolabio e del Planisferio. Firenze, 1578.
