Trepidation (astronomy)

Trepidation as used in the field of astronomy means an imagined oscillation of the equinoxes. It should not be confused with precession. In precession, the equinoxes appear to move slowly through the ecliptic, completing a revolution in approximately 25,800 years (according to modern astronomers).

The earliest known account of trepidation comes from the "Small Commentary to the Handy Tables" written by Theon of Alexandria in the 4th century CE. According to the version described by Theon (which he might not have advocated), the equinoxes oscillate through the ecliptic over a span of 8 degrees, moving 1 degree in 80 years. This seems to have been an alternative to precession. In medieval trepidation models, the oscillation is added to the equinoxes as they precess.

According to the version of trepidation that dominated Latin astronomy in the later Middle Ages, the oscillation occurred over a period of 7000 years, added to the eighth (or ninth) sphere of the Ptolemaic system. This version of trepidation is described in "De motu octavae sphaerae" ("On the Motion of the Eighth Sphere"), believed to be a Latin translation of a lost Arabic original. In Latin the book is attributed to the Arab astronomer Thabit ibn Qurra, but the attribution has been contested in modern times. "Thabit's" trepidation model was used in the "Alfonsine Tables", which assigned a period of 49,000 years to precession.

Islamic astronomers described other models of trepidation. In the West, the main alternative to "De motu octavae sphaerae" was part of the theory of the motion of the Earth published by Nicolaus Copernicus in "De revolutionibus orbium coelestium" (1543). Copernicus' version of trepidation combined the oscillation of the equinoxes (now known to be a spurious motion) with a change in the obliquity of the ecliptic (axial tilt), acknowledged today as an authentic motion of the Earth's axis.

Trepidation of the spheres: Theory of trembling celestial spheres to explain unpredicted variation in the path of the heavenly bodies.

References

*1911
* Otto Neugebauer, "Thabit ben Qurra 'On the Solar Year' and 'On the Motion of the Eighth Sphere'," "Proceedings of the American Philosophical Society" 106 (1962): 264-299.
* F. Jamil Ragep, "Al-Battani, Cosmology, and the Early History of Trepidation in Islam," in "From Baghdad to Barcelona: Studies in the Islamic Exact Sciences in Honour of Prof. Juan Vernet", Barcelona 1996.
* N. M. Swerdlow and O. Neugebauer, "Mathematical Astronomy in Copernicus's De Revolutionibus", (Studies in the History of Mathematics and Physical Sciences 10), Springer-Verlag 1984.