Trepidation
The trepidation is a historical astronomical term that describes a fluctuation in the precession of the equinoxes . It is a kind of tremor of the earth's axis that causes inaccuracies in the precession, so that the progression of the vernal equinox on the ecliptic is sometimes faster and sometimes slower. The corresponding values are to be added to or subtracted from the precession.
The trepidation was often described and discussed in the Middle Ages. Today these reports are believed to be in error; the phenomenon is not observed and is not recognized in modern astronomy.
history
Theon of Alexandria is said to have first described the theory of trepidation in the 4th century, referring to "older astronomers" who are not named. According to Theon, these would have observed the advance of the stars by 1 ° in 80 years, but only up to a limit of 8 °, then the process would reverse and run back the same 8 °. The reversal of this movement is said to have happened 128 years before Augustus. Theon decided against this view, however, and preferred Ptolemy 's model, in which the precession has a fixed and continuous value.
In the Middle Ages, several Arab astronomers cite this fluctuation, for example the book De motu octavae sphaerae , whose Arabic, albeit lost, original is ascribed to Thabit Ibn Qurra , which is not undisputed. Thabit - or the author of the book mentioned - worked out a detailed model of the spheres in which he related the fluctuation of the precession value to the change in the ecliptic skew. It is doubtful whether the famous astronomer al-Battani also accepted the trepidation.
Thabit's trepidation calculations were adopted from the Toledan tables of Al-Zarqali in the 11th century and the phenomenon became known and discussed throughout Europe. Soon, however, the reversal of the movement was rejected as wrong and a fixed precession value was assumed, to which the trepidation was then added as a fluctuation, namely this deviation should be +/- 9 ° and complete a cycle in 7000 years. This model was adopted from the Alfonsine Tables of Alfonso the Wise of Castile and turned into the theory of trepidation adopted as the standard in the Middle Ages. Georg von Peuerbach took over Thabit's model in 1472 and even Nikolaus Kopernikus tried in 1543, building on Thabit, to account for the trepidation and to combine it with the change in the inclination of the ecliptic.
Rejection
As early as the middle of the 13th century, however, there were increasing numbers of voices who considered this oscillation to be erroneous and advocated a fixed value of the tropical year and thus precession. At the end of the 15th century, Regiomontanus vehemently opposed the view that celestial bodies could occasionally perform irregular movements. Finally, Tycho Brahe checked the star data of the Almagest and in the work Astronomiae Instauratae Mechanica (1595) came to the conclusion that the precession was uniform and the small inaccuracies were to be ascribed to "random causes". From about this time on, the trepidation is considered to be erroneous, or is attributed to observation errors or misunderstandings regarding the graduation, and no longer appears in astronomical observations or calculations.