A libration of the starry sphere in the Ptolemaic system; a motion ascribed to the firmament, to account for certain small changes in the position of the ecliptic and of the stars.

a feeling of alarm or dread

Trepidation (from Lat. trepidus, "anxious") is a term meaning, in general, the fear or trembling, but used technically in astronomy for 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).