TREPIDATION

, in the Ancient Astronomy, denotes what they call a libration of the 8th sphere; or a motion which the Ptolomaic system attributed to the firmament, to account for certain almost insensible changes and motions observed in the axis of the world; by means of which the latitudes of the fixed stars come to be gradually changed, and the ecliptic seems to approach reciprocally, first towards one pole, then towards the other.

This motion is also called the motion of the first libration.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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