precession noun. [prɪ'sɛʃ(ǝ)n] ME.
[Late Latin praecessio(n-), from Latin praecess- pa. ppl stem of praecedere PRECEDE: see -ION.]
1. [By confusion.] A procession. ME-E16.

2. a. Astronomy. In full precession of the equinoxes: the earlier occurrence of the equinoxes in each successive sidereal year (due to precession of the earth's axis); the slow change of direction of the earth's axis, which moves so that the pole of the equator rotates around the pole of the ecliptic once in about 25,800 years. Also, any motion (e.g. of the equinoctial points or the celestial pole or equator) associated with the precession of the equinoxes. L16.
LARMOR precession. lunisolar precession the part of the precession of the equinoxes caused by the gravitational attraction of the moon and sun on the earth. planetary precession the part of the precession of the equinoxes caused by the gravitational attraction of the other planets.
b. Physics. A motion analogous to that of the earth's axis in the precession of the equinoxes; a rotation of the axis of a spinning body about another axis due to a torque acting to change the direction of the first axis; spec. the rotation of the spin axis of a nucleus, electron, etc., about the direction of a magnetic or an electric field. L19.

3. The action or fact of preceding in time, order, or rank. E17.

4. Phonetics. Advance in oral position. M19.

■ precessional adjective (Astronomy & Physics) E19.

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