pre·ces·sion /priˈsɛʃən/
 先行,優先,歲差


 Pre·ces·sion n.  The act of going before, or forward.
 Lunisolar precession. Astron. See under Lunisolar.
 Planetary precession, that part of the precession of the equinoxes which depends on the action of the planets alone.
 Precession of the equinoxes Astron., the slow backward motion of the equinoctial points along the ecliptic, at the rate of 50.2˝ annually, caused by the action of the sun, moon, and planets, upon the protuberant matter about the earth's equator, in connection with its diurnal rotation; -- so called because either equinox, owing to its westerly motion, comes to the meridian sooner each day than the point it would have occupied without the motion of precession, and thus precedes that point continually with reference to the time of transit and motion.

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 precession
      n 1: the motion of a spinning body (as a top) in which it wobbles
           so that the axis of rotation sweeps out a cone
      2: the act of preceding in time or order or rank (as in a
         ceremony) [syn: precedence, precedency]