nu·ta·tion /nuˈteʃən, nju-/
 垂頭,下垂,俯下,章動


 Nu·ta·tion n.
 1. The act of nodding.
 So from the midmost the nutation spreads,
 Round and more round, o'er all the sea of heads.   --Pope.
 2. Astron. A very small libratory motion of the earth's axis, by which its inclination to the plane of the ecliptic is constantly varying by a small amount.
 3. Bot. (a) The motion of a flower in following the apparent movement of the sun, from the east in the morning to the west in the evening. (b) Circumnutation.

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 nutation
      n : uncontrolled nodding