The earth has a slight nodding motion, over and above the 26000-year precession, which has a period of 18.6 years and an amplitude of 9.2 arc seconds. This nutation is caused by the 5 degree difference between the the moon and earth's orbital planes and hence a resultant gravitational tug on the earth's axis. (The orbits of the earth around the sun and the moon around the earth don't form a perfectly flat disc.)

Nu*ta"tion (?), n. [L. nutatio a nodding, fr. nutare to nod: cf. F. nutation.]

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.

 

© Webster 1913.

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