nutation

[lettered]

nu·ta·tion 

 
[noo-tey-shuhn, nyoo-]

–noun
1. an act or instance of nodding one's head, especially involuntarily or spasmodically.
2. Botany: spontaneous movements of plant parts during growth.
3. Astronomy: the periodic oscillation observed in the precession of the earth's axis and the precession of the equinoxes.