Equinox (celestial coordinates)
Nutation is the oscillation of the ecliptic plane.  It was first followed by James Bradley as a difference in the declination of stars. Bradley published this discovery in 1748. Because he did not have a correct enough clock, Bradley was unaware of the effect of nutation on the movement of the equinox along the celestial equator, although that is in the present day the more significant aspect of nutation.  The period of oscillation of the nutation is 18.6 years.