Nutation is a slight wavy motion of the earth's celestial pole which affects the regularity of the precession (q.v.). It is due to the fact that the earth is not a true sphere but bulges at the equator, and the moon attracts this equatorial excess of matter. If there were no precession, nutation alone would cause the earth's pole to be sometimes rather nearer to, and sometimes rather farther from, the pole of the ecliptic, a, small ellipse being described between these two positions. The change in position varies with the motion of the moon's nodes, and, as these nodes perform a revolution in 18f years, that is also the time taken for the earth's pole to describe its small ellipse. The total effect of nutation and precession is to give the wavy motion above mentioned, the crest and trough differing by only 18" - the length of the longer axis of the ellipse. Small as is this inequality, which is strictly the lunar nutation, a still smaller wave motion is added, due to the position of the sun. This is the solar nutation. If we imagine small but regular ripples extending over the surface of a number of waves, then a particle made to travel over these ripples and waves would be moving in a manner representative of the resultant motion of our pole.