A small oscillation of the Earth's axis of rotation about its mean position, due to the moon's attraction on the Earth's equatorial bulge, superimposed on the slow rotation of the axis round the pole of the ecliptic. It was discovered by James Bradley in 1747 from observations, and amounts to 9. 2 seconds of arc, with a period of 18 2/3 years - that of the circuit of the moon's nodes round the ecliptic.