Free precession of Earth

(8.61) |

(8.62) |

The Earth's symmetry axis subtends an angle [see Equation (8.44)] with its angular momentum vector, but it lies on the opposite side of this vector to the angular velocity vector. This implies that, as viewed from space, the Earth's angular velocity vector is almost parallel to its fixed angular momentum vector, whereas its symmetry axis subtends an angle of with both vectors and precesses about them. The (theoretical) precession rate of the Earth's symmetry axis, as seen from space, is given by Equation (8.58):

(8.63) |

(8.64) |

The free precession of the Earth's symmetry axis in space, which is known as the *Chandler wobble*—because it
was discovered by the American astronomer S.C. Chandler (1846–1913) in 1891—is superimposed on
a much slower forced precession, with a period of about 26,000 years, caused by
the small gravitational torque exerted on the Earth by the Sun and
Moon, as a consequence of the Earth's slight oblateness. (See Section 8.10.)