Both latitude and l.o.d. observations show seasonal oscillations that rise well above the noise level of the astronomical spectra (figures 1.1 and 1.2). The principal seasonal oscillation in the wobble is the annual term which has generally been attributed to a geographical redistribution of mass associated with meteorological causes. Jeffreys, in 1916, first attempted a detailed quantitative evaluation of this excitation function by considering the contributions from atmospheric and oceanic motion, of precipitation, of vegetation and of polar ice. Jeffreys concluded that these factors explain the observed annual polar motion, a conclusion that is still valid today, although the quantitative comparisons between the observed and computed annual components of the pole path are still not satisfactory. These discrepancies may be a consequence of (i) inadequate data for evaluating the known excitation functions, (ii) the neglect of additional excitation functions, (iii) systematic errors in the astronomical data, or (iv) year-to-year variability in the annual excitation functions. The semi-annual term in the wobble is much smaller than the annual term, and the astronomical evidence for it is not compelling. This could be expected from the nature of the solution (4.3.12) of the polar motion for a sinusoidally varying excitation: for equal-magnitude excitation functions at the annual and semi-annual frequencies and Q ≃ 100, the annual pole shift will be some eight times larger than the semi-annual pole shift simply because it is closer to the Chandler resonance.