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30 Observational series which include the ERP component, latitude or time results of 15 main networks using six techniques (Ast, Doppler, SLR, LLR, VLBI and CEI) have been reduced to give a combined determination of the ERP for the period 1980 Jan 1 – 1984 October 31. The systematic error of each series is modeled by a constant plus a drift, and these are adjusted together with the ERP unknowns. The accuracies and systematic errors of the 15 networks so found are also given.

In this paper a periodogram analysis is applied to the 1820–1970 data of Earth's rotation in a search for hidden periods ranging from a few years to several decades. 12 periods are found and their combination matches well the observed variation over these 150 years. Most of the periods are very close to the periods of the sunspots, the Moon and the planets. The extrapolated curve remains in good fit with data observed in 1972. The results may be useful in further investigations on the underlying mechanisms and in forecasting future trend in the variation of the Earth's rotation.