The empirical term describing fluctuations in the motion of the Earth's rotation axis with a period of about 30 years was first reported by Markowitz in 1960 (24 years). However, by this time the effect was considered as non-real but an artifac of local effects (like the southward drift of the ILS's station Mizuzawa or the changes in the star catalogues used by the ILS observational programs, see Poma 2000 and Dickman 2000 for a review).

Researchers	time span	period	amplitude	ellipticity
		(years)	(mas)
Wilson and Vicente (1980)	1900-1977	29	30	none (linear)
Dickman (1981)	1900-1979	31	27	0.92
Markowitz (1982)*	?	29	47	0.89
Okamoto and Kikuchi (1983)	1899-1979	30	34	0.87
Vondrak (1985)	1900-1984	28	26	0.93
Poma et al. (1987)*	?	31	32	0.87
Vondrak et al. (1999) HIPPARCOS+SPACE97	1899-1998	31	20	0.91
Schuh et al. (2001) OA97	1899-1992	28	--	--

We also provide pole coordinates averaged over decadal time by applying Gaussian filter. See the mean pole and its tabulated values x ("), y(") at 0.05 year interval since 1900.

References:

• Dickman, S.R. (1981) Investigation of controvertial polar motion features using homogeneous ILS data. J. Geophys. Res. 86, pp. 4904-4912.
• Dickman, S.R. (2000) Tectonic and Cryospheric excitation of the Chandler Wooble and a Brief review of the secular motion of the Earth's rotation pole. IAU Colloquium 178: Polar Motion Historical and Scientific Problems . ASP Conference series , Vol. 208. pp. 411-435.
• Markowitz, W. M. (1982), report to IAU Comm. 19 & 31. In Poma, A. (2000)
• Okamoto, I. and Kikuchi, N. (1983) Public. Int. Latit. Obs. Mizusawa, 16, pp. 35-?, In Poma, A. (2000)
• Poma, A. ; Probervio, E.; Uras S. (1987) Journal of Geodynamics 8, pp. 245- ?. In Poma, A. (2000)
• Poma, A. (2000) The Markowitz wobble. IAU Colloquium 178: Polar Motion Historical and Scientific Problems . ASP Conference series , Vol. 208. pp. 351-354.
• Schuh, H.; Nagel, S.; Seitz T. (2001) Linear drift and periodoc variations observed in long time series of polar motion. Journal of Geodesy, 74; pp. 701-710.
• Vondrak, J. (1985) Long-Period behaviour of polar motion between 1900.0 and 1984.0. Ann. Geophysicae 3, pp. 351-356.
• Vondrak, J. (1999) Secular and long-periodic polar motion as derived from combination of astrometric and space geodetic observations. Journées 1998 Système Référence Spatio-Temporels, N. Capitaine ed. Observ. Paris, pp. 195-201.
• Wilson, C. and Vicente, R. (1980) An analysis of the homogeneous ILS polar motion series. Geophys. J. R. astr. Soc., 62, pp. 605-616.