Secular polar motion - reality or artefact?

Abstract

The long-periodic variations of latitude at twelve observatories are used to determine secular and long-periodic polar motion in the interval 1901.0-1991.0. The Hipparcos Catalogue is used as a homogeneous and most accurate realization of International Celestial Reference System (ICRS) in optical wavelength. To derive polar motion components x, y, a classical approach with z-term is applied. It is shown that the observations at Ukiah after 1960 display an abnormal growth of latitude and, consequently, this part was discarded from the adjustment. The secular and long-periodic latitude variations at all selected observatories are mutually consistent with the hypothesis of secular motion of the pole of about 0.3" per century in the direction of 71°W meridian. The apparent changes of the arcs between the observatories can be easily explained by a change of z-term (i.e., the simultaneous growth of the mean latitudes of all observatories) by approximately 0.1", during the first half of the century.