The optical astrometry observations of
latitude/universal time variations made with 48
instruments at 31 observatories are used to
determine the Earth orientation parameters (EOP)
since the beginning of the century. The Hipparcos
Catalogue is used to bring more than four million
individual observations, made in the interval
1899.7-1992.0, into the International Celestial
Reference System. The Earth orientation parameters
(polar motion, celestial pole offsets and, since
1956.0, also universal time UT1) are determined at
5-day intervals, with average uncertainties
ranging from 8 mas (in the eighties) to about 40
mas (in the forties). Making use of very long
series of ground-based observations, the solution
also leads to the improvement of proper motions of
about ten per cent of the observed Hipparcos
stars, with precision of ±0.2 — 0.5 mas/yr. In
addition, 474 auxiliary parameters, describing the
rheological properties of the Earth and seasonal
deviations of the observations at contributing
observatories, are found. The new solution
provides the EOP series suitable for further
analyses, e.g., for studying long-periodic polar
motion, length-of-day changes or
precession/nutation.
Universal Time Variations in Space Weather