Modern planetary theory may be considred as a carrier of 4-dimensianal dynamical reference frame which is to be canfronted with stellar (or quasar) based fundamental frame. Of most interest is the problem of existence of secular trends between the above systems. We investigated the problem by discussing a vast set of planetary observations of different types.

We summarize the current status of the JPL ephemerides, focusing on the various data types utilized, especially the impact of the modern ranging data, and the resulting accuracies obtained. The dynamical equinox, as determined from the analysis of Lunar Laser Ranging data, is determined with an accuracy of 5 mas and the obliquity to a 2 mas level in ~1983, the weighted center of data. Knowledge of the lunar and planetary positions with respect to the dynamical equinox degrades to 10 mas at J2000. Twenty years of LLR data allow for the separation of the 18.6 yr nutation terms from the precession constant. The correction to IAU precession is found to be −2.7 ± 0.4 mas/yr, while the 18.6 yr nutation of the pole is 3.0 ± 1.5 mas larger in magnitude than the 1980 IAU series. The necessity of different reference systems and the accurate knowledge of the interconnections between frames is addressed.

Modern observations of celestial pole offsets indicate that small but significant corrections to the current IAU theories of nutation and precession can be detected. These corrections are of the order of a few milliseconds of arc. To meet the needs of those who require millisecond of arc accuracy in reference frame applications, the International Earth Rotation Service (IERS) publishes in weekly and monthly bulletins the most recent observations, along with predictions of these corrections. These appear to meet the needs of users. In view of the fact that the geophysical community has not adopted a model which describes the nutation and precession theory and that the current procedure of the IERS meets the need of high-accuracy users, there does not appear to be a need to identify a new IAU nutation theory at this time.

We present a new nutation series for the Earth (ZMOA-1990) based on (1) the rigid Earth nutation series developed by Zhu and Groten [1989], (2) the normalized response for an elastic, elliptical Earth with fluid-outer and solid-inner cores developed by Mathews et al. [1990], and (3) corrections for the effects of ocean tides and anelasticity, computed to be consistent with the Mathews et al. [1990] normalized response function. In deriving this series, only two parameters of the geophysical model for the Earth have been modified from their values computed with PREM: the dynamic ellipticities of the whole Earth, e, and of the fluid outer core, ef. The adopted values for these parameters, determined from the analysts of very long baseline interferometry (VLSI) data, are e=0.00328915 which is about 1% higher than the value obtained from PREM and 6×10−5 times larger than the IAU adopted value, and ef=0.002665 which is 4.6% higher than the PREM value. The above values were obtained from an adjustment of −0.3 ʺ/cent to the IAU-1976 luni-solar precession constant for e, and from the amplitude of the retrograde annual nutation for ef. The ZMOA-1990 nutation series agrees with estimates of the in-phase and the out-of-phase nutation amplitudes obtained from VLBI data to within 0.5 mas for the terms with 18.6 year period, and to better than 0.1 mas for terms at all other periods except for the out-of-phase terms with annual period (differences 0.39 mas, retrograde, and 0.13 mas, prograde), and for the in-phase term with prograde 13.66 day period (difference −0.25 mas).

Oceanic and fluid core effects inherent in polar motion and 1.o.d. -data were analyzed and related results are discussed in detail.

A new exact analytic solution to the hydrodynamic equations is obtained, which describes tidal motions at low frequencies in a homogeneous, incompressible, inviscid liquid core with arbitrary core-mantle topography. Some geophysical and astrometrical consequences of this solution are considered.

The numerical estimation of the deviations of the pole tide from the static one is obtained. A new hypothesis is proposed that the known interrelation between long-term amplitude and frequency variations in Chandler wobble may be attributed to the influence of turbulent friction of non-equilibrium pole tide.

The celestial system maintained by the International Earth Rotation Service is described in terms of physical properties of the fiducial objects, internal consistency of the frame, and agreement with the FK5 and dynamical systems.

The celestial frame of HIPPARCOS astrometric satellite will be materialized by the positions and proper motions of about 120000 stars relative to arbitrary origins. As the HIPPARCOS reference frame is not naturally related to fixed directions in space, it has to be linked to an inertial frame of similar quality. The technique of VLBI determines the coordinates of extragalactic radio sources precise at the milliarcsecond level in an equatorial frame. The precision expected for HIPPARCOS normal mission is 0.002ʺ for the positions, yearly proper motions and parallaxes.

Observations of the satellites Europa and Titan will be obtained from the European Astrometric Satellite HIPPARCOS. These observations will be used to obtain “observed” positions of the planets Jupiter and Saturn; these positions have the advantages, first to be given in the reference frame defined by the Hipparcos system, and second, the accuracy on these positions is better than the accuracy of the ground-based observations of these planets. To obtain the positions of the planets from those of their satellites, we have to take the calculated positions of these satellites by means of their ephemerides. The accuracy of the computed positions of Europa and Titan relatively to their primary is of the order of 0ʺ.l, better than the one of the direct observation of Jupiter and Saturn. An efficient comparison of the different ephemerides of these planets will be possible that way.

A requirement for solving Earth rotation problems is the definition of a terrestrial reference system (or systems) and the determination of a terrestrial coordinate frame (or frames). In this context it is important to determine the positions of the observing sites and their temporal variations and establish the dependence of the observations on local conditions and observing methods.

In situ concurrent observations are therefore important. In this paper we report the results of a comparison between astronomical and geodetic coordinates derived from different observations at the Cagliari Observatory and the Carloforte latitude station.

The IAU and IUGG has jointly established in 1988 an International Earth Rotation Service (IERS) which is in charge of the realization of conventional celestial and terrestrial reference systems, together with the determination of earth orientation parameters which connect them.

The theoretical definition of the terrestrial reference system which is realized by IERS through a conventional terrestrial reference frame formed by SLR, LLR, VLBI and GPS stations is presented. In particular its origin, scale, orientation and evolution with time are reviewed, taking into account relativistic and deformation effects.

Fictitious proper motioms of quasars and of their surrounding stars have been determined based on plates with an epoch difference of up to 90 years. From the fact that the true proper motions of the quasar are vanishingly small, we obtain conditions for the extragalactic calibration of the preliminary Hipparcos system. We present results for the fields of 3C 273, OQ 208, 3C 371 and 3C 390.3. With the data it is possible to achieve the link with random errors smaller than 0₺ 15 per century.

The catalogue of about 200000 stars in the zone from equator to southern pole was complied from photographic observations carried out during 1982 – 1969 years in USSR and Bolivia by Pulkovo astrograph (D=23 sm, F=230 sm, working field 4° × 4° ). The plates were measured by Zeiss Ascoreoord measuring mashines in Pulkovo, Tarija, and others observatories. The process of measurement and compilation of catalogue were carried out in Pulkovo by ES–1033 computer. The observations were made with a four–fold overlap of the sky. Reference oatalogue is SRS catalogue (FK5 system). The differencial refraction, differencial aberration and objective distortion were taken into account. The mean accuracy of catalogue FOCAT-S positions is ±0.123″.

A VLBI series of celestial pole offsets (designated as ERP(GSFC)90 R 01 in the IERS series of Earth Rotation Parameters) spanning 10 years has been used for deriving the corrections to the IAU constant of precession and to the main terms of the 1980 IAU series of nutation using the same method as Herring (1988) for constraining the corrections to the 9.3yr nutation and to the l8.6yr nutation in longitude. The estimated corrections have been found to be − 0.279ʺ/c ± 0.02 ʺ/c (formal error) for the IAU constant of precession and +3.13±0.15mas for the 18.6yr nutation coefficient in obliquity. A circular term with a radius of the order of 0.25mas at a period of 430 days has moreover been revealed in the residuals as obtained after applying the estimated corrections to the series. If such a circular term in the celestial motion of the pole is due to the free core nutation, its estimated period would correspond to a core flattening of 2.687×10−3, which is in good agreement with the one of 2.674×10−3 corresponding to the estimated amplitude of the retrograde annual nutation.

A new procedure has been devised for computing apparent places of stars in the intermediate frame (Capitaine 1990) linked to the nonrotating origin (Guinot 1979) for estimating the Earth rotation parameters (ERP) from astrometric observations. The latitude and time parameters as derived by this procedure have been compared to the parameters as derived from the classical procedure used for the reduction of Paris astrolabe observations (Chollet 1984). The consistency between the two procedures has been found to be of the order of a few 10−4ʺ, which is under the order of precision of the computations in the classical procedure. The new procedure, which is more directly related to the Earth rotation, is proposed to be used for the derivation of the ERP in the Hipparcos frame from existing astrometric observations, which is planned for the near future (IAU WG on “Earth Rotation in the Hipparcos Reference Frame”).

Two decades of LLR data and one decade of VLBI data are combined in a global analysis to yield improved estimates of the Earth’s precession and nutation. In this analysis, LLR provides a strong determination of precession, while VLBI is stronger in fixing nutation terms with short periods. In all, 24 nutation amplitudes are estimated. The largest correlation coefficient, between precession and 18.6 yr out-of-phase nutation in longitude, is 0.88. With the exception of some 9 yr and 18.6 yr terms, formal uncertainties are 0.1 to 0.2 milliarcseconds.

The main purpose of this contribution is twofold. First, we report on a preliminary assessment of the astrometric properties of the CCD system we are presently using to extend the magnitude limit of the Torino program on optical positions of QSO’s. This assessment is based on the internal consistency of the positions derived from different CCD frames of the same objects. Then, we give a first evaluation of the precision of CCD derived QSO positions after extending a plate based reference frame (secondary frame) onto the CCD images (tertiary frame).

Special emphasis is given to comparisons with plate positions of the same faint objects. It is shown that use of the CCD camera promises a factor of five (or larger) improvement in the precision of QSO positions.

The FK5 Extension, consisting of 3117 new fundamental stars selected from the FK4 Sup catalogue and the IRS list, will extend the FK5 system to about 9.5th visual magnitude. The construction of the FK5 Extension is briefly described and the main characteristics are given.

Optical fluctuations of the radiostar β Persei are seen from 13 campaigns performed with the astrolabe located at the Paris Observatory.

A longterm program of precise radio – and optical astrometry of selected radio stars has been conducted in the last decade by our observatories using the VLA and astrographs on both hemispheres. Positions of 54 stars north of −26 deg. declination including 6 MASER stars have been obtained. The program status and some results from the southern hemisphere are reported.