Observations of the length of day (LOD), corrected for the effects of variations in the angular momentum due to changes in wind velocity and atmospheric pressure, ocean tide heights and currents, and solid Earth zonal tides, were analyzed. The apparent effect of atmospheric angular momentum (AAM) on the astronomically determined LOD is found to be much less at the highest frequencies than at lower frequencies. For periods shorter than ten days there appears to be essentially no correlation between AAM and LOD. The International Earth Rotation Service (IERS) Standards (1992) model for the effects of zonal tides on the Earth's rotation, which includes ocean tidal effects, adequately accounts for the observations of the variations in the length of day with periods between five and 180 days at the present level of accuracy. A currently unexplained semi-annual variation in the length of day remains, but this may be due to the unmodeled effects of stratospheric winds. The power spectrum of the remaining variations with periods less than twenty days is essentially that of a white-noise process. The amplitudes of the remaining unexplained variations in length of day are less than thirty microseconds. This paper has been accepted by Geophysical Journal International for publication.

On the basis of the perturbation principles of the normal mode and Love numbers, the theoretical Q of the Chandler wobble is derived by assuming that the wobble energy is totally dissipated within the mantle and by using Zschau's mantle rheology model. The results show that mantle anelasticity is likely to be the most important dissipative source of the Chandler wobble energy, and the theoretical Chandler Q is 71. Finally, the parameter α of the absorption band model is calculated, and the applicability of the model is discussed as well.

The sequence of LOD in the period from 1988 to 1991 (which was published in Bulletin B by IERS) is analyzed. Using the method of multiple filter, all the periodic terms are removed. Therefore, the rapid and outstanding variations of LOD that greater than 0.1ms in May, 1989 and Jan., 1990 are shown in the residual series. Because of many bursts of solar flares in the period of Mar.8 to 16 and Nov.7 to 17, there are geomagnetic storms in Mar. and Nov.1989. The strong geomagnetic variation probably induces the rapid variation of LOD.

Some variations of geo and solar physical parameters are known to be observed simultaneously in the Earth's dynamic parameters changes. The evaluation of the influence of solar activity upon Atmospheric Angular Momentum (AAM) and Length of Day (LOD) within the short periods is made. The similarity between the amplitude and period variations of low-frequency components of polar motion parameters and geomagnetical AA-indices is discovered too.

In this paper authors pointed out that the root mean square of post-residual is two or three times larger than that of normal points. It is mainly due to the ephemeris uncertainties, i.e. the uncertainties of obliquity of the ecliptic and equinox and the inaccurary of lunar love numbers adopted. At present, with exception of the corrections of ocean tide and pole tide the theoretical modeling such as earth solid tide, lunar tide, plate motion and the relativity modeling, especially the space curvature caused by the earth, have been considered to match the present observing precision in the magnitude of centimeter. The equivalent of relativity modeling used in geocentric or barycentric reference frame is further described.

Adopting the time series of atmospheric angular momentum (AAM) from the National Meteorological Center of USA, the study of the prediction of the length of day (LOD) has been made by the Leap-Step Threshold AutoRegressive (LSTAR) model. The LSTAR model presented by the author is a sort of models for nonlinear time series analysis such as where Dj is the j-th leap-step domain of the data series Zn, and (j) if the sample number N=L×M, then Zj+(L×K) εDj and K=0,1,…,M−1. En denotes the white noise of data in the j-th leap-step domain. TSM denotes a class of models in time series analysis and the nonlinear threshold autoregressive model is used here.

Chinese Loess data over the past 2.4 million years are good proxy records for understanding past climate. Maximum entropy spectrum of magnectic susceptibility data at the locality of Luochuan, which is in Shaanxi province of China, have shown periodic fluctuations of about 21 kyr, 41 kyr, 100 kyr and 400 kyr. These periodic fluctuations are related to the long-term variations of earth's orbital elements of the obliquity, the climatic precession and the eccentricity respectively. The data series was separated into two parts to examine the variations of these fluctuations. The first part is from present to 1.1 Myr B.P.(i.e. million years before present), the second part is from 1.1 Myr B.P. to 2.2 Myr B.P. The periodic fluctuation of 93.4 kyr has the maximum amplitude in the first part of the data series, while the periodic fluctuation of 413 kyr has the maximum amplitude in the second part. The fluctuation about the 400 kyr was found in both parts of the data series.

The length of day (LOD), the atmospheric angular momentum (AAM) and the relative sunspot number (RSN) time series at 5-day intervals, which are spanning 13 years from 1976 through 1990, have been analysed in order to study the 30–70 day fluctuations. The filtered series with the 30–70 day band-pass of the three kinds of data were obtained by the Multi-Stage Filter. The five spectral peaks in both filtered series have been detected by the AR spectrum with the order of AR models as less than 3 percent of the sample number of each series. The amplitude estimates for the five periods in each series appear basically in equal distribution, and there is not a dominant one. The amplitudes of the averaging period for the five periods of LOD and AAM series decrease obviously with the increasing of sample number. The randomness of three series of periodic variations calculated from LOD, AAM and RSN was inspected by using the independence test and the link test of Statistics. Both tests for LOD and AAM are satisfied but RSN is not. From the above mentioned analysis, it is shown that the 30–70 day fluctuations both in LOD and AAM have the features of random walks in time scale and do not have a stable and periodic oscillation.

Djurovic et. al. detected 50-Day oscillation of the daily Wolf numbers and the daily apparent sunspot areas. We analysed the 50-Day oscillation and discovered the closer relations between the El Nino events and the 50-Day oscillation of solar activity and earth rotation. To verify the relation, in the present work, we collect longer and more data series. The result confirms that the El Nino events and the 50-Day oscillation of earth rotation are caused by the 50-Day fluctuation of solar activity.

The construction of the Fifth Fundamental Catalogue (FK5) is shortly described with emphasis on the system of the Basic FK5 and on the completion of the FK5 Extension. New observations have revealed systematic errors mainly in the declination system of the FK5 and in the right ascensions south of −50°. Comparisons of the new observations with the Basic FK5 and the FK5 Extension (made separately for the bright and faint stars) have shown that the FK5 Extension is essentially on the system of the Basic FK5.

In 1970 the IAU defined any object's north pole to be that axis of rotation which lies north of the solar system's invariable plane. A competing definition in widespread use at some institutions followed the “right hand rule” whereby the “north” axis of rotation was generally said to be that that of the rotational angular momentum.

A Working Group has periodically updated the recommended values of planet and satellite poles and rotation rates in accordance with the IAU definition of north and the IAU definition of prime meridian.

In this paper we review the IAU definitions of north and of the location of prime meridian and we present the algorithm which has been employed in determining the rotational parameters of the natural satellites.

At the International Astronomical Union General Assembly in 1991 the terminologies Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB) were introduced as the time coordinates of four dimensional geocentric and barycentric coordinate systems, respectively. The reasons for these time-like arguments and their applications will be discussed. The relationships between the different time-like arguments will be discussed along with their applications and relationships to the astronomical constants.

Since these time-like arguments have secular differences, there are new issues to be resolved. These new issues include the definition of the epoch J2000.0, relationship between mean and apparent sidereal time, the time-like argument for specifying constants, the definition and future use of ephemeris time, delta T values and the transition to, or future use of, TCG, TCB and TDB.

The paper summarizes the discussion on the origin of right-ascension and puts forward new arguments in view of high-precision Geodesy and Astrometry. From the movement of the Celestial Departure Point, the classical right-ascension precession might be amended by an additional term −0s.000257/century originating from the nutation-precession interaction movement. A similar term might also be introduced in the maintenance of a terrestrial reference system, while the concept of a Terrestrial Departure Point is considered. The definition of the Earth's rate of rotation in an inertial or quasi-inertial system is reviewed. A periodic erroneous term of maximum amplitude 2.65mas is pointed out in the conventional transfer relation between CRS and TRS, that can for its main part be compensated by introducing the periodic terms of Woolard's equation of the equinox.

Having analysed the real reason why the TDT second differs the TAI second, the TDT's property, unit as well as its definition, and the radical disadvantages existed in the problem of time, we present a new program on the problem of time.

Optical positions of some 30 radio stars derived from 12 months of HIPPARCOS measurements are compared with their radio positions obtained with the Very Large Array (VLA). — Once the lengths of arcs between optical and radio positions of pairs of stars are calculated the differences of the arcs are formed. They provide an estimate of the coincidence of the optical and radio emission centres. — From the comparison of optical and radio positions infinitesimal rotation angles of the HIPPARCOS frame with respect to the VLA extragalactic reference frame are determined by rigid rotations. After taking account of the relative orientation of the frames the standard deviations of the remaining residuals are approximately of the order of the VLA observation errors, thus demonstrating the reliability of the HIPPARCOS results. However, they also indicate some data noise very likely caused by the low accuracy of optical proper motions used to bridge the HIPPARCOS-radio epoch differences up to 9 years, and possible discrepancies of radio-optical emission centres of some stars.

A VLBI series of celestial pole offsets has been used for deriving the corrections to the main terms of the IAU 1980 nutation series using the least squares solution and amplitude spectra method. The results are compared with current geophysical models. The discrepancies between observations and theory are largely due to deficiencies in the nutation models. A retrograde periodic variation (at a period of 420.0 days) has been revealed. The free mode will give us futher information on the core-mantle boundary.

From the observational catalogues of the extragalatic sources, the common sources with larger observational intervals are selected. After a reduction of optical catalogue systems to the FK5, the “proper motions” of these sources are obtained in the FK5 system. With these values the correction of the precession constant has been estimated and compared with that obtained from the new techniques.

The rotational angles α1 and α2 for transformation between two CISs are derived from the differences of each two polar coordinate series. Then, their periodicity is analysed and they are fitted by trigonometric expansions. In terms of the fitting parameters each of them is developed into a sum of several terms with two nearly-diurnal ones involved.

Covariance functions of individual differences of positions and proper motions of stars in the FK5 and FK4 catalogues are given. The application of these functions for the study of the FK5 system and for its further improvement are discussed.

A total of 53,259 observations(26,793 in declination and 26,466 in right ascension) of the Sun, Mercury and Venus made with the Washington, six-inch and nine-inch transit circles from 1911 to 1971 and the Cape of Good Hope reversible transit circle from 1907 to 1959 are used to obtain equator and equinox solutions.