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Absolute proper motions of seven globular clusters have been determined with respect to extragalactic references and with accuracies of ∼ 1 mas/yr. Derived quantities and qualitative implications are described.
An investigation of the N30 along the same lines as that of the FK4 (Brosche and Schwan 1981) reveals quite similar non-standard systematic motions. Some consequences are indicated for the determination of cluster parallaxes, of secular parallaxes and of the galactic rotation. In general, we stress the necessity of always searching for every signal in the data - including those ones which are not understood and hence not ‘wanted’.
In the long run, the tidal interaction between the Moon and the solid Earth is mediated by the oceans. It produces the retardation of the Earth's rotation known as ‘tidal friction’. Due to the changing configuration of the continents, it is a non-monotonic function of time. Tides of the solid Earth dominate the short-periodic tidal effects while the exchange with the atmosphere is preponderant in climatic changes, especially with an annual signature. It is shown that the influences of the oceans within such short time-scales must be taken into account for tidal and for non-tidal variations as well if one wants to model the Earth's rotation at the cm-level corresponding to the most advanced observational techniques.
Because of their early epochs, Carte du Ciel plates are ideal first epoch plates for the study of proper motions. We have tested the astrometric precision of some plates from different epochs using the accurate astrometric star catalogues of Praesepe and Pleiades.
Hydrodynamical computations of the major partial tides in the oceans have been evaluated for the changes both in moment of inertia and relative angular momentum due to ocean currents. If the system solid Earth plus oceans is seen as an isolated system for these time scales, the oceanic variations lead to mirror-like changes in the rotation of the solid Earth. Amplitudes are of the order of 0.1 ms in Universal time. In contrast to the effects of solid Earth tide, phases are away from equilibrium phases.
Absorption lines at the redshifts of cosmic voids in Perseus-Pisces and Böotes have been detected in the ultraviolet spectra of background quasars. The detection of Si IV and C IV lines besides Ly α suggests chemical enrichment of gas in the voids. Additional observations of three sight-lines through the void in the Böotes suggests that the gas in the void is clumped in large clouds.
Proper motions of globular clusters referred to extragalactic background objects provide absolute proper motions leading to space velocities. Some results from the Bonn Observatory proper motion program for galactic globular clusters are shown. Reference stars from the Lick program linked to galaxies are used. Low orbital angular momenta for NGC 4147, NGC 5466 and NGC 6218 were detected. In a further program, proper motions of 47 Tuc and NGC 362 are currently being measured relative to the background of the SMC.
The torques of the frictional forces on the bottoms of the seas are calculated in an approximate inertial system and not in the corotating system used hitherto. In this way, the loss of rotational energy of the Earth – corresponding to the mean torque – can be compared with the value obtained from astronomical observations. The calculations are based on velocities which are computed by numerical integrations of the hydrodynamical equations. Initial results are presented for the North Sea, the Bering Sea and rough models of theworld ocean.
“Salvaging an astrometric treasure” is an european effort to retrieve the astrometric and photometric information contained in the early Carte du Ciel plates. Five institutes from three european countries are involved. The project was supported by the European Community.
During the period, there have been several major events which have effected the scope and interest of Commission 19. The most significant of these has been the dissolution of the BIH and IPMS and their replacement by the International Earth Rotation Service (IERS). The correlation of higher frequency fluctuations in the Earth’s rotation rate with changes in the Earth’s Atmospheric Angular Momentum is also significant. Many investigators now seem to believe that the “decade variations„ in the Earth’s rotation rate are caused by torques between the core and mantle caused by the uneven motions at the core-mantle boundary. These events and discoveries have made this an exciting period. It seems that the future holds more in the way of discovery due to the utilization of the more accurate and precise Earth rotation data coming from the modern observing techniques.
The reporting period has shown that Space has become a firmly established domain in observational Astrophysics, also in the low energy astrophysics area. The launching of new spacecraft is always an important addition to the capabilities of the Astronomers, but the availability of space observatories is strongly affected by the fact that they disappear as their subsystems become damaged or, for other reasons, become inoperable. The relatively short life of astronomical space facilities has generated new dynamic in the life cycle of observational tools for the astronomer, rather different from that for ground facilities. Launch failures or the final in-orbit functionality verification can also very strongly affect the availability of observational capabilities in space astrophysics. The only spacecraft designed without this built-in life time restriction, is the Hubble Space Telescope, which can be serviced by the Space Shuttle.
The triennial report of Commission 19 was composed from the contributions of its members. Space does not permit a listing of their names, but their contributions are sincerely appreciated. Unfortunately because of limited space it is also not possible to provide in this report the extensive list of publication of the Commission members. The list of publications is however available on the Commission 19 web site at maia.usno.navy.mil/iauc19.
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 study of double stars has since long been recognized as a basic key to the understanding of star formation and stellar evolution. Moreover, close visual double stars have always been systematically neglected in photometric observational programmes although they contain an important part of physically associated systems. It is then timely to organize major observational programmes of these objects for a number of good reasons:
1.The frequency of double stars is continuously reviewed and the rate of their detection is steadily increasing — both from ground–based and space observations — in such a way that they no longer can be discarded in any models of galactic structure;
2.Space observations (HIPPARCOS, HST) significantly improve the quality and the importance of stellar samples and permit to better take into account some of the selection effects;
3.The high–quality astrometric (and also photometric) data that will be provided for such systems by the space observations should be matched with accurate and homogeneous complementary astrophysical information such as colour indices and spectral classification. Such information for close visual double stars is unfortunately almost non-existent but are now being more easily accessible with the use of CCD detectors.
Since Euler in 1765 predicted the possibility of polar motion, the search for an observational confirmation went on. While some indications were found before, the proof without doubt is generally attributed to Küstner. Ironically, the aim of Küstner’s precise observations of 1884–1885 in Berlin was another one, namely an improved value of the constant of aberration. Using the Horrebow-Talcott method, he could conclude that the latitude of Berlin had decreased significantly by 02 in about one year. His findings promoted the start of international cooperation to observe the polar motion. Measurements from Honolulu in 1891 exhibited the effect in counterphase, thus omitting the last chance for a local explanation.