[en] Advances made in the field of photographic astrometry over the past three years are reviewed. Activities of the Commission members are summarized and include such topics as trigonometric parallaxes - nearby stars, proper motions and kinematics, photographic positions and catalogues, instrumentation and measurement, and reduction methods. (C.F.)

No abstract available

[en] The great potential increase in astrometric accuracy of modern observing techniques, particularly the HIPPARCOS astrometric satellite, requires a close examination of the procedures which should be used for calculating the observable directions to celestial objects, including the effects of aberration and light deflexion. In this paper the problem in a spherically symmetric static metric is considered. It is pointed out that, as usually expressed the shape of a null geodesic and hence, in general, the amount of light deflexion between two points, depends on the metric used for describing it, although it happens that the asymptotic deflexion is the same in both the standard Schwarzschild metric and the isotropic metric. A procedure is developed for calculating direction relative to a local Lorentzian flat space-time, which is independent of the coordinate metric; the direction as seen by a moving observer is then obtained rigorously from a simple Lorentz transformation. (author)

[en] The equations of motion of the rigid earth about its centre of mass are developed by the methods of vectorial mechanics and compared with the classical formulation given by Woolard. It is shown that apparent discrepancies in the coefficients of the Oppolzer terms in the motion of the axis of figure between values listed by Woolard and those recently published by Kinoshita, can largely be accounted for by approximations introduced by Woolard in his integration procedure. In particular the apparent difference between the precessions of the axis of figure and the angular-momentum vector is shown to be spurious. (author)

[en] From photographic plates covering epochs 1899 to 1976 a new determination of the absolute proper motion of the Crab pulsar gives: μsub(p) = 0''.013 +- 0''.004/yr and a position angle thetasub(p) = 293⁰ +- 18⁰, in fair accord with previous results. For a distance to the Crab nebula of 2 kpc, the corresponding tangential velocity is 123 km/s. Combining this result with a linear regression analysis of Trimble's proper motions for the filaments: (1) provides conclusive kinematic evidence linking the filaments and the pulsar, (2) predicts an outburst date (AD 1120 +- 7), earlier than previously found, and (3) indicates a divergent point for the filaments at α(1950) = 05sup(h)31sup(m)32sup(s).12 +- 0sup(s).06 and delta(1950) = + 21⁰58' 50''.0 +- 0''.9 on the AGK3 reference system. From the present position of the pulsar and the divergent point, an independent determination of the absolute proper motion of the pulsar gives μsub(p) = 0''.011 +- 0''.001/yr and thetasub(p) = 290⁰ +- 6⁰, in excellent agreement with the classical proper motion analysis. Within the observational errors, the projected direction of motion of the Crab pulsar is (1) along the major axis of the nebula, (2) toward the centroids of optical continuum (Scargle's wisps) and X-ray emission and (3) parallel to E in the vicinity of the pulsar. (author)

[en] In an attempt to investigate the statistics of stars in the solar neighbourhood, independent of the bias toward high-velocity stars introduced by selection of stars with large proper motion, a major astrometric investigation is being carried out at the RGO using UK Schmidt plates measured on the GALAXY machine. So far more than 15 000 objects brighter than B = 18 in a field near the South Galactic Pole have been measured on more than 50 plates taken between 1974 and 1977. This paper describes the methods of analysis and the results of a pilot investigation on nearly 900 stars with B < 14. The measurements show a very high degree of internal consistency, demonstrating the astrometric excellence of the UK Schmidt telescope and the GALAXY machine; errors are discussed. A correlation between parallax and large proper motion is found, as expected for stars with B-V >= 0.7, but is not evident for bluer stars. Significant parallaxes have been detected for the only two stars in the field which had been measured previously. Parallax data for 33 stars with large proper motion are reasonably consistent with photometric parallaxes determined by Eggen. (author)

[en] The high resolution Bitter pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high Tc superconductor YBa₂Cu₃O₇ are placed in a magnetic field. At 4.2 K with the magnetic field parallel to the c axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors. The structures typically consist of flux spots with short range hexagonal correlations. The patterns are interpreted to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of hc/2e. Experiments at 77 K, the only other temperature studied, showed no spatially varying magnetic structure. Recent mechanical measurements which are sensitive to the bulk modulus and dissipation within the vortex lattice show evidence for a mobility transition near 75 K, which would be consistent with this observation. This is believed to stem from materials properties rather than some more fundamental property of the new superconductors. 9 references

[en] Low-energy positrons implanted into a crystal can diffuse to the surface where they may be detected, for instance, by observing the formation of positronium. We describe the positron motion by a one-dimensional diffusion equation, and discuss the measurement of vacancy activation energies, positron diffusion constants, and positron stopping profiles. (orig.) 891 CDS/orig. 892 HIS

[en] Measurements are reported of the angular distribution of the slow positrons (e⁺) from negative work function (phi⁺) surfaces of Cu and Al bombarded by keV e⁺ in ultrahigh vacuum. In analogy with GAMMA-point electrons emitted from negative electron affinity surfaces, the majority (> 50%) of the slow e⁺ leave the surface with energy = approximately phi⁺ and velocity within approximately 20⁰ of the surface normal. This agrees qualitatively with the predictions of a simple 1 D model. (author)

No abstract available