The similarity rules to compare atmospheres with anisotropic and isotropic scattering are reviewed. Omission of a narrow diffraction peak in the scattering pattern is permitted and corresponds to a special application of the similarity rules. It is shown that the extrapolation length for conservative scattering can be found with great precision from a formula involving only ω2 and ω3. An asymptotic expression for high-order scattering in a semi-infinite atmosphere is given and it is shown by some examples that this expression can be used to find by interpolation the terms of any order. Finally, the way in which the contrast between a dark and bright area near the center of the disk of Mars is affected by an overlying haze is computed for isotropic and for anisotropic scattering.

Using Hipparcos and Tycho-2 data, together with the radial velocity information, a statistical analysis is applied to all the orbital solutions of planetary candidates, in order to avoid a bias towards small inclinations. There are hints of additional, longer period companions.

Estimates of the masses and energies of the supernova explosions that resulted in the Cygnus Loop and similar remnants are revised downward on the basis of recent soft X-ray observations.

The spiral galaxy M81 is a challenging object for testing a density-wave theory. It is of large angular size and favourably inclined to the line of sight; radio observations of neutral atomic hydrogen showing well defined spiral arms with noncircular motions are available (Rots and Shane 1975), and there is evidence from surface photometry for density waves in the old disk population (Schweizer 1976). Can these observations be unified in one density-wave model?

A study of the period–amplitude relation for RR Lyrae variables in the globular clusters 47 Tucanae, NGC 6441 and M55 confirms a recent finding that the period–amplitude relation for RRab stars with ‘normal’ light curves is not a function of metal abundance. Evolutionary effects are more important.

The stability of the three-body problem formed by the Sun, Jupiter and Saturn is investigated using surfaces of zero velocity. The results obtained with the models of the restricted and general problems of three bodies are compared with numerical integration. The system is found to be stable in the sense that Saturn will neither interrupt the (perturbed) binary orbit of Jupiter around the Sun, nor will it escape from the system. It is shown that the known classical triple stellar systems are “more stable” than the solar system, which in turn is “more stable” than the Earth-Moon system.

A satellite mission for accurate astrometry and multi-colour photometry is discussed, similar in principle to the ESA Hipparcos mission and here called the Roemer mission. The limiting magnitude will be about V = 18 mag while 13 mag is the limit of the present Hipparcos mission. Luminosities of stars up to 2 kpc away can be obtained, corresponding to a volume 10000 times larger than with Hipparcos. A mission of 5 years will provide an accuracy of 0.1 milli-arcsec at 12th magnitude for positions and parallaxes and 0.05 milli-arcsec for annual proper motions. This is achieved by a satellite using a mosaic of CCD detectors in the focal planes of two beam-combiner telescopes of 0.29 m aperture. The instrument is described and the expected performance with an input catalog of 400 million program stars is given.

PSRSalsa is a versatile open-source pulsar data-analysis project designed to obtain a comprehensive picture of the radio properties of your pulsar of choice. Here its usefulness is demonstrated through the analysis of the radio pulsar B1839–04, thereby revealing the extremely rare phenomenon of “bi-drifting” where the drift direction of subpulses is systematically different in different pulse profile components.

The rotational energy transfer of NH3, OH and D2CO in inelastic collisions with He and H2 is studied in a crossed molecular beam experiment. The molecules are prepared in a single initial state by rotational cooling in an adiabatic expansion followed by electrostatic state selection. Relative state-to-state cross sections are determined by measuring the collision induced redistribution of the population of the initial state by using state selective laser detection techniques. The results for NH3 and OH are compared to theoretical values obtained from quantum calculations. Except for NH3 — He where theory predicts a parity selection rule for transitions to the 33 and 43 states, which is not observed in the experiment, good agreement between experiment and theory is obtained.

In the context of the search of asteroid families (i.e. identification of minor planets as potential fragments of an old bigger body), the calculation of proper elements plays an important role. They are quasi-invariants of the motion, obtained by a double averaging process of the restricted N-body problem; firstly the osculating elements are averaged over the short periodic terms (namely the longitudes of the asteroid and of the perturbing planets) so to get the mean elements, and secondly, the mean elements are averaged over the long periodic terms (longitudes of the pericenters and of the nodes of the asteroid and of the perturbing planets) to obtain the proper elements.

To reach lower noise levels with the Hubble Space Telescope Fine Guidance Sensors (FGS) the photometric properties as well as the systematic effects affecting the data have been investigated.

The NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG) is the largest star formation survey of an H I selected sample. Since the selection is made without regard to optical morphology, it is not biased toward or against “interesting” types of galaxies; thus SINGG is an ideal sample for studying galaxy demographics. Of a sample of 90 extra-galactic sources observed in photometric conditions, all are detected in Hα. This indicates that dormant galaxies, those containing an appreciable ISM but no star formation, are at best rare. We have made first pass morphological surveys for starbursts, as judged by Hα surface brightness, and outflows as judged by extra-planar Hα. We find that about 15% of the sources contain starbursts, with little dependence on the neutral hydrogen mass MHI. Nearly one half of a sample ~ 35 edge-on galaxies show evidence for extra-planar Hα having a scale size of 0.5 Kpc or larger, while nearly one quarter have extra-planar Hα features 1.0 Kpc in size or larger. There is a hint that high MHI systems preferentially have displaced outflows (chimneys, or fountains) while central outflows (galactic winds) preferentially occur in low MHI systems. However, a larger sample (e.g. the full SINGG survey) is needed to confirm this trend.

A number of promising observational programs involving linear polarization measurements are discussed after consideration of the observational limits based on telescope size and instrumental accuracy. These projects include studies of interstellar extinction, circumstellar shells and extended envelopes, inhomogeneities on stellar surfaces such as nonradial pulsation, magnetic stars, and interactive binary systems.

Modeling techniques for solar magnetic fields are discussed. As an extension of the source surface model for large scale solar magnetic fields, a method based on the Green’s function approach to the Laplace equation is proposed. In the force-free field modeling, a proper definition of the boundary condition is an important but still unresolved problem. A condition based on the continuity of the Maxwell stress is newly proposed.

Spectra of Uranus were obtained with the 122 cm reflector of the Crimean Astrophysical Observatory using the spectrometer of the Main Astronomical Observatory of the Ukrainian Academy of Sciences (Kiev), in March 1969. A photomultiplier (Soviet type 79) was used in a pulse counting mode, and the dispersion of the spectrometer at the camera focal plane was 15 Å/mm.

It is customary to convert observed brightness temperatures of molecular lines to molecular column densities by assuming some excitation temperature to describe the population of the energy levels of the molecule. Observers often speak about “abundances” after dividing these column densities by the dimension of the cloud that is being studied. This procedure can be misleading because the level populations are usually not well characterized by one single excitation temperature and because clouds are probably quite centrally condensed. Some molecules are formed preferentially at high densities in the opaque cores of clouds, others reach their maximum abundance in the moderately shielded much less dense outer regions of clouds.

I outline some of the main processes that shape planetary rings. Then I focus on two outstanding issues, the role of self-gravity in the precession of narrow rings and the dynamics of Neptune's arcs. By airing these well-defined but unsolved problems, I hope to encourage others to join me in the quest for their solutions.

We present the results of X-ray and millimetre monitoring of the blazar 3C273 at 1–2 day intervals over the period 12 December 1992 to 24 January 1993. No large flares are seen in this period but variations in both wavebands of ∼ 30% on few day timescales are apparent. The ROSAT PSPC X-ray spectrum consists of 2 power-law components with the harder component dominating above 0.5 keV. There is very little correlation between the variability of the soft and hard components. The soft component does not correlate with the millimetre variations, but the hard component correlates reasonably well and leads the millimetre variations by about 10 days. These results show that the hard X-ray component cannot be a simple extrapolation of the millimetre/IR synchrotron component but may be explained as a self-Compton component in a shocked jet.