Atmospheric lensing effects deform our view of distant objects; similarly, without any doubt, gravitational lensing perturbs our view of the distant Universe and affects our physical understanding of various classes of extragalactic objects. We summarize here part of the theoretical and observational evidences supporting these claims.

After briefly reviewing the history of gravitational lenses, we recall the basic principles underlying the formation of gravitationally lensed images of distant cosmic sources. We describe a simple optical lens experiment, which was actually shown during the oral discourse, and which accounts for all types of presently known gravitational lens systems.

The various optical and radio searches for new gravitational lens systems that are being carried out at major observatories are reviewed. State-of-the-art observations of selected gravitational lens systems, obtained with highly performing ground-based telescopes, are then presented. These include several examples of multiply imaged QSO images, radio rings and giant luminous arcs.

Through the modeling of these enigmatic objects, we show how it is possible to weigh the mass of distant lensing galaxies as well as to probe the distribution of luminous and dark matter in the Universe. Among the astrophysical and cosmological interests of observing and studying gravitational lenses, we also discuss the possibility of deriving the value of the Hubble parameter Ho from the measurement of a time delay, and how to determine the size and structure of distant quasars via the observational study of micro-lensing effects.

At the end of this paper, we conclude on how to possibly achieve major astro-physical and cosmological goals in the near future by dedicating, on a site with good atmospheric seeing conditions, a medium size (2-3 m) telescope to the photometric monitoring of the multiple images of known and suspected gravitational lens systems.

Within the brief span of a decade, from 1979 to 1989, the Voyager spacecraft visited the four giant planets – Jupiter, Saturn, Uranus and Neptune – along with their satellites and their rings. The science return from these two spacecraft forever changed our views of this remote region of our solar system. Often overlooked, however, is the incremental gain in knowledge from these encounters over that which had been known in the early 1970s when the Voyager project first came into being. From a post-Voyager perspective, it is astonishing how little was known about the outer planets just a mere two decades ago. Yet, with all of the knowledge that the space program has brought us, there remain a number of unanswered questions and a great many new ones that have been posed as a result of this wealth of new information. Discussed here is summary of the results of the Voyager imaging cameras together with some of the many new questions that subsequently have been raised.

A large number of observations have accumulated which represent baffling mysteries from the standpoint of current astronomy. A sample of the most crucial of these cases is reviewed and updated. Evidence has slowly built up to the point where often each individual object is a formidable challenge to conventional theory. But, most importantly, it is now possible to appreciate the common pattern which mutually reinforces all the separate results.

These empirical discordances point directly at known weaknesses in the Big Bane theory of the origin of the universe. New data is used to introduce a proposal which accounts for both the data which gave rise to the Big Bang interpretation and which also satisfies the discordant results which have accumulated over the last quarter of a century. Though perhaps only a primitive beginning, the new interpretation demonstrates that observations need not be discarded because they disagree with current theory. The major conclusion from the cases discussed here is that the present observational paradoxes represent spectacular opportunities to make new and fundamental discoveries about the universe.

Diffuse clouds are interstellar objects through which starlight is not greatly extinguished. As a result, many studies rely on ultraviolet and visible measurements. The focus of this review is on spectroscopic work involving atomic and molecular lines. From these measurements, the physical and chemical makeup of the clouds is derived. Here we stress that technological advances have influenced our perception of diffuse clouds.

This paper summarizes the physical conditions in interstellar gas occurring outside classical atomic or molecular clouds, from a view-point which is purely observational.

Magnetic fields are present in every corner of the Universe. The Earth, the Sun and most of the planets are known to possess dipolar magnetic fields. In the Galaxy many individual objects like stars, pulsars, bipolar nebulae and supernova remnants are found to have associated magnetic fields. It seems that the rotation plays a significant role in the ability of a cosmic object to develop a magnetic field. The magnetic field of the Galaxy is observed to be oriented along the galactic plane as evidenced by both optical and radio polarization observations. Radio maps of the Galactic centre reveal poloidal magnetic fields as ‘wisps’ or ‘strings’ around Sagittarius A. Observations of nearby galaxies give us remarkable information about the large-scale magnetic fields in these building blocks of the Universe. Magnetic fields play an important role in the formation of jets of radio galaxies. Further out, in clusters of galaxies, definitive evidence has been given for the existence of intergalactic magnetic fields.

The relationship between high energy phenomena and the interstellar medium is wide. I had to make a selection, and only cover a few aspects: cosmic rays, gamma rays, correlation between radio synchrotron and infra red emission and its influence on cosmic rays; and finally some new observational results.

The dynamic subject of the Galactic center is driven mostly by high-resolution imaging both in the continuum and in the spectral line modes in a number of wavelength regimes. An attempt has been made to give a short overview of recent observations in this region, as follows: 1) Unusual characteristics of the population of molecular and dust clouds within the inner few hundred pc of the Galactic center. 2) Large-scale magnetic structures within the inner 100 pc of the Galactic center and the possible evidence for their poloidal configurations -unlike that seen in the disk of the Galaxy. 3) Stellar kinematics in the bulge and the possible evidence for a massive central source at the dynamical center and its examination as a gravitational lens. 4) Morphological and kinematic evidence for the Galactic center wind interacting with stellar wind associated IRS7, a mass-losing supergiant lying one light year away in projection from the Galactic center.

Recent observations of the several phases of the interstellar medium in galaxies reveal a wide range of structures and physical properties. The HI structure in nearby galaxies is very filamentary with a large number of shells and filaments suggestive of a close interaction between the star formation in the disk and the surrounding interstellar medium.

I review recent theoretical developments in the structure, heating, and ionization of the interstellar medium. Among the unsolved issues are: the vertical support of the gas layer; the ionization source for the H+ layer and diffuse line emission above the Galactic plane; the physics of stochastic heating of small dust grains and the resulting infrared spectra. Turbulent mixing may play an important role in the intercloud medium and diffuse emission above the disk. I conclude with some possible analogies between the intergalactic medium and the interstellar medium.

I start by general remarks on the background of the recommendations on space-time references which are submitted to you.

The need to consider time scales in a relativistic framework appeared more than 20 years age following the progress of atomic time standards. After long discussions, this led the IAU to define, In 1976, time scales which were designated, In 1979, as Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB). But soon afterwards difficulties in the interpretation of the definitions of TDT and TDB arose. It appeared that the source of these difficulties was the lack of a global approach to space-time reference systems. This point of view, first voiced by J. Lieske, gained acceptance. At the very beginning of the work of the WGRS Sub-Groups on Frames and Origins (SGFO) and on Time (SGT), It became clear the the primary mission of the SGFO and SGT was to jointly prepare general recommendations on space-time references on which they could base their specific recommendations.

Among the nine recommendations of the Working Group on Reference Frames as finalized during the Virginia Beach meeting, four are directly addressing the reference systems. They are the result of the work of the subgroup on coordinate frames and origins and other contributors whose names are given in section 6. The resulting document (Kovalevsky, 1991) was further revised by the working group as a whole and recommendations published (1991).

The IAU Working Group on Reference Systems (WGRS) Sub-Group on Astronomical Constants (SGAC) was established in June, 1989, as a consequence of resolutions adopted by Commissions 4, 7, 8 and 24 at the IAU General Assembly at Baltimore in 1988. The given missions of this sub-group were stated clearly by J. Hughes, the chairman of the WGRS.

Due to weather-related travel delays Dr. McCarthy was unable to present his scheduled paper. In his absence, J.A. Hughes gave a short talk indicating that Recommendation IX had received essentially universal approval at and since the Virginia Beach Colloquium, and that it would undoubtedly be approved by the Joint Commission Meeting as presently written, and then submitted as a Commission Resolution.

Mindful of the fact that any time and space coordinates of General Relativity Theory (GRT) are not, in general, physically meaningful and measurable quantities, one may choose between three possibilities in applying GRT to ephemeris astronomy:

1. 1) avoid coordinates completely, i.e., construct coordinate-independent theories of light propagation and solar system body motion involving the removal of all coordinate-dependent quantities from the present system of astronomical constants;

2. 2) use any coordinate system to describe observational procedures and to solve dynamics problems, provided that one and the same coordinates be used for both the kinematics and dynamics of a specific problem;

3. 3) adopt IAU recommendations specifying reference systems and time scales to be used in ephemeris astronomy.

Very early human experience has suggested a practical definition for the measurement of time: We define a unit of time by defining a standard (cyclical) process. Whenever this process completes its cycle identically, a unit of time has elapsed. This is the origin for the various measures of time in classical astronomy. Nature suggests strongly that we use as such standard processes the year (defined as a complete revolution of the earth around the Sun), the month (the completion of a revolution of the moon around the earth), and the day which again can be measured in several different ways. While the sidereal day is measured by a rotation in respect to the vernal equinox, the mean solar day is measured in respect to the mean. Sun. More recently, we have distinguished many more different ways of defining measures of time, partly in response to perceived needs of the applications, but in part also from purely aesthetic principles.

IAU Recommendations should 1) help to avoid confusion and 2) help to enhance scientific capability. Recommendation III does neither. It would create confusion while doing nothing to improve scientific results.

If Recommendation III is ever implemented, the result will be sheer chaos. This recommendation proposes that the rate of the basic ephemeris time scale be changed – by an amount of nearly one-half second per year. This implies that all existing planetary, lunar, satellite, spacecraft, asteroid and comet ephemerides would be referenced to an obsolete time scale; all existing sets of orbital elements would need a modification; that epoch J2000 (JED 2451545.0) would differ from the presently defined one by nearly 15 seconds.

In the past, the IAU has adopted standard values for some constants, primarily for use with solar system ephemerides. The constants adopted in 1976 were specifically adjusted to provide internal consistency. In each case, when constants have been adopted, the changes have reflected accuracy improvements, and the purpose has been to encourage the accomplishment of better science.

Over the past 12 years, the Working Group on Cartographic Coordinates has issued triennial reports giving the best values for the sizes and rotations for the planets and satellites. This working group now is an IAU/IAG/COSPAR working group reflecting the different organizations that have recognized the benefits of this group. This is an example of a properly functioning working group, which provides the best values on a regular basis. The IUGG also provides best estimates triennially for values of interest in geodesy and geodynamics.

Due to weather-related travel delays Dr. McCarthy was unable to present his scheduled paper. In his absence, however, Dr. K. Yokoyama, Chairman of the International Earth Rotation Service (IERS), reported on the status of the revision of the IERS Standards carried out by McCarthy who provided the following precis.

These standards are meant to be used in the reduction of observations contributed to the IERS and by those who require IERS estimates of polar motion, Universal Time and nutation to establish the most precise celestial and terrestrial reference systems. The goal of the IERS is to publish the document, an update of the previous version (McCarthy, 1989), in the beginning of 1992.