This report covers the period from 1 July 1987 to 30 June 1990. The ephemerides that have been published during this period have made use of the system of astronomical constants adopted at the 16th General Assembly of the IAU in Grenoble. At the same time that the use of the FK5 system on the J2000 epoch is increasing and catalogues are becoming available for that system, there is an IAU Working Group on Reference Systems with subgroups on astronomical constants, nutation of the Earth, time and reference frames/origin. The purpose is to define a future reference system and resolve some of the difficulties that occur in the system. It is anticipated that this Working Group will develop recommendations for consideration at the 21st General Assembly of the IAU.

The aims of this report are, firstly, to review the activities of Commission 5 since the previous report and, secondly, to draw attention to other relevant activities. It is based mainly on contributions (shortened in some cases) from the chairmen of Working Groups and Task Groups, but other items of general interest have been included.

The report on the meetings of the Commission at the IAU General Assembly in Baltimore in August 1988 was published in Trans. IAU 20B, 109–118, while the membership is listed on page K10 and the IAU Style Manual is printed as Chapter 8 on pages Siii-Sxii and S1-S50. The proceedings of Joint Discussion No, 1 on New developments in documentation and data services for astronomers were reported in Highlights of Astronomy 8, 67–100, The proceedings of IAU Colloquium No. 110 on Library and information services in astronomy, which was held in Washington, DC, just before the General Assembly, were published by the U.S. Naval Observatory in 1989. The Newsletter of the Commission was issued in June 1988 and July 1989.

The year 1990, within the triennium covered by the present report, marked a number of significant anniversaries for the Central Bureau, including 70 years since its foundation at the Brussels Observatory, 25 years since the transfer of the Bureau’s operations to the Smithsonian Astrophysical Observatory (and 22 years of Dr. Marsden’s service as Director), and the issuance of the 5000th Circular. The amount of work done at the Bureau has increased enormously in this time, but especially within the past decade, as a result of the demands of modern astronomers and astrophysicists who are responding to the technological advances that have revolutionized observing and computing methods. At the same time methods of rapid communication have undergone a transformation that still continues. Appreciation is due for the support provided to the Bureau by the Smithsonian Astrophysical Observatory, but special gratitude must be expressed to Dr. Marsden and members of the staff and volunteers for their long hours of work and dedication to the needs of astronomers around the world for rapid access to information.

During 1988–1990 Commission 7 has sponsored or co-sponsored several IAU conferences: Colloquium No. 109 “Application of Computer Technology to Dynamical Astronomy” (Gaithersburg, July 1988), Symposium No. 141 “Inertial Coordinate System on the Sky” (Pulkovo, October 1989), Colloquium No. 127 “Reference Frames” (Virginia Beach, October 1990), Colloquium No. 132 “Instability, Chaos and Predictability in Celestial Mechanics and Stellar Systems” (Delhi, October 1990). The colloquium No. 118 “Dynamics of Small Bodies in the Solar System” which was to be held in Nanjing in June 1989 had unfortunately to be postponed then cancelled. Other meetings of interest to the members of Commission 7 were the 2nd Alexander von Humbolt Colloquium on “Long Term Evolution of Planetary Systems” (Ramsau, March 1988), the Colloquium “Asteroids, Comets, Meteors III” (Uppsala, June 1989), the colloquium “Mécanique Céleste et Systèmes Hamiltoniens” (Luminy, May 1990) and the NATO Advanced Study Institute on “Predictability, Stability and Chaos in N-Body Dynamical Systems” (Cortina d’Ampezzo, August 1990).

According to the recommendations of the Paris IAU Secretariat, the present triennial commission report is intended to put special emphasis on outstanding achievements around the main objectives of the commission rather than on abstracting all the literatures relevant to the commission. The highlights of the commission during the triennium under review have been the establishment of the conventional stellar reference frame FK5, an intensive effort in extension of the stellar reference frame to a celestial network of higher star densities and fainter magnitudes, and the launch of the astrometric satellite HIPPARCOS, as are described in this report.

The period covered by this report has seen significant progress in the development of the new generation of telescopes with apertures in the 8 m plus range. The period has encompassed the major construction phase of the 10 m Keck Telescope, witnessed the commissioning of the European Southern Observatory’s (ESO) New Technology Telescope and the approval of funding for the ESO Very Large Telescope (VLT). Significant progress has been achieved in developing the necessary technology for manufacturing and figuring large mirrors. There have been major expansions of activity in the areas of active control of telescope optics and adaptive optics, and in high angular resolution interferometry with several new groups entering both fields. The use of optical fibers, particularly in the area of multiple-object spectroscopy, has continued to grow. Several telescopes can now be operated remotely and the control systems of new telescopes are being designed to facilitate remote operation.

In preparing this report, which covers the period July 1, 1987, to June 30, 1990, close collaboration has taken place between the two solar commissions (10 and 12), in order to avoid duplication and to ensure that appropriate topics are treated. Further information on solar physics may be found in the report of Commission 12. It is a pleasure to thank the reviewers who wrote the different sections of this report, which demonstrate the active and lively state of our subject. Unfortunately, two topics that we had hoped to include, namely “active regions” and “solar activity from space observations”, were unable to be completed in time, but they are to some extent covered in Sections 7 and 6.

This report was planned in cooperation with Commission 10. Both this report and the one of Commission 10 together give a coordinated overview of published developments in solar research during the past three years. For several years, the scope of Commission 12 has included not only the areas indicated by the title of the Commission but also the structure of the solar interior. This broadening of coverage has led to a proposed renaming of this Commission: Solar Structure, which is currently under consideration by the IAU Executive Committee.

Since its creation, the Commission has been keen on activating the cross-discipline interaction between astrophysics and atomic and molecular physics. The need for a variety of atomic and molecular data has become more and more important for the recent past years. This need will certainly increase still more in the next years, due to the creation of new ground based instruments and to the launch of new space missions : they will produce large amounts of high resolution spectra from the X-rays to the infrared and millimeter wavelengths involving many atoms, ions and molecules. At the 1988 Baltimore meeting there was a general consensus that the aim of the Commission is to watch over the atomic and molecular spectral and structure data, together with the energy exchange processes in atomic and molecular physics relevant for astrophysics. In particular, the Commission is concerned by the interactions between photons and atoms (or ions or molecules), including wavelengths and line transition probabilities data, and by the interactions between particles, including atomic, molecular, ionic and electronic collision cross-sections, and by related phenomena, such as line broadening, collisional redistribution of radiation and line polarization. All these informations are essential for the interpretation of astronomical observations, such as spectroscopic diagnosis and theoretical modelling of astrophysical media.

The period covered by this report, July 1987 to June 1990, witnessed a similar activity as the previous triennial interval. While the latter saw the successful encounters of several space probes with comets P/Giacobini-Zinner and P/Halley as well as the unprecedented worldwide campaigns of ground-based measurements organized by the International Halley Watch, emphasis during the 1987–1990 time period focussed mostly to the analysis of existing data, while the study of new comets continued at a slightly increased level.

It is with great sadness that we have to report the death of Harold Masursky on August 24, 1990 at the age of 66. Harold Masursky was the President of the Working Group for Planetary System Nomenclature. He was well known for his active participation to space exploration of the planets. He was one of the pioneers of planetary geology. He wrote many important contributions on planetary surfaces.

The period has been marked by the start of the new International Earth Rotation Service (IERS), which benefits from a tight cooperation between astronomers, geodesists, and specialists in satellite geodesy, as well as meteorologists. The scope of the IERS covers not only the Earth’s rotation per se, but also the conventional terrestrial reference frame, of direct interest to the International Association of Geodesy, and a high accuracy (0.001”) celestial reference frame based on extragalactic compact sources observed in Very Long Baseline Interferometry. The IERS conventional celestial reference frame is consistent with the FK5 within the uncertainties of the latter (0.04”). The IERS Standards (1989) which contain the current best estimates of astronomical models and constants are used in many fields of astronomy and geodesy.

The past triennium was characterized by important advances in several directions within the broad field of this Commission. Among the most exciting events were the Phobos 2 and Voyager 2 missions to Mars/Phobos and Neptune/Triton; their navigation was critically dependent on precise space- and ground-based observations as well as refined theoretical models of the motions in the solar system. Among many other achievements, the secular acceleration of Phobos was accurately measured and the dynamical analysis of the ring-like arcs at Neptune has just begun.

The different components of the light of the night sky are integrated along the line of sight of the observer located on planet Earth or on the near Earth’s environment. They encompass a huge scale of distances, ranging from hundreds of kilometers in the Earth’s atmosphere (airglow lines, bands and continuum), to astronomical units in the interplanetary medium (zodiacal light continuum), thousand of light years in the interstellar medium (diffuse galactic light and integrated starlight) and even much more in the extragalactic universe (extragalactic background radiation).

As usual, a number of members of the Commission have contributed reviews of their own field. These have been edited in order to reduce overlaps and keep the whole report within the size limitation. Reference numbers from “Astronomy and Astrophysics Abstracts” have been used when available, and those references not (or not yet) included in the “Abstracts” have been gathered into a list at the end of the report. As editor, the President takes responsibility for any shortcomings in the report.

Despite some much publicized disappointments, the highlights of Astrometry during the past three year period are dominated by the successful launches of the ES A Hipparcos astrometric satellite and the NASA Hubble Space Telescope. For the past 15 to 20 years the research activities of many Commission members have been monopolized by the design and development of the associated instrumentation and the preparation of catalogues of objects to be observed. Unfortunately, the successful launch of Hipparcos was followed by a failure of the apogee boost motor to ignite and place the satellite into its proper geostationary orbit. After considerable replanning, the perigee was raised to a safe level and access to additional ground stations was arranged so that observations could still be made from the 10.6 hour period elliptical orbit. Even though Hipparcos passes through the Earth’s particle radiation belts twice a day, little degradation in the solar panel output has been observed subsequent to the initial decrease in the output and a lifetime of approximately three years is predicted for the satellite.

Technology has continued to influence the methods of stellar photometry and polarimetry in the last triennium. For example, CCDs continue to displace photographic plates in stellar photometry and other detector systems for imaging and spectro-polarimetry, new array detectors for infrared wavelengths have revolutionised the way infrared photometry and polarimetry is done and small automatic telescopes and robotic observatories have experienced a remarkable growth period. The volume of data to be archived and transformed to “standard” systems has become immense and all the problems of inter-comparing different systems still remain.

The observational study of double and multiple stars is traditionally seen as comprised of several specialized subfields, each defined by particular observational selection effects. Improvements in increased precision and accuracy, higher resolution, higher signal-to-noise ratios, greater sensitivity etc.), are leading the study of double stars to a truly coherent field. Significant advances in ground-based spectroscopy and interferometry provide a framework from which we can prepare for the future. The contributions to be made from space are currently in a state of uncertainty as we await results from the Hubble Space Telescope and Hipparcos.

Comm. 27 sponsors three special services for the astronomical community: the Information Bulletin On Variable Stars (IBVS). the Archives of Unpublished Observations of Variable Stars, and the General Catalog of Variable Stars. The news from the centers organizing these activities can be found in Section 2. The subsequent sections present a summary of the developments between 1987 and 1990 in the various scientific fields written by selected experts. We are grateful to the authors for their excellent reviews.