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.