The present report covers the period 1976–1978. As with previous reports it has not been possible to write an all- inclusive account covering everything published, or even every field under investigation; the number of contributions to extragalactic research is increasing enormously fast. Summaries of the papers may be found in the Astronomy and Astrophysics Abstracts.

The report is divided into ten sections, including the four Working Groups, and they have been prepared by the Vice-President, by members of the Organizing Committee and by the Chairmen of the Working Groups.

This Report is compiled by using the reports of the Directors of the national ephemeris offices reviewing the programs for the preparation and publication of astronomical ephemerides of variuos purposes, i.e. for astronomers and space scientists, marine and air navigators, and surveyors. Some proposals of Commission members are also included. The Report may be of interest also to other I.A.U. Commissions, and mainly to Commissions 7, 8 and 31. Principal topics belonging to the Commission competence and some items to be presented to the XVIIth General Assembly of the I.A.U. in Montreal will be reviewed in following comments.

A la requîte de J. Sahade, Président du POGSI, les membres de la Commission 5 ont donné des avis sur ce problème. Quelques extraits significatifs des suggestions relatives aux solutions possibles:

As usual, this report contains contributions from a number of authors, as follows: § 2, P. D. Jackson and M. P. FitzGerald; §§ 3 and 4, F. J. Kerr and D. L. Crawford; §5, P. O. Lindblad; §§ 6A and C, R. Wielen; §§ 6B and 7, J. Einasto; §§ 6D and E, K. C. Freeman, § 6F, M Fujimoto. The layout follows previous practice, except that a new Section 7 on the galactic environment has been added. A longer version of the Report will be published by the University of Maryland and will be distributed to all members of Commission 33 and to astronomical institutions.

As noted below in the report of the Central Bureau, a number of changes have been introduced in the technical and business operations of the Bureau during the triennium. Some of the changes follow from the transfer of the IAU Minor Planet Center to the Smithsonian Astrophysical Observatory. For example the Minor Planet Circulars now offer an alternate avenue of publication for less time-urgent data concerning comets as well as minor planets. Changes at the communications center of the Smithsonian Astrophysical Observatory have made it less costly, as well as more efficient, for the Central Bureau to maintain its own communications equipment. At the same time, in the face of generally rising costs, and after careful investigation of alternatives, it has become necessary for users to assume an increased share in underwriting the operating expenses of the Bureau.

It is now recognized that diffuse matter in space plays a decisive role in the evolution of our Galaxy and of similar galaxies. Primordial gas—together with gas ejected in planetary nebulae, stellar winds, novae, supernovae, and other types of stars—has accumulated to form a complex medium containing regions with densities ranging from 10–3 to 106 particles cm-3 and with temperatures ranging from 10 K to 106 K. From time to time, part of the interstellar medium collapses to form stars. In order to understand the evolution of the Galaxy, it is essential to understand how energy, mass, trace elements, and dust grains are deposited into the interstellar medium by stars. It is also essential to understand the mechanisms that initiate star formation in certain regions, and how the ensuing collapse develops in space and time.

It is now clear that the ISM does not consist solely of the classical three components, i.e., molecular clouds, diffuse clouds, and the intercloud medium (H II regions are discussed in section 7 of this Report). High-temperature, low-density components also exist. The role of the transition regions between these components has recently been emphasized, and dynamical processes, including interchange of matter between them and the role of stellar winds and supernova remnants, have been extensively studied. Review papers or general discussions have been given by Field (18.131.115); Salpeter (17.131.143); Kahn (18.131.116, concerning hydrodynamics and MHD); and Heiles (20.156.012, on interstellar magnetic fields). McKee and Ostriker (1975, Astrophys. J. 218, 148) have given a general theory of the ISM including a hot, diffuse component; they emphasize the role of the energy and momentum input from supernovae, the evaporation of cold clouds into the hot medium, and the role of heat conduction.

This report is organized according to twelve subjects of interest in modern celestial mechanics. Emphasis is placed on the results of an exhaustive literature search in which assistance was received from several members of Commission 7. The impetus on celestial mechanics by space projects, by new observational techniques and by the use of high speed electronic computers continued to be of considerable importance. The fact that celestial mechanics became an experimental science is still too recent to be fully evaluated. The high accuracy of radar and laser observations, the new analytical and numerical techniques, the recent sophisticated statistical orbit determination methods and many other results achieved during the past three years suggest that celestial mechanics is an exploding field of science in which new and exciting results emerge. Celestial mechanics is influencing other fields of scientific endeavor and is intimately connected and embedded in almost all areas of modern astronomy and space research.

It is clearly impossible to prepare a comprehensive review of three years’ work in the field of astronomical instrumentation within the space available in this volume. The selection of topics included and the space devoted to each is therefore largely subjective; some emphasis has been placed on material unlikely to be generally available elsewnere.

During the past three years work on Clusters and Associatons has been proceeding with unprecedented vigor. Space limitations, unfortunately, make it impossible to review all of this material in detail. A listing of some of the most important recent review papers is given in section 2. Work in progress or recently published on individual clusters and associations is collected in the tables given in section 3.