The Commission has, since its inception, been devoted to the continually increasing needs of astronomy and astrophysics for reliable atomic and molecular data a) for diagnostic interpretation of astronomical observations, and b) for support of theoretical modelling of astrophysical situations. At the 1985 Delhi General Assembly, the Commission reviewed the scope of its subject matter, and considered whether it should be extended to include higher energy physical processes than are commonly treated by atomic and molecular data. It was concluded that there was no strong demand for this change, which, if implemented, would make the work of the Commission too diffuse. The appropriateness of the past working group structure was also carefully reviewed in the light of contemporary needs. The following working groups, which have evolved from those of past years, together with their chairmen was approved:

1. 1: Atomic Spectra and Wavelength Standards (excluding primary standards): W.C. Martin

2. 2: Atomic Transition Probabilities: W.L Wiese

3. 3: Collision Processes: A. Dalgarno

4. 4: Line Broadening: N. Feautrier

5. 5: Molecular Structure and Transition Data: W.H. Parkinson

The Data Center on Atomic Transition Probabilities at the National Bureau of Standards, Gaithersburg, Maryland, 20899, U.S.A. has continued its critical compilation work and maintains an up-to-date bibliographical data base. Work to revise and expand the existing NBS critical data compilations for the allowed and forbidden transitions in Fe-group elements, (Refs. A-D) has been completed. A single volume containing all these data for the Fe-group elements Sc to Ni is in press (Volume III of the NBS series of atomic transition probability tables) and is scheduled to be published in the near future, as a supplement to the Journal of Physical and Chemical Reference Data.

Of the vast array of data on electron and heavy-particle collisions that are produced each year, I select only those that have an obvious immediate bearing on astronomical research. A brief review of recent developments in atomic data for astrophysics has been published (1).

There has been quite a renewal of interest in the field of line broadening during the last three years and the proceedings of the Eight International Conference on Spectral Line Shapes (1) showed the general state of progress of this topic. It is not the purpose of this report to be exhaustive, so we will simply give a number of useful results for astrophysical purposes and indicate some new fascinating directions in this research theme.

Research in molecular spectroscopy has continued to grow over the past three years. The spectral range has expanded from the far ultraviolet to millimeter wavelengths. The report has been limited to molecular spectroscopy of relevance to astronomy and has been compiled from edited contributions sent to me in the fall of 1987.

The basic principles of heterochromatic extinction show that the approach used in the visible should not work well in the infrared, where molecular line absorption rather than continuous scattering dominates the extinction. Not only does this extinction change very rapidly with wavelength (so that stellar color becomes only weakly correlated with effective extinction), but also many of the lines are saturated (so that Forbe’s curve-of-growth effect is much more severe in the IR.) Furthermore, broadband IR colors are more undersampled than those in the visible, so aliasing errors make them correlate even less with extinction.

The period covered by this report, 1984 July to 1987 June, was of extraordinary importance for the progress of cometary physics. For the first time in the history, special space probes were launched to comets. Vega 1, Vega 2 and Giotto encountered P/Halley, providing us with the first close-up pictures of a cometary nucleus, its surface features, and with the first in situ measurements of the matter escaping from it. ICE, Suisei and Sakigake carried out measurements relevant to P/Giacobini-Zinner and P/Halley in interplanetary space. Unprecedented worldwide campaigns of ground-based observations, with the participation of about 1000 professional and 2000 amateur astronomers, were coordinated in 8 sections of the International Halley Watch. Additional measurements were made from artificial satellites, sounding rockets, and highflying airplanes. The wealth of data collected in this way, to a major extent thanks to an excellent international cooperation, represents a milestone in cometary astronomy. Another important step was the progress in processing the extensive 1983 IRAS observations of minor planets and comets, including the discovery of asteroid dust bands and cometary dust trails.

The physical study of planets and satellites is probably one of the more active fields of research of the second half of this century. This is due to space exploration by spacecraft, but also to the use of modern detectors, of large ground-based telescopes, and of powerful computers by active researchers. Planetary research (or planetology) is a pluridisciplinary domain, which requires not only the competence of astronomers, but also of geophysicists, of mineralogists, of climatologists, of biologists, of chemists, of physicists, of “pure„ mathematicians, and many other scientists. Many results are at the boundary of those of other commissions such as the 15, 20, 7, 19, 33, 40, 44, 49 and 51 ones. The study of the main results obtained during this last triennum shows a perfect complementarity between space and ground-based observations. It should be arbitrary to separate space and ground-based scientists. The have the same goal and they study the same objects. Quite often, the same individuals use both techniques, depending on the most efficient one for the problem under study. It is remarkable to see that space data collected more than ten years ago are still analysed in connection with ground-based observations. The same remarks can apply for ground-based data. In addition to that, new theoretical models, new numerical simulations and new laboratory experiments have ben recently developed. They all contribute to a better understanding of planets and satellites physics.

During the period, there have been several major events which have effected the scope and interest of Commission 19. The most significant of these has been the dissolution of the BIH and IPMS and their replacement by the International Earth Rotation Service (IERS). The correlation of higher frequency fluctuations in the Earth’s rotation rate with changes in the Earth’s Atmospheric Angular Momentum is also significant. Many investigators now seem to believe that the “decade variations„ in the Earth’s rotation rate are caused by torques between the core and mantle caused by the uneven motions at the core-mantle boundary. These events and discoveries have made this an exciting period. It seems that the future holds more in the way of discovery due to the utilization of the more accurate and precise Earth rotation data coming from the modern observing techniques.

In the past triennium members of the Commission 20 have been very active in studying positions and motions of minor planets, comets and satellites including rings by observational and theoretical investigations, as it is described in this report In fact observers have been producing tremendous amount of astrometric data, much more than we could imagine twenty years ago, when we heard many complains and appeals to observers for needs of more observations, particularly, for minor planets and satellites. Technically, several new devices to detect faint objects and to measure their positions more effectively, have been developed. Theoreticians have faced many interesting and important problems to explain observational facts with several powerful methods developed recently.

The different components of the light of the night sky have their origin in different formations of matter in the universe - encompassing a huge scale of distances ranging from a few kilometers in the earth’s atmosphere to the most distant known galaxies and beyond. Correspondingly, the borderlines to other Commissions are not very well defined and thus material relevant to Commission 21 can also be found in the reports of other Commissions on the following topics: zodiacal light and zodiacal IR emission (Comm. 22, 44), integrated starlight (33, 25), diffuse galactic light (34), extragalactic background light (47), airglow and atmospheric scattered light (50), and space-borne observations of the LONS (44). From the Commission 21 point of view the connecting link between these various fields is the special techniques utilized in the surface photometric measurements and reductions of background radiations which extend over the entire sky. One crucial problem is the separation of the LONS into its several components. The approach for solving this task is to utilize the different spatial distributions and different broad and narrow band spectral properties of each of the LONS component. Thus the successful measurement and separation of one of the LONS components requires a knowledge of the properties of all the other components. This situation has become apparent in recent years as the infrared background radiation database, provided by the Infrared Astronomical Satellite (IRAS), has been analyzed: both the zodiacal and galactic dust emissions have to be analyzed hand in hand, and both these components must be very accurately mastered before any conclusions are possible on the extragalactic component. It is also obvious that very similar problems are encountered in the ultraviolet and infrared wavelength regions as in the more traditional optical domain. Thus the techniques developed in one of these wavelength domains are directly applicable in the others.