The last three years have been marked by changes, highlights and progress. Organizationally, commission 7 has joined Division I and plans proceed for commissions 8 and 24 to merge in 2000. They have had a common vice president during this triennium. Sadly, the Royal Greenwich Observatory was closed after over 200 years, but Her Majesty’s Nautical Almanac Office has continued at Rutherford Appleton Laboratory. In St Petersburg, Russia, the Institute of Theoretical Astronomy was abolished, with some of the personnel relocated to the Institute of Applied Astronomy and Pulkova Observatory. In Paris, France, the Bureau des Longitudes was reorganized as the Institute of Celestial Mechanics-Bureau des Longitudes as part of the Paris Observatory.

The Commission has established its own website (http://ssd.jpl.nasa.gov/iau-comm4/) containing the following main topics:

The Commission presently has 88 full members and 3 consultants.

Research in Celestial Mechanics, for the past three years, has mainly focused on the understanding of Chaos on all its aspects. The always larger number of potential applications (meteors, KBO, NEA, asteroids of the main belt but also exoplanets or galactic motions) and the development of new efficient tools, like the symplectic integrators, have allowed the passage from QUALITATIVE models (for example the transfer mechanisms) to real QUANTITATIVE results (like the calculation of lifetimes). This important step has contributed to (re)create collaborations between theoreticians and observers (for example, in the prediction of catastrophic impacts) and to situate the Celestial Mechanics in a wider scientific context.

The present triennial commission report embraces mainly activities in wide angle, optical astrometry. With the successful development and application of new techniques from Earth (e.g. optical interferometry, CCD’s) and space (Hipparcos mission and new projects) the sub-division between Commissions 8 (Positional Astrometry) and 24 (Photographic Astrometry) has become questionable. During the GA at Kyoto in 1997 all steps for a merger of both commissions have been taken. The final merging will take place at the forthcoming GA in Manchester. For a more complete overview on astrometrical work done in the past triennium the reader should also take notice of the report of Commission 24.

The triennial report of Commission 19 was composed from the contributions of its members. Space does not permit a listing of their names, but their contributions are sincerely appreciated. Unfortunately because of limited space it is also not possible to provide in this report the extensive list of publication of the Commission members. The list of publications is however available on the Commission 19 web site at maia.usno.navy.mil/iauc19.

The scope of scientific interests of about 130 Commission members is much more complex than one may expect from the title of the commission. Besides traditional topics like the compilation of astrometric catalogues and the construction of an inertial reference system, more and more investigations performed by our members have been dedicated to the astrophysical interpretation of observations including among others astrometric data.

The CCTF (formerly named CCDS) held its 14th meeting on 20-22 April 1999. Following the discussions, seven Recommendations were adopted and submitted to the Comité International des Poids et Mesures (CIPM). The list is the following:

1. 1. Recommendation S 1 (1999): Mise-en-pratique of the definition of the second.

2. 2. Recommendation S 2 (1999): On stating uncertainty in comparisons involving primary frequency standards.

3. 3. Recommendation S 3 (1999): On the comparison of primary frequency standards.

4. 4. Recommendation S 4 (1999): On the use of multi-channel and multi-code GPS and GLONASS time receivers.

5. 5. Recommendation S 5 (1999): Time and frequency comparisons using GPS phase and code measurements.

6. 6. Recommendation S 6 (1999): Future global navigation satellite systems.

7. 7. Recommendation S 7 (1999): On Two-Way Satellite Time and Frequency Transfer

Additionally discussed were the present form of UTC and mostly the interest of preserving the leap second. The advantages and disadvantages of several options regarding the future use of leap seconds were compared. The CCTF, however, felt that it did not have the authority to propose any action. Then CCTF decided to ask the BIPM Director to write to the relating international bodies including IAU so as to draw their attentions to this issue while recommending the usage of TAI in case a time scale without discontinuity is needed. Also it was decided, in order to make more expedite the process, to ask the opinions of the various Commissions of the Scientific Unions.

Division II consists of Commissions 10 (Solar Activity), 12 (Solar Radiation and Structure), and 49 (Interplanetary Plasma and Heliosphere). More detailed information on IAU activities in each of these three areas can be found in the individual reports of the respective Commissions.

The Division II OC briefly considered changes to the structure of our Division in response to opportunities offered by the Executive Committee during this triennium. However, we believe that the present structure works well and change is not warranted at this time.

The Sun’s activity has been evolving in the ascending phase of Solar Cycle 23 since 1996. Similarly, the research on solar activity is also in the ascending phase of a new active period. Numerous new results have been obtained from a large amount of space and ground observations covering a wide spectral range. In particular, observations with YOHKOH, SOHO, and TRACE have revealed a multitude of phenomena and processes in the solar atmosphere which provide us a new picture of the Sun.

The focus of Commission 12 is on the solar interior, on global phenomena of the quiet Sun, and on the Sun’s radiative outputs, both spectral and total. These are the topics treated below in our discussion of Scientific Highlights. The many topics having to do with solar activity at photospheric to coronal levels, are dealt with in the report by Commission 10, while the report of Commission 49 describes research on the solar wind and interplanetary medium.

In the last decade the triennial reports from Commission 49 have covered various topics like (nonlinear) plasma processes, magnetohydrodynamic phenomena and flows in the heliosphere, solar wind composition, transient events in, and latitudinal dependencies of, the heliosphere, interstellar gas flow through the interface region, kinetic versus magnetohydrodynamic theory in heliospheric plasmas and charged dust in space plasmas. Continuing the tradition of summarizing specific aspects to give astronomers outside our own specialty a flavour of our field, we now address recent advances in understanding coronal mass ejections in interplanetary space and the inner heliospheric solar wind under quiet and perturbed conditions. We owe a great debt of gratitude to the eminent contributors for their valiant efforts in writing these succinct but clear reports and guiding us through the recent literature.

Because the division consists of many very active commissions, most activities are included in the reports of the individual commissions. This report highlights a small subset of the major achievements that are covered in detail in the reports by the commissions. Some administrative activities of the division and reports of the divisional working groups and committees are also included as subsequent sections of this divisional report.

The present report of Commission 15 has been, as usual, prepared primarily by the chairpersons of the two working groups. E. Tedesco wrote the section about Asteroids and Meteorites, with the assistance of A. Cellino, G. Consolmagno and C.-I. Lagerkvist. W. F. Huebner prepared the section about Comets, with the assistance of J. Benkhoff, H. Boehnhardt, J. Brandt, M. T. Capria, A. Cochran, G. Cremonese, M. Duncan, W. Huntress, H. Levison, and G. P. Tozzi. Moreover, the whole document has been assembled by K. Muinonen, who did the final editing, to merge the two reports and fit the document into the allotted space. Material taken from both major areas regarding the relationship between comets and asteroids has been combined into a single section.

This report includes some of the major achievements in studies of planets and satellites that have been made during the past three years.

This Comm. 20 triennial report is prepared as a short summary of the scientific progress in the fields of interest, where, by necessity, only a few highlights are to be mentioned. On the Comm. 20 webpage, the interested reader may find a more complete list of recent references to the relevant literature. Its URL at the time of writing (October 1999) is www.astro.uu.se/IAU/c20/, but mirror sites are expected to have been set up at the time of appearance of these Transactions.

During the period from July 1996 to Dec 1998, 189 photographic positions of the galilean satellites were obtained with the zonal astrograph (D=120 mm, F=2044mm). In the year 1998 the close approach of the satellite system with the star N 117881 from the HIPPARCOS catalogue was observed. The observations of 1996 were processed using PPM catalogue and the observations of 1997, 1998 in the ICRS using HIPPARCOS and ACTRC star catalogues. The results are not yet published, but are available upon request. Observations made since 1962 up to 1996 with the zonal astrograph were reprocessed and results are in preparation for publication. During the period from 1996 to 1998, 69 positions of the Galilean satellites were determined by photoelectric meridian observations with the Repsold meridian circle equipped by CCD-microscopes (F=2150 mm, D=150 mm).

It has been known that non spherical silicate particles of a size comparable to the wavelength of light, and aggregates of such particles, produce negative polarization in the backscattering region (e.g. Xing & Hanner 1997, Yanamandra-Fisher & Hanner, 1999). It has now been shown that large aggregates of small absorbing particles of fractal dimension about 2 produce a slightly negative polarization at small phase angles (Levasseur-Regourd et al., 1997). The phase-curves strongly differ from those of Mie spheroidal particles. They are likely to be due to scattering by irregular dust particles and/or fluffy aggregates of numerous submicronic absorbing particles (Levasseur-Regourd et al., 1997; Lumme et al., 1997).

The IAU Meteor Data Centre (MDC), maintained at Lund, Sweden, under the direction of Bertil Lindblad, remains a valuable resource for the meteor community. Commission 22 currently has representatives on two inter-commission Working Groups, The Prevention of Interplanetary Pollution (Chair C. S. L. Keay of Commission 22) and Near-Earth Objects (IAU C22 representative I.P. Williams). The Commission has one Working Group of its own, Amateur-Professional Co-operation in Meteors, consisting of P. Jenniskens (Chair), H. Betlem, N. Bone, P. Brown, T. Cooper, I. Hasegawa, R. Hawkes, G. Klar-Renner, V. Porubčan, J. Rendtel, J. Richardson, T. Yoshida. Meetings of the working group were held in Japan in Aug 1997 and in Ithaca, NY in Jul 1999, and circulated periodic electronic newsletters. The working group helped establish professional-amateur collaborations during the Leonid campaigns of November 1998 and 1999, facilitated amateur participation in NASA’s Leonid Multi-Instrument Aircraft Campaign, and encouraged publication of amateur observations. The IAU Commission 22 www site has the following new location: http://www.mta.ca/rhawkes/IAU22.

Professor Colomb resigned as President of the Commission because of pressing demands of work. The Organizing Committee appointed Stuart Bowyer (Vice President) as Acting President.

The Commission has proposed changing its name to “Bioastronomy: Search for Extrasolar Planets and Extraterrestial Life” to reflect our long-standing involvement in the search for extrasolar planets. The name change is pending approval of the IAU.