Pierre Bretagnon succumbed, on the 17-th of November 2002, to long and painful aftereffects of a serious operation. He was Corresponding member of the Bureau des Longitudes and astronomer at the IMCCE (Institut de Mécanique Céleste et de Calcul des Ephémérides) at the Paris Observatory.
Pierre Bretagnon was the last researcher to join the Bureau des Longitudes before I left Paris. So I could follow his scientific work from the beginning to the end. Let me sketch some highlights of his career.
I have kept the reports written by the examining board reports of his "Thèse de spécialité" (phd) and then of his "thèse d'Etat". He tackled a particularly difficult problem: to represent the planetary motions using exclusively analytical expressions consisting of periodic terms (periods from a few weeks to tens of years) whose coefficients could be functions of time. He was actually on of the first to engage in this problem simultaneously for all the major planets of the Solar system, and the first to have succeeded, even though he had to use slightly different methods for terrestrial and giant planets.
Already, the reports of the jury were laudatory. Let me quote Ray Duncombe: "I regard this (the thesis) to be a highly competent piece of work and of great significance to the programs of the National Ephemeris Offices".
In addition to theoretical difficulties that Pierre solved, he programmed an enormous number of manipulations of analytical expressions. What is now routine was, at that time (early seventies),a major challenge that he took up and completed the task that was his objective, which was inferred by Ray Duncombe: to produce ephemerides. Indeed, under the name VSOP-82, the results of Pierre Bretagnon were used in the Connaissance des Temps, replacing those prepared by Le Verrier and Gaillot, which lasted almost one hundred years!
It is interesting to note that, around 1975, André Berger used Bretagnon's expressions to reintroduce the Milankovitch astronomical theory of climatic change. His theory was also used in the reduction of the data collected by the European astrometric satellite Hipparcos. Lately, he got involved in the preliminary studies in preparation of the successor of Hipparcos: GAIA.
This work, as well as those that followed, showed off his qualities, particularly well adapted to research in Celestial Mechanics: perseverance, extreme tidiness, curiosity. To these, one must add a perfect knowledge of the domain.
But Newton's gravitation theory is insufficient, and one must perform the calculations in the framework of the general theory of relativity. This was another of his objectives that he met and became one of the worldwide specialists of the Relativistic Celestial Mechanics in faint fields. He has extensively collaborated in this domain with Victor Brumberg.
Pierre Bretagnon was also very much interested in other motions of the Earth. With Jean Chapront, he produced expressions for the precession. But he is especially well known for a remarkable theory of rotation of the solid Earth that he published with P. Rocher and J.-L. Simon. He was improving it lately by introducing non-rigidity effects. This work, which provided terms as small as a few millionths of an arcsecond, was very well perceived and is constantly mentioned among specialists of the Earth's rotation.
It is not excessive to state that Pierre Bretagnon was one of the most brilliant scientists of his generation in the difficult field of Celestial Mechanics applied to bodies of the Solar system. In line with the French tradition maintained in the Bureau des Longitudes and now at the INCCE, Pierre Bretagnon was one of the best successors of Laplace, Delaunay, and Le Verrier. All his French and foreign colleagues, who have known him, can confirm that his premature death leaves a great emptiness that will be difficult to fill.
Jean Kovalevsky