History of the Bureau des longitudes

For two centuries, the Bureau des longitudes participated to the progress of science. It is the origin of many initiatives and it has since 1961 a research laboratory with an international audience for the realization of its ephemerides. It originally consisted of ten members and five deputies and intended to increase the accuracy of longitude determination at sea. It now has the form of an academy of forty-seven members and correspondents, associated with a modern research laboratory in astronomy, the Institute of Celestial Mechanics and Calculation of Ephemerides, an official organization for the calculation and dissemination of data to the public, which include about 50 people, teachers, researchers, engineers, technicians and students preparing thesis.

In 1995, the Bureau des longitudes celebrated its bicentennial and this was an opportunity to present its past and present activities to the public. In 1998, its calculation department became an autonomous institute with a new statute set by the decree of June 2 1998 adapted to its functioning. The lines below trace the history of the Bureau des longitudes and its Calculation Department, now the Institute of Celestial Mechanics.

Foundation of the Bureau des longitudes

The Bureau des longitudes was established by a law (still in force) of the National Convention of messidor 7 Year III (June 25, 1795) after hearing a report jointly prepared by the Marine Committee, the Finance Committee and the Committee of Public Instruction. This report, read by Abbé Grégoire, very clearly states the practical aims of the operation: to regain control of the seas to the English by improving the determination of longitudes at sea (latitudes are easily determined) which can be done only by astronomical observation and by the use of reliable clocks. (See the text of the law and the report of the Abbé Grégoire).

This new organism, therefore, was, in imitation of the English Board of Longitudes created in 1714, to solve the astronomical problems of the determination of longitude, hence its name. But the duties of the Bureau des longitudes went much further, since he was entrusted with the drafting of the Connaissance des temps, an annual publication created in 1679, and the preparation of a yearbook “suitable for settling those of the Republic”, the Annuaire du Bureau des longitudes, which was to presented each year to the Legislative Body. In general, the Bureau des longitudes was entrusted with the perfection of astronomical tables. It had in its attribution the Observatoire de Paris, that of the école militaire, and all the instruments of astronomy which belong to the nation. The Bureau des longitudes had to make an inventory of observatories and report annually on its work in a public meeting. One of the members of the bureau also had to do an astronomy course every year.

The first members, ten in number, were: Lagrange and Laplace, “geometers”, brilliant representatives of the French School of Celestial Mechanics of the eighteenth century; the famous astronomers Lalande, Delambre, Méchain and Cassini; Bougainville, a famous navigator, who was missing on his journey to India for the observation of the passage of Venus, was replaced and reinstated on his return; Borda, who carried out work related to the fluid mechanics and precursor of Carnot because of his views on thermodynamics; Buache, geographer; Caroché, manufacturer of telescopes. These first members were appointed by law, for them “to appoint to the vacant places in their midst”. The members of the Bureau des longitudes first met in a room in the Petit-Luxembourg, then in the Louvre until 29^th nivôse an XII (January 20^th, 1804), at the Observatory until 1874 and at the palais de l’Institut until today.

Evolution of the Bureau des longitudes

The composition and duties of the Bureau des longitudes have been amended by various decrees. Let us note first of all the creation of its Service of calculations in 1802 composed then of 7 persons working under the orders of a member of the bureau in charge of the ephemerides. The decree of January 30, 1854, profoundly modified its duties: a commission composed of Admiral Baudin, Le Verrier and Biot, respectively president and members of the Bureau des longitudes, and the maréchal Vaillant proposed improving the functioning of the Paris Observatory by appointing a permanent director at its head and thus giving it its independence. The decree endorsed this conclusion, which now separates the two bodies and redefines the tasks of the Bureau des longitudes. In addition to the writing and publication of the Connaissance des temps and the Yearbook Annuaire du Bureau des longitudes, the bureau was “called upon to carry and provoke ideas of progress in all parts of astronomical science and the art of observing” and also “to give its opinion on the questions concerning the observations and the scientific missions entrusted to the navigators in charge of distant expeditions”. Many works were published under the auspices of the Bureau des longitudes: tables of the Sun of Delambre, tables of the Moon of Burckardt and Damoiseau, tables of the planets of Bouvard, tables of the satellites of Jupiter of Laplace, etc.

Since the decree of January 30, 1970, the Bureau des longitudes has thirteen full members, members in extraordinary service and thirty-two correspondents. These members and correspondents are mainly astronomers or geophysicists. Rather, they now operate in the various organizations to which they belong. Since the decree of 2 June 1998, the activities of the Bureau des longitudes have been entrusted to the Institute of Celestial Mechanics and Calculation of Ephemerides (IMCCE).

The Bureau des longitudes today

It is now the Institute of Celestial Mechanics and Calculation of Ephemerides (IMCCE) that publishes the Ephemerides of the Bureau des longitudes with the rigor guaranteed by its research teams.

The Bureau des longitudes and the IMCCE continue to publish the traditional ephemerides: Connaissance des Temps – Éphémérides astronomiques, Yearbook Guide de données astronomiques – Annuaire du Bureau des longitudes and Éphémérides nautiques. In addition, between 1977 and 1988, the volumes entitled “The Earth, the waters, the atmosphere” (2 editions in 1977 and 1984) were published under the name of the Encyclopedie des sciences de l’Univers; “The stars, the Solar system” (2 editions in 1979 and 1986); “The Galaxy, the extragalactic universe” (2 editions in 1980 and 1988); “Physics” (1981). In 1991, the collection of Cahiers des sciences de l’Univers was launched and several notebooks have already been published. The IMCCE also publishes several Supplements to the Connaissance des temps and published in 1997 a book of fundamental astronomy, the Introduction aux éphémérides astronomiques – Supplément explicatif à la Connaissance des temps.

The Institute of Celestial Mechanics also distributes ephemerides via the Internet. It also has an information service for the public.

Finally, the Bureau des longitudes participated in the creation of the GRGS (Research Group on Space Geodesy) with the Paris Observatory, IGN and CNES. This organization is contributing to new major scientific developments in geodesy, gravimetry, globe physics and oceanography thanks to the data collected by the numerous Earth observation satellites and, in recent years, by satellites – in particular the most recent among them the Franco-American satellite Topex-Poseidon. These new and rapidly developing studies are coordinated internationally, notably by the International Astronomical Union (IAU), the International Union of Geodesy and Geophysics (IUGG), the Committee on Space Research (COSPAR), and so on.