The Paris Observatory has 350 years

International audienceThe Paris Observatory is the oldest astronomical observatory that has worked without interruption since its foundation to the present day. The building due to Claude Perrault is still in existence with few modifications, but of course other buildings have been added all along the centuries for housing new instruments and laboratories. In particular, a large dome has been built on the terrace in 1847, with a 38-cm diameter telescope completed in 1857: both are still visible. The main initial purpose of the Observatory was to determine longitudes. This was achieved by Jean-Dominique Cassini using the eclipses of the satellites of Jupiter: a much better map of France was the produced using this method, which unfortunately does not work at sea. Incidentally, the observation of these eclipses led to the discovery in 1676 of the finite velocity of light by Cassini and Rømer. Cassini also discovered the differential rotation of Jupiter and four satellites of Saturn. Then, geodesy was to be the main activity of the Observatory for more than a century, culminating in the famous Cassini map of France completed around 1790. During the first half of the 19th century, under François Arago, the Observatory was at the centre of French physics, which then developed very rapidly. Arago initiated astrophysics in 1810 by showing that the Sun and stars are made of incandescent gas. In 1854, the new director, Urbain Le Verrier, put emphasis on astrometry and celestial mechanics, discovering in particular the anomalous advance of the perihelion of Mercury, which was later to be a proof of General Relativity. In 1858, Leon Foucault built the first modern reflecting telescopes with their silvered glass mirror. Le Verrier created on his side modern meteorology, including some primitive forecasts. The following period was not so bright, due to the enormous project of the Carte du Ciel, which took much of the forces of the Observatory for half a century with little scientific return. In 1926, the astrophysical observatory at Meudon was merged with the Paris one. A strong revival of the Observatory and of all French astronomy took place just after WW2 under the impulse of André Danjon. Radioastronomy was developed with the creation of the Nançay station in 1953, and the Observatory became very active in space research after 1963 thanks mainly to Jean-Louis Steinberg. It is presently one of the biggest astronomical institutes worldwide, with a total scientific, technical and administrative staff of 650, and many students and post-doctoral researchers. Essentially all the aspects of astronomy and astrophysics are covered, including laboratory work, especially on very accurate clocks. However, essentially all the observations are done elsewhere, particularly in international facilities such as IRAM, ESO and with many satellites and space probes