Besançon Astronomical Observatory

The Besançon Astronomical Observatory is an astronomical observatory owned and operated by the Centre national de la recherche scientifique . It is located in Besançon, France.
In the past, the Besançon Observatory participated in assessing and rating Swiss timepiece movements for accuracy. As marine navigation adopted the usage of mechanical timepieces for navigational aid, the accuracy of such timepieces became more critical. From this need developed an accuracy testing regime involving various astronomical observatories. In Europe, the Neuchatel Observatory, Geneva Observatory, Besançon Observatory and Kew Observatory were examples of prominent observatories that tested timepiece movements for accuracy. The testing process lasted for many days, typically 45 days. Each movement was tested in 5 positions and 2 temperatures, in 10 series of 4 or 5 days each. The tolerances for error were much finer than any other standard, including the ISO 3159 chronometer standard that other testing bodies such as COSC use. Movements that passed the stringent tests were issued a certification from the observatory called a Bulletin de Marche, signed by the Director of the Observatory. The General Bulletin Order stated the testing criteria, and the actual performance of the movement. A movement with a bulletin from an observatory became known as an Observatory Chronometer, and such were issued a chronometer reference number by the Observatory.

Wikipedia