François Auguste Claude

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François Auguste Claude (30 December 1858 – 15 July 1938) was a French astronomer who worked at the Bureau des Longitudes, which handles longitude and standard time. In 1899 he designed the first prismatic astrolabe. Born in Strasbourg, he chose French citizenship after Alsace was annexed by Germany in 1871 and served in the army. He worked as a designer for six years before joining the Bureau des Longitudes at the Parc de Montsouris in Paris. Although he had no university degree, he became an assistant calculator in 1884 and later director in 1929. His main achievement was a 60-degree prismatic astrolabe, first published in 1899 and later improved with hydrographer Joseph-Ferdinand-Ludovic Driencourt. It was made by Jobin of Paris and came in three sizes in the 1910s. The instrument included a magnetic compass, a circular scale to measure angles from magnetic north, and a small pool of mercury below to act as a mirror. When a star reached the target angle (60°), its image and its reflection could be aligned in the viewfinder for a precise measurement. Andre Danjon later modified the design in 1938, and it became commonly known as the Danjon astrolabe.