František Nušl (Czech pronunciation:[ˈfrantɪʃɛkˈnuʃl̩]; 3 December 1867, inJindřichův Hradec – 17 September 1951, inPrague) was aCzechastronomer andmathematician.
After high school in Jindřichův Hradec, he studied physics and astronomy in Prague, where he met among others the future president of Czechoslovakia,T. G. Masaryk, the linguistJan Gebauer and the historianJosef Goll, with whom he collaborated onOtto's encyclopedia. After 1893 he taught maths on various high schools and met with J. J. Frič, a manufacturer of precision instruments. In 1898 Frič bought a large piece of land in Ondřejov, 50 km south-east from Prague, where they established an astronomical observatory. Since 1904 Nušl was assistant professor of astronomy at theCharles University of Prague and after 1908 professor of mathematics at theCzech Technical University in Prague (ČVUT). In 1917, Nušl was among the founders of theCzech Astronomical Society, whose president he was to become later. In 1928 Frič dedicated the well-equippedOndřejov Observatory to the Charles university and to the Czechoslovak state and Nušl became its first head. He was a member and temporarily vice-chairman of theInternational Astronomical Union and participated in establishing the popularŠtefánik's Observatory on thePetřín hill in Prague. Nušl was a gifted musician, he played viola and was member of a Prague chorus. He was married and had three children.
Nušl was a very gifted experimenter, particularly interested ingeometrical optics and ingeodetical instruments. His circumzenithal, jointly developed with Frič, was an ingenious portable instrument used to precisely determine geodetical coordinates. The method was based on the properties ofmercury horizon and allowed to establish local coordinates with an error about 10 m, surpassed only much later by satellite navigation. To reduce the error caused by the observer, Nušl developed an "impersonal micrometer": instead of pushing a button in the moment of the passage of a star, the observer set up the instrument to follow exactly the movement of the star and the moment of the passage has been signalled automatically.
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