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Data mortii: 
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16 Jan 1801 
Snogebaek, Denmark 
23 May 1885 
Dorpat, Russia (now Tartu, Estonia) 
Thomas Clausen was taught by the local priest for whom Thomas looked after cattle. He taught Thomas Latin, Greek, mathematics and astronomy. Thomas studied several languages on his own, in particular French, English and Italian. Clausen became an assistant at Altona Observatory in 1824. However he had an argument with the director and left to succeed Fraunhofer at the Optical Institute at Munich. He did not carry out any duties in this post and was left on his own to undertake research into mathematics and astronomy. Clausen's work was recognised by many of the top scientists of the day including Olbers, Gauss , Bessel , Hansen, Crelle , von Humboldt and Arago . However he suffered from mental illness and left Munich, returning to Altona. There he spent two years on his own doing some of the best science of his life. During this time he was engaged in an argument with Jacobi . In 1842 he was appointed to the observatory in Dorpat, two years later receiving a doctorate for work carried out under Bessel 's supervision. In 1866 he was appointed director of the Dorpat Observatory, a post he held until he retired in 1872. Gauss was impressed with Clausen, describing him as a man of outstanding talents.
He won the prize of the Copenhagen Academy for his work on determining the orbit of the comet of 1770. Bessel described this work in the following terms: What a magnificent, or rather, masterful work! It is an achievement of our time which our descendants will not fail to credit him with.
Clausen wrote over 150 papers on pure mathematics, applied mathematics, astronomy and geophysics. Among his work in pure mathematics, he factored the 6^{th} Fermat number 2^{n} + 1 where n = 2^{6} in 1854 showing it was not prime . The first to show that not all the Fermat numbers were prime was Euler in 1732 when he showed that 2^{n} + 1 where n = 2^{5} was not prime. Clausen also gave a new method of factorising numbers. KR Biermann, in , writes: He possessed an enormous facility for calculation, a critical eye, and perseverance and inventiveness in his methodology.
Source:School of Mathematics and Statistics University of St Andrews, Scotland
