Thomas Clausen (16 January 1801 Snogback, Northern Jutland – 23 May 1885 Tartu) Director in 1865-1872

Th. Clausen was born to the family of a small Danish farmer. When he was twelve, he took care of the cattle of the local pastor G. Holst who began to teach him languages, mathematics, astronomy and history. The next teacher of the talented young man was the director of the Altona Observatory H. Chr. Schumacher who took Clausen to work as an assistant. While working with Schumacher Clausen made astronomical calculations and he could publish two of his research papers. During this process he got acquainted with the famous mathematician C. F. Gauss.

In the years 1823-34 Clausen worked in Munich at the J. Utzschneider’s optical institute where he primarily dealt with solving the problems of pure mathematics and published some articles. Soon Clausen fell ill and returned to Altona. In Tartu Mädler needed an observer and Schumacher recommended to him Clausen who arrived in Tartu in 1842 and he remained there until the end of his life. In the years 1865-72 Clausen was the director of the Tartu observatory and the professor of astronomy at Tartu University.

Clausen’s work in the field of astronomy is first of all theoretical. His calculations dealt with determining the geographical longitudes according to the movement of the Moon (1824) and the orbits of comets (mainly in 1842). Clausen has calculated the orbits of 14 comets, presented the concept of the system of comets and created the accurate methods for the use of instruments and processing observation data. As a theoretician, he explained a rare phenomenon observed in Tartu on 5 June 1849 – the halo with the triple sun as the refraction and reflection of light on ice crystals. The major part of Clausen’s scientific heritage is connected with mathematics. Clausen found all the five Hippocrates lunes (moon-shaped areas between the circular arcs), which could be expressed exactly as a quadrature, presented the theorem of the Bernoulli numbers and calculated 250 digits of π.

Clausen, who was without academic education, was elected the honorary doctor of Königsberg University (1844), the honorary member of the Royal Astronomical Society of England (1848), the corresponding member of the Academy of Sciences of St Petersburg (1856), the corresponding member of the Scientific Society of Göttingen (1856), the honorary member of St. Petersburg University (1879). He was awarded with the memorial medal of C. F. Gauss (1856).