Thomas Clausen (16 January 1801, Snogbæk, Sottrup Municipality, Duchy of Schleswig – 23 May 1885, Tartu, Imperial Russia) was a Danish mathematician and astronomer.

In 1820, he became a trainee at the Munich Optical Institute and in 1824, at the Altona Observatory after he showed Heinrich Christian Schumacher his paper on calculating longitude by the occultation of stars by the moon.

[2] He eventually returned to Munich, where he conceived and published his best known works on mathematics.

[3] In 1842, Clausen was hired by the staff of the Tartu Observatory, becoming its director in 1866–1872.

Works by Clausen include studies on the stability of Solar System, comet movement, ABC telegraph code and calculation of 250 decimals of pi (later, only 248 were confirmed to be correct).