A (very) Brief History of Anders Johan Lexell

Anders Johan Lexell (24 December 1740 – 11 December 1784) was a Finnish-Swedish astronomer, mathematician, and physicist who spent most of his life in Imperial Russia, where he was known as Andrei Ivanovich Leksel.

Lexell made important discoveries in polygonometry and celestial mechanics; the latter led to a comet named in his honor. La Grande Encyclopédie states that he was the prominent mathematician of his time who contributed to spherical trigonometry with new and interesting solutions, which he took as a basis for his research of comet and planet motion. His name was given to a theorem of spherical triangles.

Polygonometry was a significant part of Lexell's work. He used the trigonometric approach using the advance in trigonometry made mainly by Euler and presented a general method of solving simple polygons in two articles "On solving rectilinear polygons".

Lexell also was the first to calculate the orbit of Uranus and to actually prove that it was a planet rather than a comet. He made preliminary calculations while traveling in Europe in 1781 based on Hershel's and Maskelyne's observations. Having returned to Russia, he estimated the orbit more precisely based on new observations.