Anders Johan Lexell - Definition and Overview

Anders Johan Lexell (December 24 1740 – December 11 1784 (Julian calendar: November 30)) was a Swedish-Russian astronomer and mathematician. In Russian he is known as Andrei Ivanovich Leksel (Андрей Иванович Лексель). His name is also given as Anders Johann Lexell or even Johann Anders Lexell.

He emigrated to Russia in 1768. He studied the motions of comets. He computed the orbit of comet D/1770 L1 (Lexell), and it is named after him although it was discovered by Charles Messier. This comet made the closest known approach to Earth by any comet in history (although asteroids have come closer), making it the first known near-Earth object; the exact distance is not known but has been estimated to have been within 3 million km. The comet was later ejected from the solar system by a close encounter with Jupiter.

He was also the first to compute the orbit of Uranus soon after its discovery and realized from its orbit that it was a planet rather than a comet. He also found that Uranus was being perturbed and deduced the existence of another planet (the eventual Neptune), although the position of Neptune was not calculated until much later by Urbain Le Verrier.

The asteroid 2004 Lexell is named in his honour.

External links

• Biography (http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Lexell.html)
• Comet Lexell (http://cometography.com/pcomets/1770l1.html)