|known_for = ComputedCalculated the orbit of [[Lexell's Comet]]<br/>
ComputedCalculated the orbit of [[Uranus]]
|influences = [[Leonhard Euler]]
}}
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Lexell aided Euler in finishing his [[Lunar theory]], and was credited as a co-author in Euler's 1772 "Theoria motuum Lunae".<ref>{{cite book|date=1772|author=J. A. Euler|author2=W. L. Krafft|author3=J. A. Lexell|title=Theoria motuum lunae, nova methodo pertractata una cum tabulis astronomicis, und ad quodvis tempus loca lunae expedite computari possunt, incredibili studio atque indefesso labore trium Academicorum: Johannis Alberti Euler, Wolffgangi Ludovici Kraft, Johannis Andreae Lexel. Opus dirigente Leonardo Eulero|pages=775}}</ref>
After that, Lexell spent most of his effort on [[comet]] [[astronomy]] (though his first paper on computingcalculating the [[orbit]] of a comet is dated 1770). In the next ten years he computedcalculated the orbits of all the newly discovered comets, among them the comet which [[Charles Messier]] discovered in 1770. Lexell computedcalculated its orbit, showed that the comet had had a much larger [[perihelion]] before the encounter with [[Jupiter]] in 1767 and predicted that after encountering [[Jupiter]] again in 1779 it would be altogether expelled from the [[Solar system#Inner Solar System|inner Solar System]]. This comet was later named [[Lexell's Comet]].
Lexell also was the first to computecalculate the orbit of [[Uranus]] and to actually prove that it was a [[planet]] rather than a [[comet]].<ref>{{cite journal|date=1783|author=A. J. Lexell|title=Recherches sur la nouvelle planete, decouverte par M. Herschel & nominee Georgium Sidus|journal=Acta Academiae Scientiarum Imperialis Petropolitanae|issue=1|pages=303–329}}</ref> He made preliminary calculations while travelling in [[Europe]] in 1781 based on [[William Herschel|Hershel's]] and [[Nevil Maskelyne|Maskelyne's]] observations. Having returned to [[Russia]], he computedestimates the orbit more precisely based on new observations, but due to the long [[orbital period]] it was still not enough data to prove that the [[orbit]] was not [[Parabolic trajectory|parabolic]]. Lexell then found the record of a star observed in 1759 by [[Christian Mayer (astronomer)|Christian Mayer]] in [[Pisces (constellation)|Pisces]] that was neither in the [[Flamsteed]] catalogues nor in the sky by the time [[Johann Elert Bode|Bode]] sought it. Lexell presumed that it was an earlier sighting of the same [[astronomical object]] and using this data he calculated the exact orbit, which proved to be elliptical, and proved that the new object was actually a [[planet]]. In addition to calculating the parameters of the orbit Lexell also estimated the planet's size more precisely than his contemporaries using [[Mars]] that was in the vicinity of the new planet at that time. Lexell also noticed that the orbit of [[Uranus]] was being [[Perturbation (astronomy)|perturbed]]. He then stated that, based on his data on various [[comet]]s, the size of the [[Solar system]] can be 100 [[Astronomical unit|AU]] or even more, and that it could be other [[planet]]s there that [[Perturbation (astronomy)|perturb]] the [[orbit]] of [[Uranus]] (although the position of the eventual [[Neptune]] was not calculated until much later by [[Urbain Le Verrier]]).
