Lexell's comet(leks -ĕlz) A comet that was discovered by Messier in June 1770 but was named after the St. Petersburg mathematician who calculated its orbit. Prior to 1767 Lexell's comet had a period of 11.4 years; a close approach to Jupiter in 1770 changed this to 5.6 years and the next close approach to Jupiter in 1779 changed this period to a calculated 174 years. This last approach perturbed the comet so that it has a perihelion distance of 5.4 AU and it has never been seen again, always being too far away from Earth.
Lexell's Comet[′lek·selz ‚käm·ət]
A small comet that approached to within 2,000,000 miles (3,200,000 kilometers) of earth in 1770; it has not been seen since.