Edmund Halley (1656-1742) a contemporary of Isaac Newton (1642-1727) began addressing himself to the problem of cometary motion.  With Newton's help, he began cataloguing and calculating the paths of comets using Newton's principle of gravitation, and discovered that one particular comet appeared in the same path about every 76 years.  In 1705, he predicted that that comet would reappear in 1758, and it has been known as Halley's Comet since.  It's currently outside the orbit of Neptune.  The comet moves in an elliptical orbit, speeding past the Sun in about 2 years, but spending about 50 years slowly curving around its distant point, outside the orbit of Pluto.  Its furthest distance will be in December 2023.  One can calculate its distance from known formulas; however,  in 2003, the Very Large Telescopes at Paranal, Chile observed the comet.  Astronomers concur that the new telescope technology will be able to track Halley's at any point in its 76 year orbit from now on!