From around 1695 Halley made a careful study of the orbits of comets. Newton favoured comets having parabolic orbits, but Halley believed that elliptical orbits might exist.
Mula sa humigit-kumulang na 1,695 Halley na ginawa ng isang maingat na pag-aaral ng orbits ng comets. Newton pabor sa pagkakaroon ng comets paparabola orbits, ngunit Halley sumampalataya na tambilugin orbits maaaring umiiral.