In 1664–1665 he made observations of comets, and argued in favor of their following elliptical or parabolic orbits (in this he was opposed by his rival Johannes Hevelius).
V rokoch 1664 – 1665 uskutočnil niekoľko pozorovaní komét a bol zástancom teórie ich pohybu po elipse alebo po parabole (v tom bol v spore so svojím rivalom Johannesom Heveliusom).