How did astronomers in earlier times account for comets they observed?

   Once astronomers finally determined that comets occur in space, beyond the Earth's atmosphere, they tried to ascertain where a comet's journey begins and ends. Johannes Kepler, who observed the comet of 1607, concluded that comets follow straight lines, coming from and disappearing into infinity. Somewhat later, German astronomer Johannes Hevelius suggested that comets followed slightly curved lines. Then in the latter half of the 1600s, Georg Samuel Dorffel suggested that comets fol­low a parabolic course.