ABSTRACT
Before the seventeenth century, comets were thought to be random, unrelated occurrences. Astronomers who studied the motions of comets thought they moved in a straight line through space, and there was no reason to suspect a strange object appearing in the sky was the same body that had last shown up dozens of years earlier. But by 1687, when Isaac Newton (1643-1727) published Principia Mathematica, astronomers realized comets, like planets, were celestial bodies that orbited the Sun. Newton, using his new theory of gravitational attraction, showed that comets followed highly elliptical orbits, so they were only visible from Earth for short periods when they were close to the Sun.
