In his 1705 publication, Synopsis of the Astronomy of Comets, Halley calculated the effect that Jupiter and Saturn's gravitational fields would have on the path of comets. As such, it was Edmond Halley – Newton's friend and editor – who showed how Newton's theories on motion and gravity could be applied to comets. Unfortunately, he was unable to develop a coherent model for explaining this at the time. In 1687, in his Philosophiæ Naturalis Principia Mathematica, Isaac Newton theorized that comets could travel in an orbit of some sort. However, for another century, astronomers would continue to believe that comets traveled in a straight line through the Solar System rather than orbiting the Sun. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. Until the Renaissance, astronomers' believed that comets – consistent with Aristotle's views – were merely disturbances in the Earth's atmosphere. It was not until 1705 that English astronomer Edmond Halley, who used Newton's Three Laws of Motion to determine that it was periodic. However, these records did not recognize that the comet was the same object reappearing over time. Halley's Comet has been observed and recorded by astronomers since at least 240 BCE, with clear references to the comet being made by Chinese, Babylonian, and medieval European chroniclers.
And its visits have become entirely predictable! But in truth, its behavior is no different than any short-term visitor that swings by from time to time. It's appearance in the skies above Earth has been noted since ancient times, and was associated with both bad and good omens by many cultures.
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