Comets

There is a long association between comets and disastrous events on Earth. Over the ages, they have been variously blamed for floods, droughts and pestilence. As far as the ancients were able to tell, comets appear unpredictably, seemingly from nowhere, and after a few weeks vanish, never to be seen again. They were seen as a challenge to the harmony and predictability that normally reigns in the skies, and were excluded from the heavens on the authority of Aristotle. He argued that the fact that comets appear and disappear without warning, means that they must be due to events close to the Earth, the only part of the universe where he believed change was possible. According to Aristotle, comets were atmospheric phenomena involving friction between layers of the atmosphere furthest from Earth.