Comets have been observed by humanity for thousands of years, but only in the past few centuries have they been studied as astronomical phenomena. Before modern times, great comets caused worldwide fear, considered bad omens foreboding disaster and turmoil; for example, the 1066 passage of Halley's Comet depicted as heralding the Norman conquest of England. As the science of astronomy developed planetary theories, understanding the nature and composition of comets became a challenging mystery and a large area of study. Halley's Comet, reappearing every 75–76 years, was pivotal to the study of comets and their orbits. Thinkers such as Immanuel Kant in the eighteenth century hypothesized about the physical composition of comets. Today, comets are well understood as "dirty snowballs" in eccentric orbits around the Sun, but they continue to be objects of scientific and popular fascination. In 1994, comet Shoemaker–Levy crashed spectacularly into the atmosphere of Jupiter. In 1997, members of the Heaven's Gate cult committed mass suicide, inspired by the passage of comet Hale-Bopp. Since 1985, a total of 8 comets have been visited by spacecraft.
Early observations and thought
From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. Little is known of what people thought about comets before Aristotle, who observed his eponymous comet, and most of what is known comes secondhand. From cuneiform astronomical tablets, and works by Aristotle, Diodorus Siculus, Seneca, and one attributed to Plutarch but now thought to be Aetius, it is observed that ancient philosophers divided themselves into two main camps. Some believed comets to be astronomical entities; others affirmed their meteorological nature. Meteors and comets were of great importance to the native inhabitants of Mexico. Meteors were alternatively viewed as arrows of stellar gods, as their cigar butts, and even as their excrement. The arrows could hit animals or people and were feared when walking at night. Comets were conceived as smoking stars and as bad omens, e.g., announcing the death of a ruler.
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