Observational history of comets

Comets have been observed by humanity for thousands of years, but it is only in the past few centuries that they have been studied as astronomical phenomena.

Until the first century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. One very famous recording of a comet is the appearance of Halley's Comet as a terrifying omen on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.

Ancient Chinese records of comet apparitions have been particularly useful to modern astronomers. They are accurate, extensive, and consistent over three millennia. The past orbits of many comets have been calculated entirely from these records and most notably they were used in connection with Halley's comet.

In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.

A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky. However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.

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