14. Comets

Unexpected appearance

Fig. ..  Fig. .. 

Every few years, on average, an unusually bright comet will blaze forth in the night sky becoming visible to the unaided eye and sporting a graceful tail resembling long hair blowing in the wind. In fact, the word comet is derived from the Greek name aster kometes, meaning “long-haired star”. But a comet is not anything like a star. Their dramatic display emanates from a relatively small, blackened chunk of ice and dust, comparable to large city in size.

Unlike the planets, the comets can appear almost anywhere in the sky, remain visible for a few weeks or months, and then vanish into the darkness. Astronomers call this period of visibility an “apparition”. During its apparition, a comet changes its shape, often from night to night.

Fig. .. 

Comets used to frighten people, filling ancient minds with awe and terror. They upset the natural order, by moving into the otherwise placid firmament. By their unexpected arrivals, these celestial intruders seemed to upset the natural order of the otherwise placid firmament, and to presage changes in the order of things on Earth, such as the death of rulers, wars and other disasters.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University