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An Introduction to the History of Comets


The Bayeux tapestry records the bad omen of Halley's Comet in 1066 C.E.
Comets have always captured the imaginations of humans. Probably because of their unexpected appearances, huge apparent sizes on the sky, mysterious and rapid motions about the sky, and, finally, their equally mysterious disappearances, civilizations have been awed and frequently terrified by comets. Somewhat surprisingly, in the last few decades, comets have taken on new meaning and increased significance in modern astronomy as well.

In European history, comets were most frequently viewed as portents of bad times. The most famous portent may have been the appearance of Halley’s comet shortly before the battle of Hastings in 1066. While it was a bad portent for Harold of England, who was killed by an arrow in the eye late in the battle, it was a good portent for William the Conqueror, who, through this battle, mounted the last successful invasion of England.

They were a stumbling block for the great Italian astronomer Galileo, who argued that comets were phenomena of the upper atmosphere. Then he used every trick in the book to discredit Father Horatio Grassi, who correctly claimed that they were bodies in the solar system. Galileo wasn’t often wrong but he sure did badly when it came to explaining comets. Tycho had also decided, from observations, that the orbits were more distant than the moon and moved in curved orbits. Oddly, his most famous student, Kepler, who discovered the true nature of the paths of the planets, broke with his teacher in declaring that comets must have straight paths. By 1665, the orbital issue started to be resolved when Giovanni Borelli suggested that comet orbits had a parabolic shape. Most comets have either highly elliptical paths or paths indistinguishable from parabolas.

Comets were first put on a rational basis by Edmund Halley, the secretary of the Royal Society in 1690s, when he noticed that the orbital characteristics of a number of great comets of 1456, 1607, 1682, and 1705 were very similar. He decided that they were reappearances of the same comet and predicted its return in 1758. Unfortunately, he died in 1742, before his prediction was proved right by the appearance of the comet now known as Halley’s Comet was recovered on Christmas night 1758. Perhaps more significantly for physics, Halley felt that a good theory of gravitation should underlie the similarities in these orbital characteristics so he approached his friend Isaac Newton for such a theory. The time was right for such a theory and many scientists were hard at work at trying to unravel the mystery of gravity. Newton, arrogant as always, replied to Halley that he had solved this problem sometime before but not told anyone. Of course, this retort set Halley on a path of constant nagging to get Newton to publish these results. Newton certainly had worked on the problem of gravity during the plague year of 1665 but probably had not resolved all the details. Since he had been embarrassed in the past by publishing results before they were ironclad, he put Halley off, claiming he was looking for the results. In fact, he spent the next 20 years working out all the details and proofs describing gravity. He even had to invent calculus to do some of the mathematical work (although all his published proofs were completed using classical geometry).

Even in modern times, some elements of society respond badly to the appearence of bright comets. It is not uncommon for the appearence of a bright comet to trigger hysteria or mass suicides in fringe cults. Luckily, most folks enjoy them for the wonderful celestial show they put on.