Origins
The word 'comet' preserves one of the most vivid metaphors in the history of astronomy. It descends from Greek 'komḗtēs' (κομήτης), meaning 'long-haired,' which is the shortened form of 'astḗr komḗtēs' (ἀστήρ κομήτης) — literally, a 'long-haired star.' The Greek root is 'kómē' (κόμη), meaning 'hair of the head,' and the image is unmistakable: to ancient observers, a comet's luminous tail looked like streaming hair trailing behind a bright point of light as it moved across the night sky.
Aristotle discussed comets at length in his 'Meteorologica' (c. 340 BCE), but he classified them not as celestial bodies but as atmospheric phenomena — exhalations of dry, warm gas that ignited in the upper atmosphere. This classification placed comets in the same category as meteors and the aurora, and it carried enormous authority. For nearly two thousand years, the Aristotelian view that comets were sublunary (below the moon) dominated European thinking. The Danish astronomer Tycho Brahe finally overturned this in 1577, when his careful parallax measurements showed that a great comet was well beyond the moon's orbit.
The word passed from Greek into Latin as 'comēta' or 'comētēs,' and from Latin into Old French as 'comete,' entering English around 1200. In medieval English, the word was sometimes spelled 'comete' or 'comette' before settling into its modern form.
Greek Origins
The same Greek root 'kómē' (hair) gives us the modern astronomical term 'coma,' which refers to the diffuse, luminous envelope of gas and dust surrounding a comet's solid nucleus when it approaches the sun. The coma is, quite literally, the comet's 'hair' — the very feature that gave the object its name in the first place. The word 'comate,' meaning 'hairy' or 'tufted,' shares the same origin and was once used in botanical descriptions.
Comets have held a prominent place in the mythology and superstition of nearly every civilization. In the Western tradition, they were long regarded as omens — usually ill ones. The Bayeux Tapestry (c. 1070) famously depicts the appearance of Halley's Comet in 1066, shown as a harbinger of the Norman Conquest. In Chinese astronomical records, comets were called 'broom stars' (掃帚星, sàozhou xīng), an independent metaphor that captures the same visual impression as the Greek 'long-haired star' — both cultures saw something sweeping or trailing in the sky, but one imagined hair and the other a broom.
Edmund Halley's prediction in 1705 that the comet of 1682 would return around 1758 — and its triumphant reappearance on Christmas Day 1758 — was a landmark in the history of science. It demonstrated that comets followed predictable orbits governed by Newtonian gravity, transforming them from mysterious portents into ordinary members of the solar system. Halley's Comet, the most famous comet in history, carries the name of the man who demystified these 'long-haired stars.'
Modern Legacy
The poetic force of the original Greek metaphor has not been lost on later writers. In Shakespeare's 'Julius Caesar,' Calpurnia warns that 'when beggars die there are no comets seen; the heavens themselves blaze forth the death of princes' — a line that depends on the ancient association of comets with extraordinary events. The etymology reminds us that every scientific term began as someone's attempt to describe the unfamiliar in familiar terms, and the Greeks, confronted with a blazing apparition in the sky, reached for the simplest image available: it looked like a star with long, flowing hair.