The ancients regarded comets as harbingers of disaster.
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history
In imperial Chinese astrology, sudden new lights in the sky - 彗星 (comets) and 客星 ("guest stars," what we now call supernovae) - were read as 灾星, signs of impending calamity. Court astronomers were required to report sightings immediately.
usage
Modern figurative use parallels English "jinx": a person, team or object whose presence seems to bring bad luck.