It is believed by historians that the aztec ruler did not fight Cortes in 1519, because he saw a comet, which he believed to be the sign of the end of his realm.
Comet Halley was seen as a bad omen: It was believed to have caused the plague of 141 A.D., the conquest of England by William the Emperor, the attack of Genghis Kahn of the Western world, etc. Even in 1909 people got panicked, and anti-comet sickness pills were sold on the streets.
Actually I wonder if that last one would still work on the end-of-the-world rubes.
