For years, this story was repeated as an example of how absurd and superstitious religion is, especially when contrasted with science. Carl Sagan referred to it in his book on comets. But of course, you know where I'm going with this: it didn't really happen. The story appears to have been popularized by Pierre-Simon Laplace at the end of the 18th century; Laplace, in turn, apparently got it from Vitæ Pontificum, a 15th century work, by Bartolomeo Platina. The Catholic Encyclopedia article on Platina dutifully translates the relevant text as follows:
A maned and fiery comet appearing for several days, while scientists were predicting a great plague, dearness of food, or some great disaster, Callistus decreed that supplicatory prayers be held for some days to avert the anger of God, so that, if any calamity threatened mankind, it might be entirely diverted against the Turks, the foes of the Christian name. He likewise ordered that the bells be rung at midday as a signal to all the faithful to move God with assiduous petitions and to assist with their prayers those engaged in constant warfare with the Turks.Now there are a couple of things to note right away. First, there is no mention here of the Pope excommunicating the comet. Second, while the Pope had indeed issued a papal bull calling upon people to pray, and while Halley's Comet did appear in the sky at about the same time, there was simply no perceived link between the two. The bull doesn't even mention the comet. Platina just tied two events together that had no connection.
Laplace took Platina's account and suggested that Callistus sought to exorcize Halley's Comet -- and I can't help but wonder if he intended this as a metaphor. Regardless, subsequent writers took it literally, and replaced "exorcize" with "excommunicate" since all those religious terms mean the same thing anyway. The final paragraph of the afore-mentioned article summarizes this development well.
Of course, no doubt there were people who thought Halley's Comet had something to do with the siege of Belgrade. That's the kernel of truth in this story. For that matter, it may very well be true that people in the 15th century were in general more superstitious than people today tend to be. But we still have astrology. Most newspapers print the horoscope every day.
What interests me is how people who hold themselves up as skeptics were taken in by such a ridiculous story as this. Carl Sagan was, by any account, a brilliant man. Yet he uncritically repeats an urban legend in order to show how other people are gullible. What this shows, I think, is that there are no true skeptics. People are only skeptical of things that they want to be skeptical about.
For example, in his book My Life Without God, William Murray describes how his mother Madalyn Murray O'Hair would tell groups of atheists that religious people were so stupid that nobody realized sex led to pregnancy until the 19th century. This is difficult to write without chuckling, but her throng of skeptics bought it. O'Hair herself attended seances, and believed she could talk with dead people. Murray wrote that, as far as he knew, his mother never tried to reconcile this with her belief that there is no afterlife. The skepticism with which she and her fans approached religion was obviously not consistently applied.
The point of all this is that we should be skeptical of our skepticism. The reason why an intelligent person like Carl Sagan could be taken in by such a silly story as a Pope excommunicating a comet is because it fit with his views on the nature of science, the nature of religion, and the relationship between the two. Madalyn Murray O'Hair and her followers were completely contemptuous of religion and religious believers, so any claim that justified this attitude, no matter how insane, was plausible to them.
I have different biases: I am skeptical of the claim that science and religion are opposed to each other. This makes me prone to accept stories that seem to affirm this bias without showing them the same level of critical analysis that I would show to a story that contradicts it. I have to examine myself to determine, as far as possible, what my biases are, and how they might be influencing my beliefs.
(cross-posted at Quodlibeta)