An urban legend of science history has been that Pope Calixtus III excommunicated Halley’s Comet to counter its baleful influence on the outcome of the siege of Belgrade (July 4–22, 1456), which was encircled by the forces of Ottoman Sultan Mehmed II. In actuality, Halley’s Comet was prominent in Rome’s skies in June. On June 29 the pope ordered in a bull that loud bells should be rung three times in the afternoon and that on the first Sunday of each month there should be general processions and sermons about the cruelty of the Turks. On July 4 the Turks began their siege, and the comet disappeared from view on July 8. The Turks were defeated on July 22, and the news reached Rome on August 6. Platina (1421–81), in his Lives of the Popes, said that Calixtus ordered the processions “to avert the wrath of God” presaged by the comet. However, the bull did not mention the comet, and no one writing in 1456 considered the bull and the comet connected. Platina’s error was compounded by later scientists, such as the 18th-century astronomer Pierre-Simon, marquis de Laplace, who said that Calixtus “exorcised the comet,” and François Arago, who first mentioned excommunication, in 1832.