Excommunicating Comets

Jim at "Quodlibeta" has a post on the widespread story that Pope Callistus III excommunicated Halley's comet. Of course, the story doesn't even make any sense in its own right; Halley's comet is presumably not a regular partaker of Eucharistic communion, and therefore is not even the sort of thing that could be excommunicated. (Take that, comet! You are no longer allowed to participate in the Mass!) But, then, people don't always think stories through. He suggests the probable history of this curious little anecdote. It sounds very similar to the excrement-in-heaven story, which started out as a joke and began to be taken seriously.