In his Ecclesiastical History of the English, the eighth-century historian Bede repeatedly mentions the controversy between the Irish and the Roman churches over the correct calculation of Easter. As Bede sees it, the culminating moment in this battle comes at the synod of Whitby when both sides present their arguments before king Oswy, who decided in favor of the Roman method. From our distance, the argument may seem rather silly—an argument over how many angels can dance on the head of a pin. (The standard answer to that question, by the way, is "as many as God wants.") Looking more closely, we might notice that beneath the surface debate is really a question of power—who gets to define the rules for the most important Christian celebration. In any event, for the historian interested in establishing a chronology of the Middle Ages, the debate over how to calculate Easter is invaluable. We owe the AD system of counting years to an early Easter calculator, and the Church's continuing concern that Easter be celebrated correctly ensured that various regions all adopted the same count of years—the one we still use today.