Pierre d'Ailly

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Pierre d’Ailly was a French theologian, churchman, and cardinal who held that the stars carried real information about history and that reading them was no offence to Christian faith. Born at Compiègne in 1351, he rose through the University of Paris to become its chancellor, then Bishop of Cambrai, and in 1411 a cardinal; he was among the dominant voices at the Council of Constance, the assembly that ended the Great Schism of the papacy. He died in 1420. By the standards of his office he was an entirely orthodox man — which is part of what makes the rest of his work striking.

For d’Ailly belonged to a learned medieval tradition that took astrology seriously as a science, and he defended it in a long sequence of treatises. The art he practised was not the casting of personal horoscopes so much as what its proponents called the astrology of great events. The doctrine reached him from the Arabic astronomer Abū Maʿshar, by way of Roger Bacon: the slow conjunctions of the outer planets — Saturn and Jupiter above all — governed the rise and fall of religions, dynasties, and ages of the world. D’Ailly set out to reconcile this with sacred history. In works such as the Concordantia astronomie cum theologia he argued that the conjunctions could be correlated with the events of scripture and used, cautiously, to estimate when the present age might end. His own calculations pointed toward a great conjunction near the year 1789. He was careful to insist that the stars incline rather than compel, and that they could not bind the freedom of God or the human will.

That reservation is the crux of his position, and it is where the registers must be kept apart. The Church’s settled teaching condemned the claim that the stars fix human choices; d’Ailly accepted the condemnation and worked within it, holding only that the heavens disclosed God’s general plan for nations and times. Whether that distinction is coherent is a question scholars still debate. What is established is that he made it in good faith, as a senior prelate, and that his synthesis of conjunctionist astrology with Christian eschatology was widely read for more than a century.

His longest afterlife came by an unintended route. His cosmography, the Imago Mundi, gathered the geographical and astronomical learning of his day, and a printed copy of it passed into the hands of Christopher Columbus, who covered its margins with notes. From d’Ailly, Columbus drew arguments that the ocean between Europe and Asia was narrow enough to cross — a conviction that helped launch the voyage of 1492. The cardinal who computed the end of the world thus also supplied, by a misreading of distances, part of the reasoning that opened, at catastrophic cost to the peoples already living in it, what Europe would call a new world.

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