Perhaps the most anxious question that has been asked of the astronomer is when the world is to come to an end. It is a question which, of course, he has no power to answer with truth; but it is also one that has often been answered in good faith. It has perhaps been somewhat natural to ask such a question of an astronomer, partly because his science naturally deals with the structure of the universe, which might give some light as to its future, and partly because of his connection with astrology, whose province it was supposed to be to open the destiny of all things. Yet the question has been answered by others than by astronomers, on grounds connected with their faith. In the early ages of the Church, the belief in the rapid approach of the end of the world was universally spread amongst Christians. The Apocalypse of St. John and the Acts of the Apostles seemed to announce its coming before that generation passed away. Afterwards, it was expected at the year 1000; and though these beliefs did not rest in any way on astronomical grounds, yet to that science was recourse had for encouragement or discouragement of the idea. The middle ages, fall of simple faith and superstitious credulity, were filled with fear of this terrible catastrophe. As the year 1000 approached, the warnings became frequent and very pressing. Thus, for example, Bernard of Thuringia, about 960, began to announce publicly that the world was about to end, declaring that he had had a particular revelation of the fact. He took for his text the enigmatical words of the Apocalypse: "At the end of one thousand years, Satan shall be loosed from his prison, and shall seduce the people that are in the four quarters of the earth. The book of life shall be open, and the sea shall give up her dead." He fixed the day when the Annunciation of the Virgin should coincide with Good Friday as the end of all things. This happened in 992, but nothing extraordinary happened. During the tenth century the royal proclamations opened by this characteristic phrase: _Whereas the end of the world is approaching_.... In 1186 the astrologers frightened Europe by announcing a conjunction of all the planets. Rigord, a writer of that period, says in his _Life of Philip Augustus_: "The astrologers of the East, Jews, Saracens, and even Christians, sent letters all over the world, in which they predicted, with perfect assurance, that in the month of September there would be great tempests, earthquakes, mortality among men, seditions and discords, revolutions in kingdoms, and the destruction of all things. But," he adds, "the event very soon belied their predictions." Some years after, in 1198, another alarm of the end of the world was raised, but this time it was not dependent on celestial phenomena. It was said that Antichrist was born in Babylon, and therefore all the human race would be destroyed.