From November 1680 through March 1681, the Great Comet of 1680 streaked across the night sky for all to see, eliciting widespread fascination and contributing to important scientific insights, including Isaac Newton’s development of the law of gravity. Increase Mather, a prominent Puritan minister and future president of Harvard College, preached several sermons about the comet and its religious implications. Like many in his day, Mather viewed comets as signs—often of impending judgment—from God. He soon released Kometographia, which dealt not only with the nature of comets but also presented a voluminous list of comets throughout history and the calamities that followed. In this copy, a prior reader completed some arithmetic inside the front cover and wrote, “so many Christians slain by Diocletian about 303,” a reference to the persecution of Christians under the emperor Diocletian, which, according to Mather, was presaged by a blazing star across the sky.