One evening early in September he was riding back toward London from the north. He'd been up in Woolsthorpe helping Isaac run the numbers on his planetary orbit theory, but with inconclusive results, because they did not know exactly how far from the center of the Earth they were when they stood on the ground and weighed things. He had stopped in at that plague-ridden town of Cambridge to fetch a new book that claimed to specify the crucial figure: how big around was the Earth? and now he was going down to visit his father, who'd sent him an alarming letter, claiming that he had just calculated a different crucial figure: the exact date (early in September, as it happened) that the world would end.