It was even the same time of day; the blue sun rising in the east behind him and the yellow sun bright in his face as it touched the western horizon before him. He remembered how the yellow sun had been like the front sight of a rifle, set in the deepest V notch of the western hills-- But now, exactly a year later, it was not in the V notch. It was on the north side of the notch. He looked to the east, at the blue sun. It seemed to him that it, too, was farther north than it had been although with it he had no landmark to check by. But there was no doubt about the yellow sun: it was going south, as it should at that time of year, but it was lagging behind schedule. The only explanation Lake could think of was one that would mean still another threat to their survival; perhaps greater than all the others combined. The yellow sun dropped completely behind the north slope of the V notch and he went on to the caves. He found Craig and Anders, the only two who might know anything about Ragnarok's axial tilts, and told them what he had seen. "I made the calendar from the data John gave me," Anders said. "The Dunbar men made observations and computed the length of Ragnarok's year--I don't think they would have made any mistakes." "If they didn't," Lake said, "we're in for something." Craig was watching him, closely, thoughtfully. "Like the Ice Ages of Earth?" he asked. Lake nodded and Anders said, "I don't understand." "Each year the north pole tilts toward the sun to give us summer and away from it to give us winter," Lake said. "Which, of course, you know. But there can be still another kind of axial tilt. On Earth it occurs at intervals of thousands of years. The tilting that produces the summers and winters goes on as usual but as the centuries go by the summer tilt toward the sun grows less, the winter tilt away from it greater. The north pole leans farther and farther from the sun and ice sheets come down out of the north--an Ice Age. Then the north pole's progression away from the sun stops and the ice sheets recede as it tilts back toward the sun." "I see," Anders said. "And if the same thing is happening here, we're going away from an ice age but at a rate thousands of times faster than on Earth." "I don't know whether it's Ragnarok's tilt, alone, or if the orbits of the suns around each other add effects of their own over a period of years," Lake said. "The Dunbar Expedition wasn't here long enough to check up on anything like that." "It seemed to me it was hotter this summer than last," Craig said. "Maybe only my imagination--but it won't be imagination in a few years if the tilt toward the sun continues." "The time would come when we'd have to leave here," Lake said. "We'd have to go north up the plateau each spring. There's no timber there--nothing but grass and wind and thin air. We'd have to migrate south each fall."