"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 [p. 59] 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."
"Yes ... migrate." Anders's face was old and weary in the harsh reflected light of the blue sun and his hair had turned almost white in the past year. "Only the young ones could ever adapt enough to go up the plateau to its north portion. The rest of us ... but we haven't many years, anyway. Ragnarok is for the young—and if they have to migrate back and forth like animals just to stay alive they will never have time to accomplish anything or be more than stone age nomads."
"I wish we could know how long the Big Summer will be that we're going into," Craig said. "And how long and cold the Big Winter, when Ragnarok tilts away from the sun. It wouldn't change anything—but I'd like to know."