other 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."

"We'll start making and recording daily observations," Lake said.
"Maybe the tilt will start back the other way before it's too late."

* * * * *

Fall seemed to come a little later that year. Craig went to the south as
soon as the weather permitted but there were no minerals there; only the
metal-barren hills dwindling in size until they became a prairie that
sloped down and down toward the southern lowlands where all the
creatures of