Nobody seems completely sure how that happened-the usual guess is something to do with the high concentration of heavy elements, particularly radioactives. That resulted in a hard, heavy core that formed early, and a mantle that stayed hotter and more liquid than usual, and somehow that let the core get pulled to one side by Eta Cass A's gravity-or possibly by Eta Cass B, during a pass.
Or, just possibly, it got thrown off-center because a comet or something hit the planet-the system has plenty of comets.
However it happened, it happened. Epimetheus had a normal rotation when it first coalesced, but the offset core slowed it down in a hurry. It stopped spinning evenly, slowing down each time the core passed the sunside, until finally it was hardly moving at all.
But it takes time for something the size of a planet to grind to a halt, even with its own core acting as a giant brake. It takes a lot of time. It doesn't just stop in a few hours, or a few years, or even a few centuries. And Epimetheus is very young.
It's almost done rotating; the experts all agree that it's on its very last spin before it stops with the core permanently offset toward the dayside. But that last turn is a slow one. It's been going on for centuries, and it'll still be going on a thousand years from now. A thousand years is nothing on a planetary scale.
By then it'll be really slow, though, just a few centimeters a day.
Meanwhile, Nightside City is going to swing out onto the dayside, and it's not going to swing back. It'll move out into the full sunlight, where the ultraviolet eventually kills all unprotected, unmodified terrestrial life; it'll swing on, moving slower and slower, and eventually, thousands of years from now, it will stop.
And the city will stop well beyond the sunrise terminator, out there in the sun, far enough out that the crater wall's shadow won't mean a thing. It will never get anywhere near reaching the sunset terminator; it won't even reach mid-morning. When the rotation stops the planet will be tidelocked, and the city will be on the dayside to stay.
They figured this out, way back when, and they shrugged and forgot it; after all, it was a long way off, a hundred years away. Nightside City grew and flourished and everybody had a good time.
But those hundred years slipped away, like data scrolling across a screen, and the dawn got closer and closer, and before we knew it we were all just waiting for the sun.
And everyone in the city knew this; we had grown up knowing it. It had all been checked and rechecked a hundred years ago. We all knew the rate of movement, the distances to go, everything. When I was eight my friends and I worked out the exact dates that the sun would shine in our respective bedroom windows-but we were eight, and it was just a game.
Looking up at that blue sky and red horizon it wasn't a game anymore. It was death, disaster, the end of the world, and there was nothing I could do that would change it.
The end of the world, I said, but no, that wasn't what it was, not really. The nightside would still be habitable; most of the mines that were being worked could still be worked. People could live on the day side in suits or domes or underground. It wasn't the end of the world, not even necessarily the end of the city; it was just the end of the night.
That was the other ancient song I remembered: "End of the Night."
All I ever knew was the night. I had never lived anywhere but Nightside City, never wanted to, and Nightside City had never known anything but night.
The city's whole economy lived on the night; if anything did survive in the crater after the sun rose it would need an entirely new reason for its existence. It was the night that made unshielded life there possible. It was the night that gave the tourists something worth visiting. Without the nightlife, the miners would have no reason to come to the city instead of launching cargo on-site.
But the dawn was coming, coming one hundred and thirty-eight centimeters closer every day-every twenty-four hours, I should say. We had always used standard Earth time, since the Epimethean day lasted forever. And real daylight was coming. That scared the hell out of me.
My cab was coming, too, settling to the curb in front of me, dropping down from a flashing swarm of advertisers and spy-eyes and messengers. Above them, like another layer of floaters, a sudden, silent spatter of meteors drew a golden spray across the sky-there's still a lot of debris in the Eta Cass system.
I looked at the red in the sky and I felt that warm wind and I shivered; then, because I had business, I stepped into the cab.
The cab's interior music was sweet and slow, I noticed as I settled onto the seat. I liked it.
"Where to?" the cab asked.
"Third and Kai," I told it. "And there's no hurry, so keep it smooth."
"Got it," it said. It lifted and cruised toward the Trap, smooth as perfect software.
An advertiser came up to the window beside me, purring seductively about the pleasures of a night at the Excelsis and trying to focus a holo in front of me. Its chrome casing glittered in moving bands of red and white sparks as it caught the lights in passing.
"Lose it," I told the cab. "I hate advertising."
The cab didn't say anything, and I didn't feel anything but a slight jerk, but suddenly the advertiser was gone. It was a slick little move, and I got curious and looked at the cab's identification.
It was a Hyundai, of course-I hadn't seen any other make in years-but the model number was one I'd never seen before, a whole new series, and I found myself wondering what it was doing in the city. Who was buying new cabs?
I hated myself for asking that; I wanted to believe that somebody had enough pride in the city to buy new cabs for the last few years. I wanted to believe it-but I couldn't. Nightside City was going to hell, and we all knew it.
But maybe somebody knew something I didn't.
All my life I'd been hearing schemes to save the city- put up a dome, go underground, cut the crater loose and haul it back to the other side of the planet. They all had one thing in common: no one was willing to finance them. Nightside City had always made money, but not that much money.
Besides, everybody knew it was the weird ambience of the city that drew the tourists, the wind and the darkness and the night sky with its meteors and a comet every year or two, and Eta Cass B lighting everything dull red. It was the presence of a breathable atmosphere on a planet that was mostly bare rock, still so young the ground almost glowed in spots. Put that underground, or under a dome, and what's to see? And on the dayside there is no darkness; you can't even see the stars, any of them at all.
As for the miners, they weren't about to come out into the daylight for anything. If they had to go to a domed or buried city for their sprees, they'd build their own, safe on the nightside.
Now, cutting the entire crater loose and hauling it back -that might work, but think of the cost! Not to mention the legal complications, or that the whole city would probably have to be evacuated while the job was done, or the difficulties of figuring out where to put it, or that in cutting under the crater you'd be awfully close to going right through the crust and opening the largest damn volcano Epimetheus ever saw, which might not be good for the planet's long-term stability. Epimetheus is delicate. The impact that made the city's crater in the first place didn't punch through the crust into raw magma, but the experts say it came close-very close.
All the same, the scheme got some attention now and then, but the conclusion was always the same.
Nightside City wasn't worth it. The cost would be much higher than any possible profits.
If the city wasn't worth saving, it couldn't be worth much of an investment. Everything in Nightside City had to be considered strictly short-term.
So who was buying new cabs and bringing them in from off-planet?