Landfall: Tales From the Flood/Ark Universe by Stephen Baxter

families collected the other Orbs, and longer still until they were united enough to join them into the necklace of the Fourteen … The only way an Orb could have been lost here is if a Founder came this way. ’
    Xaia nodded. ‘And if she or he came this far, they would surely have gone further.’
    Teif, alarmed, stood directly before her and stared into her eyes. ‘Lady, don’t even think about it. The season is already late. If we go on, we will be caught by the winter.’
    Xaia looked at him, and laughed, and closed her fingers around the Orb. ‘Throw that fool back to his family. Break camp. We’re going back to the coast. And in the morning, we go on.’
    Manda howled like a dog. ‘North?’
    ‘North!’

VI

    ‘Earth II is unstable.’
    Thom Robell paced the streets of Orklund, aides at his heels, Proctor Chivian at his side. It was September, close to the autumn equinox, and the weather was pleasant, temperate, though clouds covered the sky, and a light rain made the pavements of Thom’s home city gleam.  
    At this time of year, with both the world’s poles looking away from the sun, the roll of the planet delivered day and night of equal lengths, about fifteen hours each. It was said that this time of year was the closest the climate of Earth II came to emulating that of old Earth itself. But everybody knew this was the last of the good weather. In a month the snow would start to fall, and in just two months the sun would disappear altogether, for eighty long days, and the coldwinter would set in. So, all over the city, people were preparing, bottling food, laying in fuel for fires, strengthening the stone walls of their houses, preparing the cellars dug into the ground where the soil would retain some of the warmth of summer even in the winter’s depths. It was an important time of the year, essential for survival. It was too late even to think about starting the Proctor’s absurd Library project; everybody was too busy for that. Winter was coming.  
    And Xaia was not yet home.
    Thom tried to focus. ‘“Earth II is unstable.” What can that possibly mean, Proctor?’
    The Proctor sternly matched Thom pace for pace. Thom sensed that he wasn’t going to give up today. Perhaps he had given himself a private target of the equinox to convince the Speaker to cooperate. ‘It comes from the work of Jan Stanndish, Speaker. Who has cultivated your son, at my suggestion, in the hope of finding a way to your ear. I’m sorry if that seems cynical – we are desperate, Speaker. I don’t use that word lightly.’
    ‘Because the world is unstable?’
    ‘Yes! That is Stanndish’s conclusion, the outcome of page upon page of mathematics – I will not pretend that I follow it all. Some say Stanndish is the most brilliant scholar we have produced since the Founders’ generations. And what he has been analysing is the motion of Earth II itself.’
    Thom frowned. ‘What is there to understand? Earth II spins like a top on its axis. And it follows a circular orbit around its sun. The planet’s spin axis is tipped over so that it lies in the plane of the orbit. At the solstices one pole or another points directly at the sun, and half the world is light, and half dark -’
    ‘Almost. The orbit is an ellipse – low eccentricity, not quite a circle. And the axis is a few degrees away from the plane of the ecliptic -’
    ‘Into the sea with your nitpicking! How it is unstable, man?’
    ‘If it were alone in this solar system, if there were no other planets, Earth II would be perfectly stable, yes. But it is not alone. You are aware that further from the sun orbit two giant worlds, balls of gas we call Seba and Halivah, off in the dark.’ He glanced at the cloudy sky. ‘They are remote, but massive, and their strong gravity plucks at Earth II. You have a child. Did he ever play with spinning tops? If you poke a top with your finger -’
    ‘It wobbles.’
    ‘Yes. And that, we think, is what is going to happen