MUCH HAS BEEN made of the central conceit of Karen Thompson Walker's The Age of Miracles, and for good reason: It's the best sort of science fiction, the kind in which a single impossibility becomes possible, irrevocably changing the world as we know it. Walker's twist, subtle at first and increasingly consequential, is that Earth's rotation has inexplicably begun to slow. Days and nights are lengthening, first by minutes and then hours. Crops suffer. The earth's magnetic field withers. Birds fall from the sky. The government institutes a policy of "clock time"—24-hour days maintained without regard to when the sun rises or sets, meaning that, as days and nights grow longer, entire waking periods are spent in darkness, and entire nights ("white nights") pass in bright, skin-scorching sunlight. It's an ingenious premise, one that encompasses enough of our contemporary anxieties—climate change, food shortages, environmental devastation—to seem familiar, yet is improbable enough to retain a quality of mystery. The slowing Walker envisions isn't quite a metaphor, but it's not quite reality, either—it's a fiction, one that works because of the ways in which it's like life, and the ways that it isn't.

But The Age of Miracles is equally notable for the protagonist Walker has created to witness this world: 11-year-old Julia, the only child of two loving parents, a girl subject to the whims and cruelties of other sixth graders as surely as the planet suffers from the changing of the earth's rotation. Her mother hoards peanut butter and her father works long shifts at the hospital; Julia pines for the attention of a floppy-haired skateboarder, and tries not to wonder why her best friend has abandoned her. With Julia, Walker has pinpointed the exact moment when a girl looks like a child, but is beginning to think like an adult, and she renders the perspective with grace and understanding.

The Age of Miracles is squarely in the Ray Bradbury tradition of science fiction that looks forward and backward at the same time; it posses the golden-hued nostalgia of Dandelion Wine, and the gentle fatalism of "The Last Night of the World," in which a couple goes quietly to bed, knowing they won't wake up. (Walker acknowledges her debt to Bradbury with an explicit reference to his story "All Summer in a Day," which imagines a world in which the sun shines for only two hours every seven years.) Aimee Bender is another obvious influence—not because either Bender or Walker is "precious," a charge too often lobbed unfairly at both, but because Walker, is also a careful writer whose sentences are precise and emotionally loaded.

But The Age of Miracles drifts rather than escalates, as though plot, too, has been stretched beyond recognition by the earth's slowing. This, perhaps, is why Walker's book ended up in the literary fiction category, even though in different hands its premise and protagonist could just as well have earmarked it for the so-hot-right-now genre of apocalyptic YA fiction. Julia is a girl witness to the slow erosion of her species, possessed of the knowledge that all human efforts must fade away. "We passed the ruins of an ancient gas station where one pump remained, rusted red," Walker writes. "Beside it stood a humble sun-bleached structure leaning heavily to one side, without its roof. There was a certain heartbreak in that scene. Someone had built those walls. Someone had once felt some kind of hope for the future of this place. Now you could see right through the cracks in the walls to the sky on the other side." Not with a bang but a whimper.