第10章 The Sidereal Defect

Time at an observatory is a strange composite of two clocks that are not the same. One is the clock a person wears, which keeps the ordinary day, twenty-four hours of sunlight and dark. The other is the clock the sky keeps, which is shorter by nearly four minutes, because the Earth turns against the stars a little faster than it turns against the sun. Every telescope on Earth is built to follow the second clock, the star clock, and the difference between the two is one of the first things a survey must be able to trust. Viveka had trusted it her whole career. She had never once had to check it, because the star clock is not an instrument; it is the sky, and the sky is the thing the instruments measure. On the fifteenth night she checked it, and it was wrong. It began as a curiosity about the object. The object did not turn with the sky. So the object, if it was real, was keeping a different time from the sky, and the difference, if the object were real, should be measurable. She sat down to measure it as an exercise, a thing to do with a number that had no other use, and the exercise turned over in her hands and became the whole night. She pointed the mirror at a reference star near the object, one of the survey's brightest calibrators, and tracked it for an hour, and then, having tracked it, she turned the mirror off the track and left it fixed to the ground, and let the star drift across the field the way the sky carries everything across everything. A star at a fixed telescope drifts at the rate of the star clock, fifteen degrees of sky per hour, a quarter of a degree per minute, and the survey had measured that drift so many times that its number was a constant in the reduction code. She measured it again that night, the old way, with the mirror parked and the field open, and she did it four times, at four different points in the sky, and each time she got the same thing. The sky was turning a little too slowly. The defect was small. It was one part in about four thousand, which over an hour was a fraction of a second and over a night was a few seconds and over a year would have been minutes, which is to say that over a year the station's records and the sky's own motion would have drifted apart by an amount no one would notice unless they were measuring the sky for its own sake, which was the only thing this station did. She went back through the year's data and looked for the defect there, and it was there, in every field, patiently, the same fractional amount across every night of the season: the sky turning slow, the record keeping ordinary time, the two of them separating like ships. Except the object. She measured the object's own bearing across four hours and found that it did not share the sky's defect. It held its place above the southern rim, fixed, unaffected, and if she used the object's own rate as the reference rather than the field stars, the sky's slow turning became a thing that was not slow at all, but rather a thing that had been measured against the wrong zero, and the object was the only correct zero on the mountain. That was the moment she understood it could go either way, and that the direction mattered, and that she had no instrument on the station that could tell her which. Perhaps the sky was turning slowly, and the object was the one honest thing in the field. Or perhaps the sky was turning exactly as it always had, and the object was the thing that was wrong, and everything she called ordinary time was being told to her by an object that was not a star. Or perhaps the instrument, the clock, the mirror, the mountain, the dark, or her own senses had begun to keep a rate that no longer matched the rate of anything outside her skull. She went down the corridor with the measurement in her head, and on the way she passed the galley, where Kolstad was mending a boot at the table, and he looked up and said good afternoon, and she said good afternoon, and for a moment the ordinariness of it was so complete that she nearly turned back and put the whole thing away. That was the thing about a place that has begun to change: it does not look changed. It looks like a kitchen with a man mending a boot in it, and the only evidence against it is a number in your own handwriting, and a number in your own handwriting is the easiest thing in the world to disbelieve when the alternative is the man with the boot. She went to Saskia that afternoon, in the dome, and asked her to check the clock. The atomic clock was Saskia's province, and Saskia was proud of it the way she was proud of the mirror. "It is the best clock on this continent," Saskia said, without looking up from her work. "It is a reference clock. It does not drift. People drift. The clock does not." "I measured a defect in the field stars' rate last night." Saskia stopped working and looked at her for a moment. "You measured a defect in the field stars' rate," she repeated. "Skoglund. Every star in the sky comes out of the same sky. If the sky turned slowly, the survey would have caught it decades ago. If the sky turned slowly only for you, then it is not the sky that is slow. It is you and the sky that are out of step, and the one who is out of step is standing here." She said it without unkindness, because Saskia thought in mirrors and mechanisms and trusted the mechanism first, and Viveka, who had spent twenty years trusting the mechanism first, found that she had nothing with which to answer her. She had learned the two clocks as a student, on a small refractor at a low site, and she remembered the night she had understood them, because her teacher had put a hand on the tube and said that every instrument in the world was a way of asking the sky what time it was, and that the answer was always a comparison, and that a comparison was only as good as the two things held up to each other. She had carried that sentence through every site since. Now she held the field stars against the null of the object, and the two clocks of the observatory against each other, and the sentence had turned over in her hands and become a question she had no way to answer. If the answer is always a comparison, what happens when the thing you compare against is the thing you are trying to measure? That night she sat the watch and did not look at the field stars. She fixed the mirror on the object and let the sky run past it, and she watched the numbers come down from the encoder, and the numbers said the object was holding still while everything turned behind it, and the clock on the wall said the minutes were ordinary, and the two of them disagreed by an amount too small to see and too large to explain, and Viveka sat between them in the dark and understood that from now on, whatever she did, she would be doubting one of them, and that she would never again know which. ---