Chapter 4 The Star It Covered

The occultation came four nights later. She had not been looking for it, because the odds of catching an occultation of a background star by a single faint object, on the first attempt, were small enough that she had dismissed the idea as a diagnostic she would set up and never see. So she did the ordinary thing with the ordinary patience of her trade: she put a watch on field seven, added a note to the reduction that flagged any frame in which a reference source lost more than half its light, and went back to her own work. The days passed. The survey took its deep fields on a schedule, and field seven came up every third night, and the object sat in its small corner of the field, and the sky rotated over the mountain, and nothing happened. She reduced, she filed, she slept in the afternoons, she ate with the others in the galley and listened to Kolstad not talk, and she began, in the way of anyone who lives in a small place, to know the others as a set of habits rather than as people: the way Lisbet stacked her cups, the way Saskia hummed while she aligned, the way Emeric's laugh arrived a half second after everyone else's, as though he kept it in a drawer and fetched it out. On the fourth night it happened. The reduction flagged it before she did. A reference source in the field had gone missing for a fraction of one exposure: its light, steady in every frame for thirty-five years, had dropped to nothing and then returned, and the drop lasted forty-one seconds, and it happened exactly when the survey's geometry put RV-417 between the mirror and the missing star. Lisbet stayed at the console with her, and the two of them measured the drop a fourth time, independently, and got the same forty-one seconds, and then they sat in the quiet of the room and let the number stand between them the way a stone stands in a stream. She sat very still and looked at the graph. A star cannot go out. A star's light is enormous and old and practically changeless on the scale of forty seconds. There was one ordinary way for a star's light to be interrupted, and that was for something solid to pass in front of it, and the something had moved across the line of sight, and the something was in the field, and there was only one candidate. An occultation is a ruler. If you catch one, you know the occulting body is nearer to you than the star it hid, because light travels in straight lines and a nearer thing can block a farther one, and a farther thing cannot block a nearer one. The missing star was a background star, part of the great field, its distance a matter of the galaxy's own measure, and RV-417 had passed in front of it. Which meant RV-417 was nearer. Which meant RV-417 had a distance, a finite one, a distance from this mountain to a solid body in the sky. Which meant it should show a parallax. She rebuilt the arithmetic on paper, then on the console, then on paper again, and every route came to the same wall. The object had to be nearer than the background star; the object showed no parallax; and if it was nearer than the background star and had no parallax, then the Earth's own swing across three hundred million kilometers had produced no shift in its position, and there is no object in the universe for which that is true except objects that do not move relative to the Earth at all. Things that do not move relative to the Earth are things attached to the Earth. Things attached to the Earth are not in the sky. She checked the drop twice. She checked it a third time with the raw frame open beside it, pixel by pixel, so that she could see with her own eyes the little line of the background star's light go flat and then come back, and the flat part sat exactly under the object's position, as neatly as a shadow under a stone. Lisbet found her at the light table at three in the morning, the graph still on the screen. "You look like you've seen something." "I have." Viveka turned the screen. "A reference star disappeared for forty-one seconds. Here's the light curve. Here's the geometry. RV-417 crossed it." Lisbet read the figure the way she read everything, fast and thoroughly, and Viveka watched the speed leave her face. "That's an occultation." "Yes." "An occultation means near." "Yes." "And near means parallax." Lisbet looked up. "But it has no parallax. You showed me the plates." "Yes." "Then one of your three measurements is wrong." Viveka had been waiting for someone to say that, and she was almost grateful for it. "Then find it," she said. "Please. Take the raw frames. Take the reduction. Take my arithmetic and break it. If I have made an error, I want it found tonight." Lisbet worked until dawn. She was good and she was fast and she wanted the error to exist because the alternative was worse, and at six in the morning she sat back with her eyes red and said the thing that Viveka already knew. "There's no error. The star went out. The object crossed it. The object sits in the catalogue with zero parallax." She rubbed her face. "Long before the occultation there had been three nights of nothing, and she spent them the way a careful person spends an interval between anomalies: she looked for the error the way a locksmith searches a door, patiently, from every side, and refused to be interested in the strange parts until the ordinary parts were exhausted. She calibrated the astrometric reference set against the oldest and steadiest stars in the frame. She re-derived the plate scale from the survey's own standards. She checked the clock against the encoder and the encoder against the mount and the mount against the sky, and each check came back clean, and each clean check narrowed the range of excuses she had left. So we have a thing in the sky that is nearer than the stars and farther than everything. That can't be." "It can," Viveka said, "if the sky is not where we think it is." Lisbet did not sleep that morning either, and the two of them met in the galley at ten, hollow-eyed, and ate bread and did not talk about the object, and then talked about it in low voices, and then did the thing that people who have seen the same impossible thing always do, which is to take turns being the one who believes it. For an hour one of them could be ordinary while the other held the weight, and then they would trade, and the arrangement was poor and it worked, and by noon they had made a plan between them: Lisbet would take the occultation and re-reduce it from the frames and the arithmetic both, and Viveka would go to the archive and look for whether anyone, in thirty-five years, had ever written the object's altitude down. Lisbet laughed, once, the way people laugh when they are tired and frightened at the same time. Then she stopped laughing and looked at the window, because the station's bell was ringing for breakfast and the first light was coming up over the Cinderveld, and it was doing the thing it did every morning at this hour: it turned the far edge of the plateau the color of a struck match, and then it reached the ridge, and then it climbed into the sky, and the sky above the mountain held its blue without a single seam.