Greg Egan - The Clockwork Rocket

She wound the drive’s spring, lowered the bench to make more room for herself, then lay down beneath the telescope. A selection of eyepieces sat in a rack beside her; she picked one with a modest magnification that would allow her to view Sitha’s trail all at once, and inserted it into the holder.

With three eyes closed, she peered through the telescope, adjusting the focus. So soon after sunset, most of the sky would just look gray, but she’d expected a little of Sitha’s trail to be showing in her field already. She checked the time again, and did a few calculations; she should have been seeing something. She reached over and put her hand on the azimuth wheel; there was some play in it, rendering the narrow engraved markings she’d carefully aligned nothing more than rough landmarks. Painstakingly, she nudged the wheel back and forth, until a wisp of red and orange appeared in the corner of the field. The time was getting closer; she kept making adjustments until the whole trail was visible.

The clock chimed; Yalda released the brake on the tracking drive. The mechanism was not sophisticated enough to follow the star for an indefinite period as it circled the celestial pole, but the telescope’s steady movement from the current location to the predicted one would take most of the burden off the observer for one chime, allowing her to keep the trail centered with just a few corrective nudges.

With the hard work done, Yalda finally relaxed and let herself marvel at the telescope’s power. Even in the gray twilight, Sitha’s trail was already brilliant and clear. Most bright stars were bright because of their proximity, and that in turn usually meant that their trails were short; a close neighbor of the sun was rarely rushing by with great haste. But Sitha was an exception, a brilliant oddity fast enough to spread its colors wide. When she made her measurements, it would be her first choice.

Yalda squeezed out of the way and let Renato check the result of her efforts; he needed to prop himself up on the bench to reach the eyepiece. He remained there, perfectly still, for what must have been a full lapse. Then he climbed out and put a hand on Yalda’s shoulder.

On his palm he’d written, Well done. You’ll be fine.

Renato insisted on sleeping outside and giving Yalda the debris-free bed in the living quarters; she would have had no qualms about sharing it with him, but she decided it would be presumptuous to expect him to feel the same way. The clean white sand had a peculiar, slippery texture, but the stone base certainly kept it cool, and Yalda surrendered to her weariness with luxurious rapidity.

She woke before dawn and unpacked the cart so Renato could use it to take his own notes and equipment back down the mountain. When he had departed, the muffled sound of her footsteps in the thin air took on an eerie, distant quality; she could not expect to see another person for the next three stints. She’d asked Ludovico for four stints, assuming that he’d grant her two at the most, but he must have mistaken the curious familiarity of her Meconio essay for some kind of genuine resonance with his own views. Either that, or he was wise to the whole scam and simply enjoyed watching people scrambling about trying to satisfy his whims.

Yalda set up her equipment in the observing hut, and spent the morning testing and aligning it; there were parts of the task that were actually easier in daylight. In the afternoon she forced herself to sleep; she needed to nudge herself into a cycle of nocturnal wakefulness, but it was hard to relax when her first observations were just a few bells away.

She woke around sunset, ate half a loaf, then went to the hut while it was still light. In time, she hoped to be able to operate the telescope’s machinery by touch and memory alone, but for now she was better off starting each session with a clear view of her surroundings, giving her a chance to get oriented.

With her own bulky contraption clamped over the telescope’s eyepiece holder there was no longer room for the observing bench; she’d taken it out and put it in the office. She cued up Sitha and checked the image by flipping down a mirror that diverted the light into an ordinary eyepiece; it didn’t take long to center the trail, as she’d done the previous night. Then she raised the mirror and let the same light pass into her purpose-built optics. Sliding her body around on the floor, she peered into the second eyepiece. In this view, the trail was replaced by a broad elliptical blur—more compact than the long streak she’d started with, but still multicolored and not remotely point-like.

She reached into the side of the device and began adjusting the distance between two lenses. The principle was simple enough: if a clearstone prism spread a narrow shaft of white light into a fan of colors, the very same fan fed back through the prism would have to emerge as a single, sharp beam. Sitha’s trail provided a ready-made fan, albeit a far from perfect match. A system of lenses could magnify the overall angular width of the star trail, and then a flexible mirror could tweak the detailed progression across the colors. Yalda’s first task was to get the width right: to shrink the blurred ellipse as much as possible by changing the magnification alone. Then she could tinker with the shape of the mirror to perfect the transformation.

That had been the plan; the reality wasn’t so simple. Once she started moving the pegs that deformed the mirror, she realized that she was still altering the overall size of the color trail. In theory, it might have been possible to make the two kinds of adjustment independently, but with nothing actually enforcing that the idealization was irrelevant.

Yalda spent a few pauses cursing her stupidity, then reached back to adjust the lenses again. The ellipse became a little less broad, but also grew thicker in the other direction. The clock chimed; it was time to change the tracking parameters.

Improvements came painfully slowly. When Sitha drew too close to the horizon to be followed—more than a bell before dawn—Yalda was still not happy with the results. Rather than choose another star and start again from scratch, she decided to call it a night; this way she could preserve all the Sitha-specific adjustments she’d made, ready for a further round of refinements.

Trudging back toward the living quarters, she stopped to look up at the sky, at all the burning worlds rushing by. Sitha was just one transient neighbor among this staggering multitude. How could anyone hope to wrap the stars in mathematics and draw them into their mind? She was a child fumbling with a clumsy toy, pretending that it granted her magical powers, while this vast, magnificent procession continued on its way, entirely oblivious to her fantasies.

Yalda slept until mid-afternoon, then she sat in the office planning a new strategy. If she disciplined herself when she reshaped the mirror—always making changes in pairs that largely canceled each other’s effects on the overall spread of the color fan—it might save her some unnecessary adjustment of the lenses.

Two bells later, lying on the floor of the hut with her skin chafing and her fingers cramped, she allowed herself a chirp of celebration, unperturbed by its distorted tones. Sitha’s trail had finally shrunk to an almost circular patch of light, only slightly bluer on one side than the other.

It was time to make use of Nereo’s trick. Yalda slipped a mask into the light path that blocked the center of the image, leaving only the faint halo that surrounded the bright core. As her eyes accommodated to the much dimmer partial image, it became easier to see the changes caused by each slight movement of the mirror’s pegs.

Half a chime later, a single, fine adjustment plunged the view into total darkness. Yalda was ecstatic; Sitha’s image was now smaller than the mask that was blocking it!