Planet where giant manned machines travel in convoy across surface, tracking some astronomical phenomenon and a girl runs away from her family to find someone (her brother?) who's travelling on one of these machine "caravans". There are elite experts on board who know how to identify the celestial target, whatever it is, can't remember. The machines follow specific routes and someone on board has to be scanning the sky at all times in case the phenomenon appears. Any ideas? Cheers Fred
This is Absolution Gap by Alastair Reynolds.
The girl is Rashmika Els and she is looking for her brother Harbin Els who had gone to work on one of the Cathedrals and disappeared. The route followed is called the Permanent Way.
The Cathedrals were built on Hela, the moon of the gas giant Haldora, to observe Haldora. Haldora is occasionally blinking out of existence for less than a second, for reasons that do not become apparent until partway through the story, and the observation to monitor for these events is done by various groups of religious fanatics, who are infected by various strains of a religion-carrying virus originating from a man named Quaiche.
As mentioned in DannyMcG's comment, Quaiche has had his eyes modified to aid watching:
Beneath the eyeshades, hugging the skin like a second pair of glasses, was a skeletal framework. Around each eye were two circles from which radiated hooks, thrusting inwards to keep the eyelids from closing. There were little sprays built into the frames, blasting Quaiche’s eyes with moisture every few minutes. It would have been simpler, Grelier said, to have removed the eyelids in the first place, but Quaiche had a penitential streak as wide as the Way, and the discomfort of the frame suited him. It was a constant reminder of the need for vigilance, lest he miss a vanishing.
A comment by Daniel Roseman mentions a chasm, and indeed the book is set in the same fictional universe as Reynold's novel Chasm City, though this is on a different planet. There is a chasm on Hela, whose name is the titular Absolution Gap. It intersects the Permanent Way, and how the Cathedrals cross it is a part of the tale.
The story links to other stories in Reynolds' universe, including such plot elements in common as the Inhibitors, the aforementioned religion-carrying virus, a group of characters who have survived the Inhibitors destroying their stellar system, and Yellowstone the planet where Chasm City is.
Sounds like it could be Christopher Priest's The Inverted World.
It's about a city slowly moving along on a track that's built ahead of it and dismantled behind it. The city is following "The Optimum" - an energy source which causes time to dilate ahead and behind it. From Goodreads:
The city is winched along tracks through a devastated land full of hostile tribes. Rails must be freshly laid ahead of the city & carefully removed in its wake. Rivers & mountains present nearly insurmountable challenges to the ingenuity of the city's engineers. But if the city does not move, it will fall farther & farther behind the optimum & into the crushing gravitational field that has transformed life on Earth. The only alternative to progress is death. The secret directorate that governs the city makes sure that its inhabitants know nothing of this. Raised in common in creches, nurtured on synthetic food, prevented above all from venturing outside the closed circuit of the city, they're carefully sheltered from the dire necessities that have come to define human existence. Yet the city is in crisis. People are growing restive. The population is dwindling. The rulers know that, for all their efforts, slowly but surely the city is slipping ever farther behind the optimum. Helward Mann is a member of the city's elite. Better than anyone, he knows how tenuous is the city's continued existence. But the world he's about to discover is infinitely stranger than the strange world he believes he knows so well.
The phenomenon in question could be the Optimum itself, or the sun, which, due to the Optimum's warping effect, is an inverted sphere - something like a spindle or spinning top.
The main character, Helward, is a scout who has to find the safest/easiest route for the city as it follows the Optimum. The workforce building and dismantling the tracks are recruited/enslaved from the people outside the moving city.