story identification - Planet where giant manned machines travel in convoy across surface, tracking some astronomical phenomenon

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

Answer

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.