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The calendar islands.
At the very centre of the archipelago, above the orbital plane of the Eye, are four islands which act as the clock used by all of Thet. The middle island is suspended directly above the anomaly and kept in place electromagnetically; this contains a bank of lasers that measure the distance and rotation of the other three, and a computer that converts these data into usable time information. That time signal is then transmitted to a series of outposts throughout all of Thet.

The Horologium was constructed in 11 tgc by the Lyrisclensiae, originally as an experiment to study how regular the orbits of islands were. While several phenomena causing fluctuations were identified in the course of the experiment, once they were corrected for, it was found that the movements of the islands provided exceptional regularity and reliability; much moreso than methods based on measuring radioactive decay, pulsars, or the gradual change of cosmological parameters like the speed of light. The three islands correspond to the day, month, and year of the new Thessian calendar, and can be read visually like the hands on a clockface by an individual with adequate knowledge of the aberrations.

The Horologium and the various debris islands surrounding it are sometimes called Minor Belt 0A. No other "0" belt designation is in use.