It was not easy for the Romans to keep a close track on time. They did not work by minutes, let alone seconds, their smallest unit being the hour. Even that was not standardised, but varied according to season and location. By day, a Roman hour was a 12th of the time between sunrise and sunset, and by night a 12th of the time between sunset and sunrise. The length of this hour therefore depended on the time of year and on how far north or south you were.
But in the 3rd century BC, the Greeks had begun to develop clocks which took account of the seasons and the latitude, and which displayed the time with a pointer on a scale. The final and most sophisticated version, the ‘anaphoric’ clock evolved somewhere around 100 BC. It was picked up by the Romans and described by the 1st century BC architect Vitruvius. Its usual Latin name was horologium, although that can also apply to a sundial or simple clepsydra.
Fragments of bronze discs have been found from two anaphoric clocks, both of perhaps the 2nd century AD. One, from Salzburg in Austria, was about 170cm in diameter and is engraved with a map of the constellations. The other, from Grand in the Vosges in north-eastern France, is only 35cm across and much more crudely made.
What of the new discovery of a ‘Roman calendar’ at Vindolanda reported in CA 224? The bronze fragment was found in the fort’s granaries, although it had surely ended up there merely as a piece of scrap, and had probably started life in the headquarters building. It was interpreted as part of a calendar for keeping track of the date by moving a peg one hole forward every two days. But to anyone familiar with anaphoric clocks it was immediately obvious it was something much more important and exciting than a calendar.
It is the first evidence for an elaborate clock in Roman Britain. The holes are precisely as on the Salzburg and Grand discs. Like them, it is labelled with a month name (September) and abbreviations for the Kalends, Nones, Ides, and equinox. The clock it came from seems to have been more down-market than the others. The lettering was punched, not engraved. There was perhaps not even a complete ecliptic disc, but only a narrow bronze ring nailed to a wooden backing. And with a diameter of perhaps about 35cm, it was small. Even so, it was a very sophisticated instrument.
The above extracts are from an article which can be read in full in Current Archaeology 228.