It was not easy for the Romans to keep a close track on time. They were not, as we are, inveterate clock-watchers. 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.
Other than estimating from the position of the sun in the sky or by the length of shadows, the Romans had two ways of telling the time. One was the sundial – which was not much use in cloudy weather and even less by night. The other was the water-clock or clepsydra – in its simplest form a bowl of water with a small hole at the bottom which emptied in a given time.
The simple water-clock was fine for measuring, say, the period allocated for a speech in the law-courts. But it did not empty at a constant rate: the fuller it was – the greater the head of water – the faster the outflow, and the emptier, the slower. Moreover, short of a hit-and-miss adjustment of the size of the hole with wax, it could not cater for seasonal hours. One Roman remarked that it was easier to find agreement between philosophers than between clocks!
But already 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.
A working model of the heavens
Water was fed from a header tank into a regulator tank. This was kept full to overflowing, which meant that under the constant head, the water emerged from the bottom at a constant rate. It fell into a receiver, in which was a float that, by means of a cord or fine chain with a sandbag as counterweight, turned an axle once every 24 hours. The end of the axle carried a bronze disc engraved with a map of the stars in stereographic projection (that is, the dome of the heavens represented on a flat surface) centered on the north celestial pole and extending to the Tropic of Capricorn. The ecliptic (the path of the sun through the sky) was represented on the disc by a circle of holes, ideally 365 in number. A peg to indicate the sun was plugged in the hole appropriate to the day (that is, in its proper place relative to the stars) and was moved forward by hand one hole per day. Thus, it simulated the sun’s annual passage through the heavens, just as the turning disc simulated the daily rotation of the heavens.
In front of the disc was a fixed grid of wires. One of them represented the horizon as seen at the place where the clock was located. This is where latitude mattered. Both the disc and the grid designed for a clock in Southern Italy, for instance, would be very different from those designed for one in Northern Britain, where the sun’s path through the sky is different, and the horizon is in a different place relative to the stars. An Italian clock in Britain would be wildly inaccurate.
The other important wires represented the hours, 12 above the horizon, 12 below. At the beginning of the day, the sun peg behind the grid climbed up above the horizon wire. One seasonal hour later, it passed the first hour wire, and so on until after 12 hours it dropped below the horizon. Thus it mimicked the actual position of the sun in the sky; and it did so even by night when the real sun was not visible. Provided the disc turned once in exactly 24 hours and the peg was advanced one hole per day, this elegant clock automatically adjusted itself to the changing seasons. It was a working model of the heavens.
Clocks with complex displays were expensive, but they were far from unknown as civic ornaments, and could be found in the homes of the well-heeled. We rarely hear exactly how they worked, but the anaphoric type is often the most likely used. It is plausibly suggested, for example, that the clock in the famous Tower of the Winds at Athens, built about 50 BC, was anaphoric. An inscription of the 1st century AD records that the small town of Boutae (Annecy) in France was presented with ‘this clock together with its building and all its statues and railings, which cost 10,000 sestertii plus a further 4,000 sestertii for a slave to service the clock’. Daily servicing would include ‘winding’ it – recycling the water from the receiver to the header tank and returning the float to the bottom of its travel – and moving the sun peg.
What is more, 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. It has no map, and it extends only as far as the ecliptic (i.e. it depicts the great circle on the celestial sphere representing the sun’s apparent path through the year). The complete disc, extending out to the Tropic of Capricorn, was probably of wood, with this smaller bronze disc attached to it. But on both the Salzburg and Grand examples the ecliptic is marked by a circle of 182 or 183 holes, the sun peg being moved every other day. And on both discs the circle of holes is labelled: at Salzburg with the months and the signs of the zodiac, at Grand with the months and the fixed points in the month – Kalends, Nones, and Ides – plus the equinoxes and, no doubt, the solstices.
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.
But what was it doing in a workaday military context on the very margin of the empire? Surely such things were not standard in auxiliary forts? The unexpected sophistication may be the whim of some technically-minded commanding officer whose fancy had been tickled by a similar clock he had seen elsewhere. The army, however, did need reasonably accurate timekeeping. According to Vegetius, the military writer of the 4th century, ‘the night-watches are divided into four parts by the water-clock (clepsydra) so that nobody has to watch for more than three hours a night’.
Normally, we may guess, this was the simple outflow clepsydra, which was apparently standard equipment in the army. Julius Caesar, when in Britain in the summer of 54 BC, ‘found by accurate measurement with a water-clock that the nights are shorter than on the continent’. But, because the length of the hour varied with the season as well as with the latitude, three-hour stints for watches were more complicated to measure than that.
Here the anaphoric clock would prove its worth. And at the auxiliary fort of Remagen on the Rhine an inscription records that in AD 218 ‘Petronius Athenodorus, prefect of the First Flavian Cohort, repaired at his own expense the horolegium [misspelt] that had stopped telling the time and was broken down through age’. This is an elegant and elaborate inscription which names the emperor and the governor: too grandiose, it would seem, to be referring to the repair of a common-or-garden sundial or clepsydra.
In this respect, then, Vindolanda was doubtless not unique; and we would hardly expect otherwise. However, as Andrew Birley declared in CA 224, the meagre 8cm strip of bronze, which he thought was a calendar, ‘ranks with the famous writing tablets as one of the rarest objects ever found at Vindolanda’. Now we know that it was actually a clock, his words are even more true.
Clock or calendar?
Not all experts agree with Michael Lewis’s analysis. Parapegmata – ‘peg-calendars’ – seem to have been relatively common in the Roman world. These were essentially no different from the ‘perpetual calendars’ still used today, and involved moving a peg forwards each day to represent the progression of time: they were, in other words, nothing more than a method for labelling the current day.
Cicero makes reference to such a peg-calendar when he writes the following to his friend Atticus: ‘I reached Laodicea on the day before the Kalends of Sextilis [later called August]. From this day you will move the year’s peg (clavum anni).’ He had previously written: ‘I think I will be in my province on the Kalends of Sextilis. From that day, if you love me, let the year’s parapegma be moved on.’
However, most known parapegmata are on stone or plaster, all have daily holes, and none is circular in shape. Moreover, the inscriptions on peg-calendars comprise a mixture of astronomical, meteorological, and nundinal (i.e. recording market-days) information. The Vindolanda fragment – like the finds from Grand and Salzburg, which are certainly from clocks – is made of bronze, is circular in shape, has holes only for every other day, and is restricted to limited astronomical information.
A particular point in the controversy is the contrast between daily and every-other-day holes. Supporters of the clock theory argue that it would be unnecessary to make a peg-calendar with holes only every other day, and that this would almost certainly lead to trouble caused by people forgetting whether they had changed the peg the day before or not. Whereas, they argue, the ecliptic circle on the bronze disc of an anaphoric water-clock simply did not have room for 365 holes.
Andrew Birley, Director of Excavations, The Vindolanda Trust
Daryn Lehoux Associate Professor and Queen’s National Scholar, Queen’s University, Kingston, Ontario
Michael Lewis, ‘Theoretical Hydraulics, Automata and Water Clocks’, in Örjan Wikander (ed.), Handbook of Ancient Water Technology, Brill, 2000.
ALL photos: The Vindolanda Trust, unless otherwise stated.