Measuring time in Antiquity; the water clocks from Karnak, Egypt & Amphiaraeion, Oropos, Greece

This post reproduces information, sourced from two published papers, on the interesting topic of ‘time measurement‘ in Antiquity. In this one, we have neglected sundials and focused solely on water-clocks.


Abstract A very well preserved ancient water clock was discovered during excavations at the Amphiaraeion, in Oropos, Greece. The Amphiaraeion, a famous religious and oracle center of the deified healer Amphiaraus, was active from the pre‐classic period until the replacement of the ancient religion by Christianity in the 5th Century A.D.. The foretelling was supposedly done through dreams sent by the god to the believers sleeping in a special gallery. In these dreams the god suggesting to them the therapy for their illness or the solution to their problems. The patients, then threw coins into a spring of the sanctuary. In such a place, the measurement of time was a necessity. Therefore, time was kept with both a conical sundial and a water clock in the form of a fountain. According to archeologists, the large built structure that measured the time for the sanctuary dates from the 4th Century B.C.

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Amphiaraus was one of the most important heroes of the Thebaean‐Argolic cycle and one of the noblest and respected figures in Greek mythology. He was born in Argus as a descendant of the great foreteller and psychiatrist Melampous (Ovid. XV, 244), from whom he inherited the art of oracles and healing knowledge. According to Pausanias (8, 45, 7), Amphiaraus participated in the hunt of the Calydonian Boar, while according to
Apollodorus he also participated in the Argonautic Expedition (Apol.Bibl. I, 9, 16).

The sanctuaries devoted to the worship of Amphiaraus were called “Amphiaraeia”.
At least twelve of them are known. The “official” Amphiaraeion in Oropos, some thirty miles from Athens, is the most famous of them. It was placed in the southeast of Oropos and was the main sanctuary of that ancient city. The sanctuary was established there in the 5th Century B.C. after a successful oracle was given (the original sanctuary of Amphiaraus in Thebes, known from Herodotus, was in decline). The Oropos Amphiaraeion was an official dream‐oracle and holy healing place.

The first explicit reference to the existence and operation of the Oropos Amphiaraeion
is found in Aristophanes’ comedy Amphiaraos.

There is also a large complex built for the believers on the right bank of the creek. Its most significant buildings are a hostel with 11 rooms, an agora and a hydraulic clock or water‐clock‐like fountain: a large built device measuring the time for the sanctuary, which dates, according to Leonardos (1918), to the 4th Century B.C. The dating is based largely on construction methods and materials.

During the excavations, a well-preserved conical sundial was discovered – even the gnomon was recovered, a rare occurrence.

αρχείο λήψης

The invention of the water clock is attributed by ancient tradition to Hermes Trismegistus, who was allegedly a time keeper and father of all sciences. The Greek word for the water clock, clepsydra, is a composite word: it means “thief of the water”; it was used to measure time mostly during the night or on cloudy days, when sundials could not be used.

The water clock was known to Egyptians since the 18th Dynasty (Pogo, 1936; Cotterell et al., 1986), while Babylonians knew it at least since the 12th Century B.C. (Neugebauer, 1947).

In India a similar device, jala‐yantra, is mentioned around 300 B.C. (Fleet, 1915). The ancient Chinese are also known to have used water clocks; however, the oldest reference to it in China is in the book Lou ‐ Shui ‐ chuan ‐ Houn ‐ t’ien ‐ i‐ chi, which means Method of rotating a sphere with ring by the water dripping from a water clock, a much later work since it was written by Chang Heng in 90 A.D. (Needham et al., 1986). J. H. Breasted (1934) writes that the most ancient water clock is named after the Egyptian pharaoh Amenhotep III and thus it can be dated from circa 1400 B.C.. Yet, the oldest Egyptian water clock for which there is written information belongs to the 16th Century B.C. (around 1550), and was constructed for the Egyptian priest and astronomer Amenemhet,
who probably was the true inventor of such devices.

Water clocks are mentioned in ancient Greece, too. The Greeks used clepsydrae since the age of Thales (636‐546 B.C.); they were known to Empedocles, Anaxagoras, Aristophanes (450/444‐385 B.C.) and Aristotle (Athenaion Politeia, or The Athenian constitution, 67, 2). Plato had constructed a most precise water clock for the night, known as the “Plato’s night clock” (Η. Diels, Antike Technik, Berlin 1914, pp. 199‐200).

In summary, the water clock is one of the most ancient instruments for the measurement of time. Its function was based on the steady and continuous flow of water between two vessels, or, in the case of the large built clepsydra of the Amphiaraeion, on the continuous flow (emptying) of a large rectangular structure, a large water tank.

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The whole clepsydra’s building (tank, corridor and stairs), is made of carved porous stone of the region, isodomically built (i.e. with stones of equal size).

The architect of our team, M. Katsiotis, observed that the inner surface of the tank is covered with thin waterproof mortar. The whole building lies lower than ground level. The rectangular tank has a narrow staircase in its exterior and a corridor (most probably a “service area”). The water from the tank was running at a very low draining rate from a faucet at its bottom. As the level of its water dropped, an index inside the tank also lowered and a time inscription could be read.

αρχείο λήψης (1)

The tank was filled probably through a pipe. When the tank was full, a a slave in charge of the clepsydra, the “eph’ hydor”, descended the nine exterior steps to the base of the tank and regulated the outflow rate; he was given orders by the priests for starting or stopping the water flow of the clepsydra. The simplest mode of operation would be to fill the tank at night or at the beginning of every day and starting the water flow early in the morning; the level of the water would fall slowly until the evening. For the permanent closing of the hole they used a conical wooden plug.

By examining this large water clock in situ we observed that on the bottom of the tank there is an oval cavity about 38 cm in diameter by 16 cm deep, with an opening in the center, now sealed. When the eph’ hydor slave removed the tap from the outer hole, the water would flow through the opening and was directed through another pipe to the adjacent creek. The Oropos clock shows no sign of renovation or remodeling; there is but a single outlet pipe. It is very similar, as Armstrong and McK. Camp (1977) point out, to the water clock in the Athenian Agora.


The proper functioning of the clepsydra was based on the steady flow of the water, which was regulated at the drain hole. Perhaps there were indications on the mortar of the water level, which would indicate the time; however, we did not discern any trace of them. So, we believe that most probably there was a float on the water in the tank: an index on the float would then show the water level by pointing at a calibrated vertical tablet. As was usual in antiquity, the float probably had a statuette on it, and either its hand or a stick held by the statuette pointed at the tablet. Indeed, such a tablet, made of marble and measuring 0.77 m × 1.25 m, which bears special line inscriptions, was discovered and is kept in the yard of the Amphiaraeion museum. The archaeologists
consider it an integral part of the clepsydra, since there is a basewith a notch, which would be the natural position of the tablet. Depending on the water level in the tank, the float would show one of the lines on the tablet that would correspond to the hours of day and night.

These horizontal lines on the tablet are carved in equal distances on its surface; on its upper part there are also some vertical lines for an unknown purpose. The equal distances of the horizontal lines are also curious, because it is known that generally in antiquity hours had different durations, depending on whether it was winter or summer: Almost all ancient people shared the notion that in any season, there were 12 hours (of varying length) between sunrise and sunset. A summer day hour was longer than a winter day hour. Greeks followed the same system. Therefore, it would be expected that the spaces between the lines indicated by the float on the marble tablet should be different from one month to the next and, moreover, the index should run through the 12 hours of day and night at the proper rate.

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This problem could be solved with the use of more than one tablets – and they probably exist, although in fragments. Otherwise, they could measure the hours in a different way: For example, the scale of the marble tablet could be altered by adding or removing parts depending on the hour duration. It is also pssible that the marble tablet found is not related to the clepsydra.

Indeed, the use of a water clock as an horological instrument had some basic problems to be solved. First, its operation was controlled by a human and not automatically. And second, the water flow was not steady, but it depended on the level of the water inside the main tank. In order to achieve a constant rate of flow, independent of the water level, the main tank was usually wider at its upper part. This is perhaps the reason that the Amphiaraeion water clock main tank was built wider in its upper part (verge) by 7 to 12 cm, by cutting the uppermost rows of stones: At the preserved top, the tank measures 0.85 m square, whereas at a point approximately 0.75 m below the top it has decreased
to 0.75 m, from which point to the bottom the walls are almost vertical. In any case, the ancients did not manage to eradicate this source of error in time measurement. The relation between the water flow rate through a narrow hole at the bottom of a vessel and the water level in that vessel was studied by Archimedes and the Alexandrine engineers Ctesibius, Philon and Hero(n). They introduced simple mechanisms, i.e. hydraulic siphons, to control the level (and therefore the flow rate).

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The main difficulty was the division of the day and the night into 12 unequal hours. The hours of day and night were equal only at the equinoxes and their difference continuously varied according to the season of the year. So the scale of the hours should also be variable. The astronomers‐engineers of antiquity tried to overcome this difficulty in the following way: the hour scale was placed in the lower vessel, where the water was collected after passing through the narrow hole. This lower vessel assumed a cylindrical
shape. A float inside it could then measure the water level in the respective hours. In this way the water clocks were replaced by the hydraulic mechanical clocks and humanity passed into a different era concerning time measurement.

(Source: “The large built water clock of Amphiaraeion”, by E. Theodossiou, M. Katsiotis, V.N. Manimanis and P. Mantarakis – Mediterranean Archaeology and Archaeometry, Vol. 10, No. 1, pp. 159‐167 – 2010)

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Summary The invention and establishment of the water clock in Egypt, at first glance, seems to be one of the best-documented developments in the history of ancient technology. A closer look at these clocks, however, reveals that their form and function have not yet been described sufficiently. Meanwhile, acquisition of three-dimensional data enables novel analysis of the preserved examples scattered all over the world. Regarding the fragmentary condition of most of the clocks, 3D scans are indispensable to investigate developments and functions of particular examples more closely and to ascertain the knowledge that existed about fluid dynamics around 1500 BC.

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It is often overlooked that in contrast to a stopwatch, the construction of a properly functioning water clock requires not only a high level of theoretical knowledge and practical abilities, but also a context in which the demand for such a clock exists, as well as the conditions to enable time measurement. In short, the amount of knowledge required before development of a water clock could begin was far more advanced than it appears at first glance. For example, time units had to be defined: in the case of ancient Egypt, twelve hours per night/day were the smallest measureable units. With regard to antiquity in general, this meant dividing the shifting time period between sunrise and sunset into twelve parts and operating with so-called unequal or seasonal hours. As a consequence, a clock in antiquity had to show different hours over the course of the year (and, in an ideal case, each day): long daylight hours in summer and short daylight hours in winter, and of course vice versa at night. Only for a very limited period at the equinoxes in spring and autumn are the hours of day and night equal. Therefore, the geographic latitude had to be considered too, since the latitude determines the rising and setting of the sun. To put it the other way around, determining the running time of such a clock allows us to to determine its appropriate latitude, or the latitude of its original site location. The removal of such a clock from the particular latitude for which it was manufactured would, inevitably, result in an incorrect display.

A working (stable) calendar is an absolute necessity in order to determine regularities concerning the increase and decrease in the length of the hours over the course of the year reciprocal to a specific latitude; it provides a clear concept not only of periodic months, but also of each month, with corresponding hours of an appropriate length.

Only two devices were available for time measurement in antiquity before the invention of the mechanical clock, which took place at some point in the fourteenth century AD. Pliny refers to the differences between these devices: whereas sundials only work on sunny days, a water clock has the potential to operate independently from external circumstances. The operation of a sundial requires only sunshine and some kind of shadow-caster, combined with a few calculations, to form a time-measuring instrument. A water clock, by contrast, involves extending beyond observation, thus, creating a higher degree of abstraction: first, it requires the conceptual development of a device that is independent from its surroundings, and then it requires the conditions for the device’s creation.

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Examined first by G. Daressy in an article in 1915, the Karnak clepsydra undoubtedly constitutes the oldest preserved water clock, originating from the time of Pharaoh Amenhotep III (1379–1342 BC).

The tomb of an Egyptian official named Amenemhet was discovered by fellaheen in 1885 at Sheikh Abd el-Qurna in western Thebes, is now lost. The only items preserved from it are a small fragment of the inscription, now in the Egyptian Museum Berlin, and two copies made immediately after the discovery. Amenemhet, who lived under the pharaohs Ahmose I, Amenhotep I, and Tuthmose I, around 1500 BC,27 explains in his inscription that he has recognized that the length of the night increases and decreases from month to month. For this reason, he has constructed an Mrht – an “instrument for telling time.” This device, he claims, shows the hours precisely, has astronomical depictions on the exterior, and has no predecessors (although he had consulted older texts beforehand); its water runs out through a single exit. The significance of this inscription was revealed a few years later, when the aforementioned discovery of the Karnak Cachette brought to light the remains of a vessel that met all these conditions.

The Karnak clepsydra was found broken in pieces, and was made of alabaster. Its shape is reminiscent of a large flowerpot; the outside of this vessel has characteristic depictions in three horizontal rows and a vignette of pharaoh Amenhotep III. The vignette allows the clepsydra to be dated to the middle of the fourteenth century BC. The uppermost row shows decans and anthropomorphic representations of stars and planets depicted in barks. Below, in the middle row, are the more prominent constellations of the northern sky and deities on both sides. The bottom row has six frames, each displaying the king, flanked by two of the twelve gods of the months. The outflow aperture is located between two of the frames.

Twelve scales of various length, with hour markings, are inscribed on the inside of the vessel. Above each scale, on the rim of the vessel, the name of the corresponding month is inscribed, with the god of that month depicted on the outside. The months containing the two solstices – and therefore the longest and shortest hours of the year – correlate with the longest and the shortest scale, respecitvely, while the months containing the equinoxes are represented by the medium-length scales. The lengths of the other scales follow accordingly. At sunrise or sunset, the vessel could be filled with water, which flowed out gradually from the small aperture near the bottom of the vessel. The hour was obtained by comparing the dropping water level to the scales on the inside.

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Each of the Hellenistic pieces copies the depiction of the Karnak clepsydra accurately; some of them show the complete pattern of the clepsydra in three rows, while others reduce the decoration to the bottom row. Both versions existed in parallel in Hellenistic times.

As simple as the water clock seems to be, on closer examination, it depicts a certain ingrained knowledge of fluid dynamics. Borchardt was the first to recognize that the shape of these water clocks revealed the application of a fundamental theorem in fluid dynamics, described for the first time in 1643 by the Italian scientist Evangelista Torricelli, and now known as Torricelli’s Law. It states that the velocity v of a liquid flowing under the force of gravity out of an opening in a tank is jointly proportional to the square root of the vertical distance h between the liquid surface and the center of the opening and the square root of twice the acceleration caused by gravity, 2g.

The exceptional importance of the Egyptian water clocks is that their design demonstrates the practical application of this theorem more than three thousand years before its theoretical formulation.

The problem for such an outflow water clock lies in ensuring constant water pressure inside and a steady outflow rate. The solution presented by the Egyptian water clocks is as simple as it is brilliant: reducing the circumference of a vessel and, hence, the water surface, to the shape of a truncated cone means that the sloping sides of the vessel (at a ratio of 1 to 3) can provide constant water pressure inside the vessel and consequently a steady outflow rate. This is the exact reason a cylindrical vessel is unsuitable: the sinking water level would result in diminishing water pressure and therefore a declining outflow rate.

By applying Torricelli’s Law, Borchardt tried to calculate the actual accuracy of the Karnak clepsydra, as well as whether the designers of this clock had succeeded. Unfortunately, the outcome was disappointing. A vessel that would be able to manage a steady outflow has to have the shape of a fourth-order parabola, and the Egyptian water clocks lacked precision in this regard: the vessels were too narrow at the top and too wide at the bottom. This would have caused the clocks to run too fast in the first half of the period of time to be measured and too slowly in the second. His calculations brought Borchardt to the realization that the Egyptian water clock was not able to display time correctly.

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It appears that there was a deliberate acceptance of the loss of accuracy, at least in Roman times. One must take this realization into consideration before imposing modern standards on the clocks’ accuracy.

In a frequently overlooked article, published in 1978 in a remote journal, a German astrophysicist* reported on a series of experiments with a plaster copy of the Karnak clepsydra. By simply filling the vessel and recording the course of the water flow, as well
as the effects of cohesion and surface tension, it became apparent that, contrary to earlier assumptions, the clock displayed the time quite precisely. The clepsydra may have been an average of ten minutes too slow in the first six hours, and too fast in the second six, leaving it running around ten to twenty minutes fast after twelve hours, but no other clock around 1350 BC could have revealed this lapse.

*(“Neue Ergebnisse zur ägyptischen Zeitmessung. Die Inbetriebnahme und Berechnung der ältesten erhaltenen Wasseruhr”, by Ludolf von Mackensen)

As important as the discussion about the accuracy is, however, other aspects of these instruments have also been neglected for too long, such as their use in practice. Why was it so important for the Egyptians to have a clock available? Conveniently, the reason is written on the clepsydra itself. There to tell the time when the sun and stars are not visible, in order to make offerings at the right time.

The absence of this type of clock in later contexts seems to imply that the production of the outflow water clock came to an end because it could be replaced by more technologically advanced types of clocks. A closer look paints a different picture.

A bronze vessel in the Archaeological Museum in Frankfurt provides clear evidence for the survival of this clock type.66 Although its shape and material are different, the basic features are the same and characterize the piece as an outflow clock. Instead of a truncated cone, it has the shape of a bowl, with a specific inclination to meet the flow requirements. Inscriptions on the rim give the names of the months, the equinoxes, the solstices, the calends, the nones, and the ides. Drilled into the rim are 368 holes for the days, and two holes can be found at the bottom: a large one, with traces of a different material, and a very small one made of gold that served as the outflow aperture. The time was indicated by the sinking water level against twelve scales on the inside. Unfortunately, the accuracy of the clock has not yet been examined. Another inscription on the outside reveals that it was dedicated to a Gallo-Roman sanctuary. This Roman clepsydra can be dated to the second century AD for epigraphic reasons.

Recently, another fragment has come to light in a remote area of the Roman Empire. In the fort of Vindolanda at Hadrian’s Wall, a small bronze stripe was discovered in the remains of a granary dating to the second/third century AD. The inscriptions on this stripe have led to its interpretation as a calendar or as part of a bronze disc from an anaphoric clock.68 Seen in comparison to the rim of the clepsydra in Frankfurt, however, it proves to be a fragment of another Roman outflow clock. The origin of these Roman pieces is still recognizable, as a look at a fragment of an Egyptian forerunner in the Musei Capitolini at Rome shows. This type of clock was obviously such a success that even in the face of more advanced devices, and despite the end of antiquity, it continued to be used. Even a medieval Arabic manuscript in the British Library contains a description of how to build such an outflow clock, which attests to a much more persistent tradition of this type of clock than previously thought.

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The study of the use of water clocks to measure time in Greek and Roman antiquity suffers from one major problem: until now, such investigations have relied almost entirely on written sources. Sophisticated devices like the Ctesibius clock left no traces and survived only in descriptions. Yet, nearly thirty outflow water clocks ranging from 1400 BC to AD 300 have been preserved in various states of fragmentation.

Modern technology offers a multitude of possibilities to more thoroughly investigate this precise ancient measurement device. Three-dimensional scans offer a unique opportunity to examine the preserved remains with unprecedented precision. Instead of approximated measurements and reconstructed values, these scans allow an exact analysis of these vessels shaped like truncated cones or bowls, and of their scale systems.

(Source: “The Karnak Clepsydra and its Successors: Egypt’s Contribution to the Invention of Time Measurement”. by Anette Schomberg)

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Research-Selection for NovoScriptorium: Philaretus Homerides


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