The clocks of the ancient Greeks

Hydraulic clocks (Clepsydras)

• The hydraulic clock of Archimedes

It was a complex hydraulic clock with many automatically moving objects. It consisted of the central storage container which supplied the water. The water went through a smaller container which ensured the stability of the water level (with a conical valve on a float), leading to the outflow nozzle. The supply of the flow was regulated depending on the date, turning the nozzle on a calibrated semicircular disc (so that the hypsometric difference of the nozzle hole and the level of water were altered, consequently, altering the time duration of the hour on the particular day). On the two columns of its facade, two rings (and two statuettes) indicated the hours that had been covered and the hours which remained respectively. On each hour, the pupils of the human eyes on a mask changed colour and a spherule fell into another container from the automatic opening of a crow's beak, with a bang. Simultaneously, the water fell into a volumetric container which, on the hour, was automatically reversed and two small snakes slid towards the birds on the tree that cried out frightened.

SOURCES: "Stamatis, Archimedous Apanta", "E. Wiedemann-F. Hauser, (Nova acta 100) Uhr des Archimedes"

• The hydraulic clock of Κtesibios (an automation miracle)

It was a marvel of automation, since the clock was able to operate continuously, without human intervention, indicating the 365 different hours of the year. The water from a spring supplied, through a spillway, the upper bronze container. This, in turn, supplied the smaller intermediate container which was a constant level controller through a system it contained (a conical valve on the float to interrupt the flow). Then a dripper supplied the tall bronze container, drop by drop, with a constant water supply. With the rising of water in it, the float rose and, through a shaft, a statuette with a pointer rose at the same pace. The pointer indicated the hour of 24 on a rotating drum containing a trace of hours of day and night depending on the date. At the end of the 24 hours, the water exceeded the side-built siphon and drained rapidly. On the descent of the float an ingenius drive system was activated in the ratio of 1 in 365 (which consisted of a toothed rule, a pawl, two gears and a worm gear) that ensured the rotation of the drum calibrated in 1/365 of its circumference as the pointer of the statuette now indicated the exact time of the next day.

• The ''anaphoric'' clock

A cam disc - on which a drawing represents the sky and the zodiac cycle – is rotated behind a bronze grid. The grid consists of 7 homocentric circles defining the month intervals and 24 curved rods defining the hours according to the "hour – month" diagram ("analemma"). The movement is achieved through a pulley and a flexible chain with a counterweight and a float weight which is lowered or lifted through the isochronous (=equal time) descent or ascent of the water level. This isochronous descent or ascent is assured because of the isochronous water outflow through a self-regulated controller of the constant level of the Ktesibios type.

Every day a pointer is located successively on the corresponding one of the 365 holes of the disc periphery, which define the days of the zodiac signs, and marks the 12 daily and 12 nocturnal non-isochronous hours according to the season.

SOURCES (about all clocks): "Vitrouvius, On arcgitecture, IX", "Ptolemy, About anallemma-Astronomica, Apotelesmatica", "Hipparchus, Των Αράτου και Ευδόξου φαινομένων εξήγησις", "Pap. Oxy. 470. 31-85", "Diodorus of Sicily, History", "Αthenaeus, Philosofers at dinner", "Diogenes Laertius, Lives of eminent Philosophers", "Strabon, Geography", "Apollonius, Conics", "Pliny the Elder, Natural history", "Sharon Gibbs, Greek and Roman sundials", "D. G. Price, Portable sundials in antiquity", "Herman Diels, Antike Technik", "Astronomic measuring instraments, Οικονόμου-Νικολαντωνάκης-Νίτσιου"

                

Portable sundials

• Portable ringed sundial  (the portable clock of Parmenion)

Three articulated rings are hung at an upright position with its external ring with an east-west orientation. The central ring comprises of two semicircles (bearing 2 pairs of engravings of 7 grooves defining the months for 4 different geographical latitudes-towns). The inner ring bears a hole on its periphery, while around it there are engravings dividing it into 12 equal parts which define the 12 hours.  During the clock operation, the semicircle with the data of the area is turned by 90 degrees and the inner ring is put on the current month; thus a bright spot indicates the hour on the opposite side.  The clock can also be used at midday of the equinoxes in order to find the latitude of a place, the azimuth and the star height. (Fractions of the instrument were found in the Philippes’ Octagon).