- Alexander Jones
The Antikythera Mechanism (National Archaeological Museum, Athens, inv. X 15087) was a Hellenistic gearwork device for displaying astronomical and chronological functions. Substantial but highly corroded remains of the instrument were recovered from an ancient shipwreck (see Figure 1).
The most complex scientific instrument to have survived from antiquity, it resembled the sphaerae or planetaria described by Cicero (1) and other Greco-Roman authors. The date of its construction is in dispute but must have been earlier than the middle of the 1st century bce and can scarcely have been before the end of the 3rd century bce. It is an invaluable witness for ancient mechanical technology at its most advanced level (see mechanics) as well as for Hellenistic astronomy.
Archaeological Context and History of Research
The remains of the Mechanism were recovered during the pioneering but unscientific salvage of antiquities carried out in 1900–1901 by sponge divers working for the Greek government at the site of a Hellenistic shipwreck off the northeast coast of Antikythera (see archaeology, underwater). The wreck is dated by coins and ceramics to within about a decade of 60 bce.1 The ship, which was exceptionally large, carried a cargo of luxury objects, including bronze and marble statuary and fine glassware, as well as passengers, and its intended course was from the Aegean to destinations further west, likely including Epirus and Italy. Together with the other materials from the wreck, the fragments of the Mechanism were deposited in the National Archaeological Museum, where their distinctive features—toothed gears and engraved Greek inscriptions—first drew scholars’ attention in 1902.
The Mechanism remained little more than an object of intermittent speculation until the investigations (by autopsy and later radiography) of D. J. de Solla Price. Price’s classic 1974 monograph Gears from the Greeks presented an essentially correct description of the device’s original exterior layout as well as a tentative and only partially successful reconstruction of its functions and gearwork. Subsequent research by A. Bromley and M. T. Wright and by the Antikythera Mechanism Research Project, employing autopsy, X-ray radiography, linear and computed X-ray tomography, and reflectance transformation imaging, corrected and greatly extended Price’s work, and by 2008 a consensus reconstruction was achieved for practically all the gearwork and exterior dials relating to chronological cycles and the motions of the sun and moon. Recent studies have been devoted to details of the mechanical construction, improved transcriptions and interpretations of the inscriptions, and confirmation of the hypothesis (urged by Wright, among others) that the Mechanism bore a planetarium displaying the motions through the zodiac of the five planets known in antiquity.2
Appearance and Operation
When intact, the Mechanism was box-shaped, with its front and back faces consisting of bronze plates approximately thirty-two centimetres tall by seventeen centimetres wide. The top, bottom, and sides of the box were wooden, and the front-to-back dimension was probably at least ten centimetres. On the right side, about halfway up, a shaft protruded through a perforation in the frame, probably ending in a knob or crank. This was the input by which motion was imparted to the Mechanism. Turning the shaft one complete turn clockwise represented going forward in time by about seventy-eight days (a number that is not known to have any special significance), while counterclockwise turning represented going back in time at the same rate. The shaft drove concealed gearwork that led to various axles through the centres of various dials on the front and back faces. Pointers mounted on these axles and revolving along the inscribed scales of the dials constituted the Mechanism’s outputs. In principle, any setting of the Mechanism obtained by turning the input knob corresponded to a specific date, and each output either situated that date according to a chronological framework or represented some aspect of the configuration of the heavenly bodies observable from the earth on that date.
The back face probably bore five dials, each having a single pointer displaying the passage of time through chronological cycles whose meaning was made apparent by means of texts inscribed on the dials’ scales (see Figure 2). The two main back dials had unusual spiral scales.3 One, occupying much of the upper half of the plate, represented a lunisolar civil calendar—a variant of the calendar of Corinth such as was probably used in one of the cities of Epirus—regulated by a nineteen-year “Metonic” intercalation cycle that determined which years had intercalary months and how many days each month comprised (see calendar, Greek, Meton). The rules governing this cycle were similar to those described by Geminus in chapter 8 of Introduction to the Phenomena. The other, in the lower half, indicated lunar months in which solar or lunar eclipses could occur, according to a 223-month “Saros” cycle, with the approximate times of the associated conjunctions or oppositions of the sun and moon. A smaller circular dial showed the four-year cycle of the Panhellenic athletic festivals including the canonical periodos (the Olympic, Pythian, Nemean, and Isthmian games), as well as the Naa of Dodona and the Halieia of Rhodes. Another small dial, no longer extant but referred to in one of the inscribed texts, counted nineteen-year periods in a seventy-six-year “Callippic” cycle (see Callippus), while a third gave a correction term to the times inscribed on the eclipse prediction dial according to the triple Saros or exeligmos cycle mentioned by Geminus and Ptolemy (4) (Almagest 4.2).
The front face bore a single dial with multiple pointers representing the sun, moon, and the five planets (see Figure 3). It had two concentric circular scales. The inner scale was graduated into the twelve signs and 360 degrees of the zodiac, according to which the pointers indicated the longitudes of all seven heavenly bodies. The outer, which could be set manually in any orientation relative to the zodiac scale, was graduated into the months and days of the 365-day Egyptian calendar year; the Egyptian date was marked by either the sun’s pointer or a separate pointer representing the mean motion of the sun. The moon’s pointer was attached to a revolving disc with an inset particolored ball that rotated to display the current lunar phase. The other pointers also probably bore small spherical appendages at appropriate distances from the dial’s centre, so that they represented the visible bodies and constituted a moving illustration of a schematic geocentric cosmology.4
In addition to the brief, mostly single-word texts along the dial scales, the front and back faces bore more extensive texts that took up much of the space around the dials.5 The inscriptions were engraved in tiny letters—letter height ranging from about 1.2 to 3 millimetres—otherwise resembling those of Hellenistic lapidary inscriptions (see epigraphy, Greek).
On the front, above and below the planetarium dial, was a parapegma listing annually recurring phenomena including first and last appearances of constellations, the solstices and equinoxes, and the sun’s entries into the zodiacal signs, keyed to the sun’s longitude by a system of alphabetic index letters on the zodiac scale. The texts around the edges of the back face were similarly indexed supplements to the eclipse predictions on the lower spiral dial, describing the eclipses in terms of colors, magnitudes, and changes of direction that may have pertained to winds or to the obscuration of the eclipsed body during the eclipse.
There were also two inscribed plates that seem not to have been integral parts of the Mechanism but may have functioned as covers protecting its faces. The text on the so-called back cover was a systematic inventory of the Mechanism’s external features (see Figure 4).
The surviving portion of this text describing the front face provides the conclusive evidence for the presence of pointers for the five planets on the front dial. The front-cover text, so far as it is preserved, describes in detail the astronomical cycles of motion of the planets through the zodiac and relative to the sun that were simulated by the gearwork.
The principal mechanical parts of the Mechanism were disk-shaped gears with triangular teeth. An engaged pair of such gears transferred motion from one axle to another while modifying the rate of rotation by the ratio between the gears’ tooth counts. Trains of engagements made it possible to represent astronomical periodicities of great accuracy. Some were expressed as equations of hundreds of years with hundreds of cycles of motion. This variety of gear technology is attested to in the surviving Greco-Roman mechanical literature, for example, in Vitruvius, De Architectura, Book 10, and Heron, Dioptra, though only in much simpler applications.
On the other hand, the Mechanism’s crown gears and epicyclic gearing (gears mounted eccentrically on other gears) are not paralleled in the textual sources. A combination of these principles in the lunar phase display operates as a differential gear, subtracting the sun’s rate of motion from the moon’s. A further noteworthy element in the gearwork leading to the output of the moon’s longitude is a gear driving another gear by means of a pin riding in a radial slot in the second gear. This introduces a periodic variation in the moon’s speed in accordance with the simple eccentre and epicycle hypotheses of lunar motion known from Ptolemy’s accounts of Hipparchus’s theoretical works. Similar use of slotted components in the lost gearwork for the planets would have accomplished the display of their alternating direct and retrograde motion, as described in the front-cover inscription.
By the standards of modern clockwork, the saw-toothed gears of the Mechanism are inefficient and crudely executed. In other respects, however, the design and metalworking of parts are complex and delicate, as exemplified by the extant variable-radius pointer of the upper spiral dial and the lost system of concentric pipes by which seven distinct rotations were transmitted along a single axis to the front dial.
Relations to Ancient Astronomy
The functions of the Mechanism were limited to phenomena governed by time on a scale ranging from single days up to cycles lasting several decades, but within this scope, it offered a remarkably comprehensive visualization of the range of topics embraced by astronomy in the Hellenistic period. It had numerous points of contact with the most comprehensive surviving text of popularized astronomy of the period, Geminus’s Introduction to the Phenomena, as well as with Greek astronomical lapidary inscriptions and papyri. These points include the regulation of Greek calendars through mathematically structured intercalation cycles (cf. Geminus ch. 8, IMilet inv. 84+inv. 1604), the “wandering” Egyptian calendar year (cf. Geminus, ch. 8), the Saros and exeligmos cycles and eclipse prediction (cf. Geminus ch. 18, PBerol. 13146–13147), and astrometeorological parapegmata (cf. Geminus ch. 17, the parapegma appended to Geminus’s work, PHib. 1.27, and IMilet inv. 456A–D and N).
The Mechanism’s pin-and-slot device for varying the moon’s rate of motion in longitude, as mentioned above, is a mechanical equivalent of the epicyclic and eccentric hypotheses (see astronomy) that, according to Ptolemy, Hipparchus (3) assumed for the moon. The design may in fact reflect Hipparchus’s influence both in the underlying geometrical hypothesis and in the assumed range of variation. The periodicities reflected in the gearwork, however, which were derived entirely from the nineteen-year and Saros cycles, are less accurate, though more mechanically convenient, than those that Hipparchus appropriated from Babylonian astronomy.
The Mechanism casts special light on two facets of Hellenistic astronomy. Its treatment of eclipses combines elements that are variously crude, sophisticated, and strange.6 The Saros cycle by which the spiral professes to predict recurrences of eclipses of similar character is poor in comparison to the methods of eclipse prediction practised in contemporary Babylonian astronomy, with its errors becoming increasingly significant as the cycle is iterated. By contrast, the specific predictions of possible eclipses and their times inscribed on the dial were originally computed by a fairly sophisticated mathematical theory for a specific range of dates beginning, most likely, in 205 bce (the terminus post quem for the Mechanism’s construction). And lastly, the surviving supplementary descriptions of the eclipses inscribed around the back plate, all pertaining to solar eclipses, relate to phenomena such as disk colour and—probably—wind directions that, from a modern perspective, seem unsuitable for scientific prediction but relate to the astrological interpretation of eclipses as ominous phenomena (e.g., Hephaestion of Thebes, Apotelesmatica 1.21–22; see astrology).
The gearwork that led to the planets’ pointers is lost, with the possible exception of one isolated gear, but the front-cover inscription provides a fair amount of evidence for the underlying planetary theory, an aspect of Hellenistic astronomy that is otherwise very poorly represented in our sources. The text does not treat planetary motion at a theoretical level but gives time intervals in days between key stages in each planet’s apparent motion such as its conjunctions or oppositions with the sun and its stationary points. The cycles for a planet were assumed to be unvarying; there was thus no representation of zodiacal anomaly such as is found in Babylonian mathematical astronomy. (According to Ptolemy, Almagest 9.2, Hipparchus faulted the Greek planetary theories current in his time for this very defect.) The numbers of days assigned to the various intervals appear to be derived from simple epicyclic or eccentric models. The two legible statements of long-term periodicities are very accurate, and distinct from their known Babylonian counterparts.
Other Ancient and Medieval Geared Astronomical Devices
References to mechanical planetaria occur in various Greek and Latin authors from the 1st century bce onward.7 The majority do not reflect personal acquaintance with such devices, and some are fanciful. In De Natura Deorum 2.88 (composed in 45 bce), Cicero mentions a sphaera possessed by Posidonius (2) that was a planetarium in which a rotary input drove simulations of the varied motions of the sun, moon, and planets. He had probably seen this device during his youthful sojourn in Rhodes around 78 bce, and his recollection of it likely also shaped his accounts of the mechanical sphaera that he ascribes to Archimedes in De republica 1.22 and Tusculan Disputations 1.63. The name sphaera is usually supposed to have signified that these were representations of the spherical heavens, not that they were themselves globe shaped; if so, the Antikythera Mechanism was a sphaera.
Nothing definite can be said concerning the history of development of the technology of astronomical gearwork leading up to the Mechanism. In particular, the role of Archimedes is obscure, though some plausible memory of an Archimedean astronomical machine seems to be reflected in Cicero’s semi-fictional references, and in late antiquity Archimedes was reported to have written a book on sphairopoiia, “sphere-making” (Pappus, Collection 8.3). Broadly speaking, the literary allusions to sphaerae confirm the physical evidence of the Mechanism’s construction that it was not unique, but also imply that such objects were rare and regarded as the acme of human ingenuity. They belonged to the categories of didactic instruments and wonder-working devices, only incidentally if at all as computational tools for technical astronomy.
Several philosophical and scientific writers of the 2nd century ce, including Theon of Smyrna (The Mathematics Useful for Reading Plato, ed. Hiller p. 180), Ptolemy (Planetary Hypotheses 1.1), Galen (De usu partium 12.5), and Sextus Empiricus (Adversus mathematicos 9.115) refer to planetaria as devices with which they expect their readers to be familiar at least in principle. Ptolemy’s criticisms of planetaria—that they are astronomically incorrect and in any case mimic only the appearances as seen from the earth without revealing their true causes—could have been applied to the Antikythera Mechanism. It is not clear whether the tradition, or the technical resources that made it possible, survived into late antiquity. The sole surviving ancient gearwork device from the time after the Mechanism is a portable sundial with remains of a rather crude geared calendrical display (London Science Museum inv. 1983-1393, c. 500 ce), similar to devices later produced in the Islamic world and medieval Europe.8
- Antikythera Mechanism Research Project. The Inscriptions of the Antikythera Mechanism. Special Issue of Almagest: International Journal for the History of Scientific Ideas 7.1. Turnhout: Brepols, 2016.
- Freeth, T., Y. Bitsakis, X. Moussas, J. H. Seiradakis, A. Tselikas, H. Mangou, M. Zafeiropoulou, R. Hadland, D. Bate, A. Ramsey, M. Allen, A. Crawley, P. Hockley, T. Malzbender, D. Gelb, W. Ambrisco, and M. G. Edmunds. “Decoding the Ancient Greek Astronomical Calculator Known as the Antikythera Mechanism.” Nature 444 (2006): 587–591. See supplementary information.
- Freeth, T., A. Jones, J. M. Steele, and Y. Bitsakis. “Calendars with Olympiad Display and Eclipse Prediction on the Antikythera Mechanism.” Nature 454 (2008): 614–617. Supplementary Notes amended June 2, 2011.
- Jones, A. A Portable Cosmos: Revealing the Antikythera Mechanism, Scientific Wonder of the Ancient World. New York: Oxford University Press, 2017.
- Price, D. Gears from the Greeks. Transactions of the American Philosophical Society N.S. 64.7. Philadelphia: American Philosophical Society, 1974.
- Wright, M. T. “The Antikythera Mechanism and the Early History of the Moon-Phase Display.” Antiquarian Horology 29 (2006): 319–329.
- Wright, M. T. “The Antikythera Mechanism: Reconstruction as a Medium for Research and Publication.” In Reconstructions: Recreating Science and Technology of the Past. Edited by K. Staubermann, 1–20. Edinburgh: National Museums of Scotland, 2011.
- Wright, M. T., A. G. Bromley, and H. Magou. “Simple X-Ray Tomography and the Antikythera Mechanism.” In Archaeometry in South Eastern Europe: Second Conference in Delphi, 19th–21st April 1991. Edited by I. Liritzis and G. Tsokas, 531–543. Rixensart: Council of Europe, Belgium, 1995.
1. N. Kaltsas, E. Vlachogianni, and P. Bouyia, eds, The Antikythera Shipwreck: The Ship, the Treasures, the Mechanism. Exhibition Catalogue (Athens: Kapon Editions, 2012).
2. M. T. Wright, “A Planetarium Display for the Antikythera Mechanism,” Horological Journal 144 (2002): 169–173 and 193.
3. M. T. Wright, “Counting Months and Years: The Upper Back Dial of the Antikythera Mechanism,” Bulletin of the Scientific Instrument Society 87 (2005): 8–13.
5. Antikythera Mechanism Research Project, “Inscriptions of the Antikythera Mechanism.” Special issue of Almagest 7.1 (2016).
6. C. C. Carman and J. Evans, “On the Epoch of the Antikythera Mechanism and its Eclipse Predictor,” Archive for History of Exact Sciences 68 (2014): 693–774; and T. Freeth, “Eclipse Prediction on the Ancient Greek Astronomical Calculating Machine Known as the Antikythera Mechanism,” PLoS ONE 9.7 (2014): e103275.
7. M. G. Edmunds, “The Antikythera Mechanism and the Mechanical Universe,” Contemporary Physics 55 (2014): 263–285.
8. J. V. Field and M. T. Wright, “Gears from the Byzantines: A Portable Sundial with Calendrical Gearing,” Annals of Science 42 (1985): 87–138.