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Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism


The Antikythera Mechanism is a unique Greek geared device, constructed around the end of the second century bc. It is known1,2,3,4,5,6,7,8,9 that it calculated and displayed celestial information, particularly cycles such as the phases of the moon and a luni-solar calendar. Calendars were important to ancient societies10 for timing agricultural activity and fixing religious festivals. Eclipses and planetary motions were often interpreted as omens, while the calm regularity of the astronomical cycles must have been philosophically attractive in an uncertain and violent world. Named after its place of discovery in 1901 in a Roman shipwreck, the Antikythera Mechanism is technically more complex than any known device for at least a millennium afterwards. Its specific functions have remained controversial11,12,13,14 because its gears and the inscriptions upon its faces are only fragmentary. Here we report surface imaging and high-resolution X-ray tomography of the surviving fragments, enabling us to reconstruct the gear function and double the number of deciphered inscriptions. The mechanism predicted lunar and solar eclipses on the basis of Babylonian arithmetic-progression cycles. The inscriptions support suggestions of mechanical display of planetary positions9,14,15, now lost. In the second century bc, Hipparchos developed a theory to explain the irregularities of the Moon’s motion across the sky caused by its elliptic orbit. We find a mechanical realization of this theory in the gearing of the mechanism, revealing an unexpected degree of technical sophistication for the period.

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Figure 1: The surviving fragments of the Antikythera Mechanism.
Figure 2: A schematic view of the mechanism to illustrate the position of major inscriptions and dials.
Figure 3: The ‘pointer-follower’ lunar month indicator of the upper back dial.
Figure 4: Reconstruction of the back dials.
Figure 5: New reconstruction of the gear trains.
Figure 6: The ‘Hipparchos’ lunar mechanism mounted on gear e3.

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This work was financed by the Leverhulme Trust, the Walter Hudson Bequest, the University of Athens Research Committee and the Cultural Foundation of the National Bank of Greece. For essential support we thank the Ministry of Culture, Greece (P. Tatoulis), and the National Archaeological Museum of Athens (N. Kaltsas). We acknowledge help and advice from J. Ambers, J. Austin, G. Dermody, H. Forsyth, I. Freestone, P. Haycock, V. Horie, A. Jones, M. Jones, P. Kipouros, H. Kritzas, J. Lossl, G. Makris, A. Ray, C. Reinhart, A. Valassopoulos, R. Westgate, T. Whiteside, S. Wright and C. Xenikakis. Author Contributions T.F. carried out most of the CT analysis of structure and its interpretation. Y.B., A.T. and X.M. read, transcribed and translated the inscriptions. H.M and M.Z. catalogued the fragments, provided guidance on X-ray examination, and measured the fragments with J.H.S. R.H. led the team (D.B., A.R., M.A., A.C. and P.H.) that built and operated the Bladerunner CT machine, and provided CT reconstructions and advice. T.M., D.G. and W.A. built, operated and provided software for the PTM. M.G.E. was academic lead, and undertook the statistical analysis. T.F. and Y.B. organised the logistics of the experimental work, with inter-agency liaison by X.M. and J.H.S. The manuscript was written by T.F. and M.G.E. including material from Y.B., A.T., X.M., J.H.S., H.M. and M.Z. T.F. designed the illustrations.

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Correspondence to M. G. Edmunds.

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Supplementary notes

This file contains supplementary notes giving a key to fragment identification for Figure 1 of the main text and the dimensions of the fragments; further notes giving details of the script of the characters, their dating and the Greek text and its provisional translation from the Front Door inscriptions, the Back Door inscriptions and the Back Plate inscriptions near the Lower Back Dial. Also further notes giving a table to compare gear nomenclature and the gear tooth count estimates with previous estimates and to tabulate measured radii. Some notes are given on the individual gears and on the tooth count estimation procedure, including the effects of uncertainty in determining the centres of the gears. The gear train ratios are explained on the basis of simple Babylonian period relations. The equivalence of the epicyclic gearing and pin-and-slot mechanism to Hipparchos’ theory of the moon is proved. (PDF 1525 kb)

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Freeth, T., Bitsakis, Y., Moussas, X. et al. Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism. Nature 444, 587–591 (2006).

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