ARCHAEOLOGY DECODING AN Ancient Computer New explorations have revealed how the Antikythera By Tony Freeth mechanism modeled lunar motion and predicted eclipses, among other sophisticated tricks f it had not been for two storms 2 ,000 years scriptions. The discovery was a shock: until KEY CONCEPTS apart in the same area of the Mediterra- then, the ancients were thought to have made ■ The Antikythera mecha- Inean, the most important technological ar- gears only for crude mechanical tasks. nism is a unique mechani- tifact from the ancient world could have been Three of the main fragments of the Anti- cal calculator from sec- lost forever. kythera mechanism, as the device has come to be ond-century B.C. Greece. The fi rst storm, in the middle of the 1st cen- known, are now on display at the Greek Nation- Its sophistication sur- tury B.C., sank a Roman merchant vessel laden al Archaeological Museum in Athens . They look prised archaeologists with Greek treasures. The second storm, in A.D. small and fragile, surrounded by imposing when it was discovered in 1900, drove a party of sponge divers to shelter bronze statues and other artistic glories of an- 1901. But no one had an- off the tiny island of Antikythera, between Crete cient Greece. But their subtle power is even more ticipated its true power. and the mainland of Greece. When the storm shocking than anyone had imagined at fi rst . ■ Advanced imaging tools subsided, the divers tried their luck for sponges I fi rst heard about the mechanism in 2000. I have fi nally enabled re- in the local waters and chanced on the wreck. was a filmmaker, and astronomer Mike Ed- searchers to reconstruct Months later the divers returned, with backing munds of Cardiff University in Wales contacted how the device predicted from the Greek government. Over nine months me because he thought the mechanism would lunar and solar eclipses they recovered a hoard of beautiful ancient make a great subject for a TV documentary. I and the motion of the moon in the sky. Greek objects—rare bronzes, stunning glass- learned that over many decades researchers ware, amphorae, pottery and jewelry—in one of studying the mechanism had made considerable ■ Inscriptions on the mecha- the fi rst major underwater archaeological exca- progress, suggesting that it calculated astronom- nism suggest that it might vations in history. ical data, but they still had not been able to fully have been built in the One item attracted little attention at fi rst: an grasp how it worked. As a former mathemati- Greek city of Syracuse (now in modern Sicily), undistinguished, heavily calcifi ed lump the size cian, I became intensely interested in under- perhaps in a tradition of a phone book. Some months later it fell apart, standing the mechanism myself. that originated with revealing the remains of corroded bronze gear- Edmunds and I gathered an international col- Archimedes . wheels—all sandwiched together and with teeth laboration that eventually included historians, just one and a half millimeters long—along with astronomers and two teams of imaging experts. —The Editors plates covered in scientifi c scales and Greek in- In the past few years our group has reconstruct- 76 SCIENTIFIC AMERICAN December 2009 © 2009 SCIENTIFIC AMERICAN, INC. DECODING AN Ancient Computer ed how nearly all the surviving parts worked and cated on a 365-day calendar dial in the front. ANCIENT GREEKS knew how to what functions they performed. The mechanism (The dial could be rotated to adjust for an extra calculate the recurring patterns calculated the dates of lunar and solar eclipses, day every four years, as in today’s leap years.) At of lunar eclipses thanks to obser- modeled the moon’s subtle apparent motions the same time, the crank powered all the other vations made for centuries by the Babylonians. The Antiky- through the sky to the best of the available knowl- gears in the mechanism to yield the information thera mechanism would have edge, and kept track of the dates of events of so- corresponding to the set date. done those calculations cial signifi cance, such as the Olympic Games. A second front dial, concentric with the cal- for them—or perhaps for the Nothing of comparable technological sophistica- endar, was marked out with 360 degrees and wealthy Romans who could tion is known anywhere in the world for at least with the 12 signs representing the constellations afford to own it. The depiction a millennium afterward. Had this unique speci- of the zo diac [see box on pages 80 and 81]. These here is based on a theoretical men not survived, historians would have thought are the constellations crossed by the sun in its ap- reconstruction by the author that it could not have existed at that time. parent motion with respect to the “fi xed” stars— and his collaborators. “motion” that in fact results from Earth’s orbit- Early Pioneers ing the sun—along the path called the ecliptic. German philologist Albert Rehm was the fi rst Price surmised that the front of the mechanism person to understand , around 1905, that the probably had a pointer showing where along the Antikythera mechanism was an astronomical ecliptic the sun would be at the desired date. calculator. Half a century later , when science In the surviving fragments, Price identifi ed historian Derek J. de Solla Price, then at the the remains of a dozen gears that had been part Institute for Advanced Study in Princeton, N.J., of the mechanism’s innards. He also estimated described the device in a Scientifi c American their tooth counts—which is all one can do given article, it still had revealed few of its secrets. that nearly all the gears are damaged and incom- The device, Price suggested, was operated by plete. Later, in a landmark 1974 study, Price de- turning a crank on its side, and it displayed its scribed 27 gears in the main fragment and pro- output by moving pointers on dials located on its vided improved tooth counts based on the fi rst front and back. By turning the crank, the user x-rays of the mechanism, by Greek radiologist JEAN-FRANCOIS PODEVIN AND TONY FREETH could set the machine on a certain date as indi- Charalambos Karakalos. www.ScientificAmerican.com SCIENTIFIC AMERICAN 77 © 2009 SCIENTIFIC AMERICAN, INC. © 2009 SCIENTIFIC AMERICAN, INC. [THE PLACES] Rome S E L E U C I D ROMAN REPUBLIC PERGAMON KINGDOM Athens Babylon Corinth Syracuse RHODES Antikythera SICILY CRETE The Greek and Roman worlds, circa 145 B.C. PTOLEMAIC KINGDOM Where Was It From? The Antikythera mechanism was built around the middle of the 2nd century B.C., a time when Rome was expanding at the expense of the Greek-dominated Hellenistic kingdoms (green). Divers recovered its corroded remnants (including fragment at left) in A.D. 1901 from a shipwreck near the island of Anti- kythera. The ship sank around 65 B.C. while carrying Greek artistic treasures, perhaps from Pergamon to Rome. Rhodes had one of the major traditions of Greek astronomy, but the latest evidence points to a Corinthian origin. Syracuse, which had been a Corinthian colony in Sicily, is a possibility: the great Greek inventor Archimedes had lived there and may have left behind a technological tradition. Tooth counts indicate what the mechanism with computer scientist Allan Bromley of the calculated. For example, turning the crank to University of Sydney. They took the fi rst three- give a full turn to a primary 64-tooth gear rep- dimensional x-rays of the mechanism and showed resented the passage of a year, as shown by a that Price’s model of the mechanism had to be pointer on the calendar dial. That primary gear wrong. Bromley died in 2002, but Wright per- [ T H E A U T H O R ] was also paired to two 38-tooth secondary sisted and made signifi cant advances. For exam- Tony Freeth’s academic back- gears, each of which consequently turned by ple, he found evidence that the back dials, which ) ground is in mathematics and 64/38 times for every year. Similarly, the motion at fi rst look like concentric rings, are in fact spi- Freeth mathematical logic (in which he relayed from gear to gear throughout the mech- rals and discovered an epicyclic mechanism at the holds a Ph.D.). His award-winning anism; at each step, the ratio of the numbers of front that calculated the phase of the moon. career as a fi lmmaker culminated in a series of documentaries about gear teeth represents a different fraction. The Wright also adopted one of Price’s insights, increasing crop yields in sub-Saha- motion eventually transmitted to the pointers, namely that the dial on the upper back might be ran Africa, featuring the late Nobel which thus turned at rates corresponding to dif- a lunar calendar, based on the 19-year, 235- Peace Prize Laureate Norman ferent astronomical cycles. Price discovered that lunar-month cycle called the Metonic cycle. This ); COURTESY OF COLIN FREETH ( Borlaug. Since 2000 Freeth has the ratios of one of these gear trains embodied calendar is named after fi fth-century B.C. as- returned to an academic focus with fragment — research on the Antikythera mech- an ancient Babylonian cycle of the moon. tronomer Meton of Athens although it had anism. He is managing director Price, like Rehm before him, suggested that been discovered earlier by the Babylonians —and of the fi lm and television produc- the mechanism also contained epicyclic gear- is still used today to determine the Jewish festi- tion company Images First, and ing—gears spinning on bearings that are them- val of Rosh Hashanah and the Christian festival he is now developing a fi lm on selves attached to other gears, like the cups on a of Easter.