Mysterious ancient gear wheels. Complex astronomical calculations. Precise manufacturing. It sounds like a script from a television show about ancient aliens. But these are terms used to describe a 2,000-year-old device found off the coast of the island of Antikythera in the Aegean Sea. The researcher Tony Freeth wrote of the discovery: “If the ancient Greek scientists could produce these gear systems 2,000 years ago, the whole history of western technology would have to be rewritten.”
The shipwreck is a tale of two storms. About 60 B.C., a huge cargo vessel bearing immense riches was sunk in about 100 feet of water in a dangerous area of the sea. Sponge divers found the lost treasure in 1900 after surviving a treacherous storm themselves. Among the findings pulled from the sea bed over nine months was a complex, clock-like device known as the Antikythera Mechanism.
The archaeological expedition would yield one of the greatest troves of ancient marine artifacts ever found, including marble and bronze statues, coins, pottery and gold jewelry. But the greatest of all was the multi-geared mechanical astronomical calculator that has come to be known as the world's oldest analog computer. When pulled from the water it was very likely in one piece, but it wasn't long until the object had disintegrated into 82 corroded fragments. It was curated and is on display at the National Archaeological Museum in Athens. (For more on early analog computers, see The Analytical Engine: A Look Back at Babbage's Timeless Designs.)
Carbon dating of the rest of the objects seems to place them all in the first century B.C., but the Antikythera Mechanism may have already been an antique by this time. It is believed that the ship was traveling from Asia Minor with what archaeologist Dr. Brendan Foley surmised may have been the dowry of a wealthy woman. The mechanism itself is thought to be much older. (For more on old (but not quite as old) technology, see New Technologies That Are Older Than You Think.)
Initial investigation in 1902 by archaeologist Valerios Stais at the museum discovered the presence of gears in the lump and held the opinion that the mechanism was too complex to have come from the same period as the other items. It did not receive serious treatment again until 1974, when Derek de Solla Price published his paper “Gears from the Greeks. The Antikythera Mechanism: A Calendar Computer from ca. 80 B.C.” Further studies by Michael Wright, of the Science Museum and later with Imperial College of London, brought the research much further. Currently, the Antikythera Mechanism Research Project, led by Tony Freeth and others, continues to make significant advances in understanding the device.
The Antikythera Mechanism was made by the Greeks. Their contributions to art, philosophy, and architecture are known to all, but their advances in technology may come as a surprise. The device has at least 30 gears, and may have had as many as 50 or 60. Similar devices were mentioned by Cicero (106–43 B.C.), and the technology has been attributed to the mathematician Archimedes (287–212 B.C.) by some, and to the astronomer Hipparchus (c.190–c.120 B.C.) by others. The first possible eclipse noted by the mechanism seems to refer to the date 205 B.C., so some believe that the device was older than that. It seems clear that the machine discovered in the Aegean Sea was the result of many years of development, and may have been a miniaturized version of larger prototypes from long before its construction.
The mechanism had remarkable capabilities. It could show lunar cycles, predict lunar and solar eclipses, and show the positions of the Sun, Moon and five known planets. It even accounted for the quadrennial athletic contests. Mike Edmunds of Cardiff University considers the find “more valuable than the Mona Lisa.” According to Tony Freeth, the mechanism “upsets all our ideas about what the ancient Greeks were capable of. It rewrites the history of technology.”
The math bears some investigation. The aim of the device is to calculate astronomy using period relations, and the key to that was the intelligent design and manufacture of interactive gears. The number of teeth on the gears aligned with the body of knowledge gained from the Babylonians. A study of the mechanism is a study of astronomy.
At issue for some of the calculations are the Saros cycle (a period of approximately 6,585.3 days, or 18 years 11 days 8 hours, that applies to both lunar and solar eclipses), the 19-year Metonic cycle, the 76-year Calippic cycle and the 54-year Exeligmos cycle. NASA offers a clear summary of the Saros cycle with modern precision. It is a natural harmony between three of the Moon's orbital periods:
- Synodic Month (new moon to new moon) = 29.530589 days = 29d 12h 44m 03s
- Anomalistic Month (perigee to perigee) = 27.554550 days = 27d 13h 18m 33s
- Draconic Month (node to node) = 27.212221 days = 27d 05h 05m 36s
One Saros cycle is the equivalent of:
- 223 Synodic Months = 6585.3223 days = 6585d 07h 43m
- 239 Anomalistic Months = 6585.5375 days = 6585d 12h 54m
- 242 Draconic Months = 6585.3575 days = 6585d 08h 35m
The Greeks were indebted to the Babylonians for much of their cosmology – including eclipses described on clay tablets in astronomical diaries – but they also looked to the innovations of their own mathematicians and scholars in the study of astronomy. Meton of Athens introduced the Metonic cycle in 432 B.C. The Metonic cycle was the basis for the Greek calendar. Here we are talking about 235 synodic months, or 19 years, a period in which the phase of the moon returns to approximately the same date of the year. The Calippic cycle extends to four Metonic periods, or 76 years (4 × 19). It was proposed by Calippus in 330 B.C., and is seen to be an improvement on the Metonic calculations. The Exeligmos cycle is the triple Saros, or 54 years. This period constituted true eclipse repeats, or the appearance of eclipses with similar properties.
We won't attempt to run all the figures here. But it is noteworthy that a gear with 223 teeth was responsible for calculating the eclipses of the Moon and Sun. The numbers 19, 53, 127 and 223 are all prime numbers, and play important roles in the calculations. The device even compensated for epicycles of the planets and the variable motion of the Moon using a pin-and-slot mechanism. To investigate further, study “The Cosmos in the Antikythera Mechanism” by Tony Freeth and Alexander Jones. You will also benefit from two sessions recorded on video by the Computer History Museum on the subject.
The Antikythera Mechanism was an ancient portable computer. It was housed in a small wooden box with dimensions 340 mm × 180 mm × 90 mm. As Freeth put it: “The Antikythera Mechanism was small, light and portable. They managed to cram nearly all their knowledge of astronomy into this small-geared device. It was 'the theory of almost everything' in a box, very similar to today's modern laptop computer.”
The front of the box had two large dials. The outer dial showed the 365-day Egyptian calendar. To account for the fact that there are 365 ¼ days in the year and no leap year was used, it was simple just to remove the pin from the dial and move it over one notch (equaling one day). The inner dial displayed the Zodiac calendar, which comprises twelve divisions of celestial longitude. The front of the device is thought to have used dials to determine the locations of the five known planets: Mercury, Venus, Mars, Jupiter and Saturn.
The back side of the computer contained two spiral dials. The top one displayed the 19-year Metonic calendar, divided into 235 lunar months. An inner dial showed the 4-year Panhellenic Games and the 76-year Callippic cycle. The bottom dial displayed the 223-month eclipse prediction cycle, based on the Saros cycle. Glyphs on the dial made further predictions about the lunar and solar eclipses, including the time of day and possibly even the radiant colors. Within that dial is the exeligmos dial. Michael Wright demonstrates the device using his own model of the Antikythera Mechanism. A hand crank advanced all dials through the various calendars and cycles. What is amazing is that in theory (allowing for potential gear slippage) this ancient computer appears to have been able to calculate the Saros, Metonic, Calippic and Exeligmos cycles and make other determinations with stunning accuracy.
Without modern conveniences such as we have, the heavens were on full display for the citizens of the ancient world. Keeping track of Sun, Moon and stars was very important to them. Theirs was a geocentric universe, understood by the celestial models of Aristotle and Ptolemy. The inquiring minds of the Greeks were thirsty for knowledge about their world and everything in it. The phases of the Moon told them when to plant crops, fight battles, hold religious festivals or travel at night. The night sky had guided the ancients from time immemorial, and now they were able to predict it by scientific means. Researcher Alexander Jones summed it up this way: “Here was Greek genius at its height, the great and divine cosmos, represented though mechanism, by scientists who wished to show there was no mathematical challenge beyond their abilities.”
(To learn more, watch BBC's documentary "The 2000 Year-Old Computer - Decoding the Antikythera Mechanism.")