It’s no secret that a great deal of Western civilization was informed by the ancient Greeks. They revolutionized mathematics and geometry, developing astronomy along the way. They built ornate statues, beautiful temples to the gods, and amphitheaters for live entertainment with astonishing acoustics. The influence of the ancient Greeks shaped almost every field of human knowledge, from the arts and architecture to politics, philosophy, science, and technology.
Like the Babylonians, the Greeks paid close attention to the night sky. Our nearest celestial neighbor, the Moon, was particularly important to them from a planning perspective. For instance, debts might be due on the new Moon. By heeding the Moon’s phases and taking note of eclipse cycles, they found that their harvests were more fruitful, and they had fewer incidents at sea.
As savvy and well-rounded as ancient Hellenistic culture appears to have been, it’s not unreasonable to imagine that the Greeks could have created some kind of computing machine to make their Moon-centered scheduling easier. Based on fragments from in a shipwreck that was discovered in 1900, it seems they did exactly this. Based on scientific dating of the coins and pottery found in the wreck and inscriptions on the bronze remnants, historians and scientists believe the Greeks created a mechanical computer capable of calculating the positions of the Sun and the Moon on any given day. This marvelous device is known as the Antikythera mechanism.
The mechanism was housed in a wooden box and controlled with a knob on one side. It is believed that the front of the box was a display made up of a set of concentric rings with graduations, and that each ring corresponded with one celestial body. Pointers attached perpendicularly to output gears moved around the rings as the knob was turned, showing the paths and positions of these celestial bodies over time. This Earth-centric planetarium also displayed the phase of the Moon as well as the positions of the five major planets known to the ancient Greeks—Mercury, Venus, Mars, Jupiter, and Saturn.
Provided the scientific dating of the coins and pottery found among the shipwreck is correct, the Antikythera mechanism also marks the earliest appearance of the differential gear. It is believed that the designer used a pin and slot arrangement to join two gears of differing tooth counts in order to model and compensate for the irregular, elliptical orbit of the Moon. Through a complex series of gearing ratios, this ancient computer could predict solar and lunar eclipses, displaying models of them at the user’s fingertips just as they would happen in the sky.
Storms and Shipwrecks
It’s a wonder the Antikythera mechanism was discovered at all. In 1900, a group of Greek sponge divers were sailing back to Symi, an island in the Rhodes region of Southern Greece. Their ship was in a channel north of Crete, near the small island of Antikythera. They became caught in a storm and were forced take shelter around the island’s main port of Potamós. Once the storm passed, they decided to scout the area for sponges before returning home.
The divers didn’t find any sponges off the coast of Antikythera, but they did find treasure. Among the steep rock shelf below laid the remains of a large ship. Scattered among the ancient timbers, partially obscured by rock and silt, the divers could see the disembodied heads, arms, and legs of large bronze and marble statuary. After recovering what they were able to haul, the captain took note of their bearings and the two ships set sail for Symi.
At eight miles square, Antikythera is a fraction the size of Kythera, the island it opposes in the Sea of Crete. Because of its dimensions, location, and low levels of human activity, the island has long been a major stop for migratory birds. Antikythera has seen a lot of fluctuation in usage over the last few thousand years, and the current population is around fifty inhabitants. Because of its staggering cliff faces and craggy shoreline, the tiny island has been a big hazard for all of maritime history.
The sponge divers and crew spent the next six months figuring out what to do about the treasures they had found. Rather than loot the wreckage site, they decided to notify the standing authorities about their discovery. The ship’s captain went to Athens with a bronze arm that his crew had found in the wreck. Almost immediately, the government sanctioned an official recovery mission.
It was agreed that the crew of sponge divers who made the discovery would revisit the site and turn whatever they found over to the Greek government. They ended up recovering the largest collection to date of artifacts from classical antiquity. They brought back scores of treasure from the ancient Hellenistic period, including corroded bronze fragments of something they couldn’t identify. All of the artifacts went to the National Archaeological Museum in Athens.
Believe it or not, the bronze fragments that comprised the Antikythera Mechanism more or less sat around unnoticed at the museum for eight months after the exploration. This was due to the sheer volume of bronze brought into the museum from the wreck. It required a lot of sorting and re-sorting as statues and other pieces were catalogued and reconstructed by the staff. During one of these re-shufflings, someone noticed inscriptions and graduation markings on one of the fragments, and they began to receive attention befitting the oldest known mechanical computer.
Shortly after the exploration of the shipwreck in 1901, it was reported that the fragments of the mysterious object comprised some sort of astrolabe, a type of inclinometer used to locate the positions of celestial bodies. A naval historian named Konstantin Rados contested this theory, arguing that it was too complex of an instrument to be a mere astrolabe. Albert Rehm, a scholar of ancient language and textual interpretation, loosely compared it to the Sphere of Archimedes, a device the ancient Greek mathematician used for computing the volume and surface area of a sphere with relation to those of a cylinder.
Gears from the Greeks
The first in-depth analysis of the Antikythera mechanism was performed by a British science historian and Yale professor named Derek de Solla Price. He began his study of the fragments in the 1950s, using still photographs and radiographs to make sense of the gear ratios. Price’s examination continued unabated into the 1970s. In June of 1974, he published his findings with the American Philosophical Society in a monograph called Gears from the Greeks: The Antikythera Mechanism: A Calendar Computer from Ca. 80 B.C. The 72-page labor of love is Price’s full inquiry into the matter, ranging from the happenstance of the shipwreck’s discovery and early explorations of the mechanism to all that he finds conclusive and inconclusive about its origins, inner workings, meaning, and the shortlist of possible creators.
Price leaves no fragment unexamined, but he was limited by the technologies available at the time. In Gears from the Greeks, he writes that every visible cog was so corroded that not a single one could yield an accurate tooth count. He nevertheless took the task on, working with artist Beverly Pope to create the intricate line drawings you see reprinted here.
Through extensive use of radiography, Price came to the conclusion that the mechanism contained at least 27 gears. It’s now believed that the complete mechanism contained at least 30 gears.
Price was sure that if he could get an accurate count of any of the gears’ teeth, he could begin to unlock the mysteries of the mechanism. This was quite a difficult task to undertake, given that he was working with two-dimensional x-rays of gears that meshed here and overlapped there in a very tight configuration. Undaunted, he literally traced around the gears to count the teeth. Price believed the largest gear was made with 223 or 225 teeth and represented the Sun. He wasn’t sure of this gear’s exact significance, and proposed that a gear representing the eclipse cycle would have 223 teeth, while a gear standing for the Metonic cycle would have 235 teeth.
Price also counted a gear with 127 teeth, and supposed that it could have been used to follow the moon’s movement around the Earth. This number is significant as it is equal to half the number of Moon orbits in a 19-year solar cycle. Scientists believe that the mechanism’s creator did this to simplify the operation, and that a multiplier gear converted the number to 254.
No one had visited the site of the wreck since the initial dredging in 1901. In 1976, an expedition led by Jacques Cousteau recovered many more objects that helped provide clues to the age of the Antikythera mechanism. Among the ship timbers and bronze figures, Cousteau and his team found bronze and silver coins from the Asia Minor colonies of Pergamon and Ephesus, which are now part of Turkey. A coin expert named Panagiotis Tselekas was able to date these coins as having been struck between 70 and 60 BC. Cousteau’s team had also recovered pieces of pottery and many large wine jugs, which experts were able to date to 65-50BC.
All of the available evidence points to the likelihood that the ship was an immense trading vessel belonging to the Roman Empire. At the time, only a few ports in the Mediterranean such as these three were big enough to handle a ship of its enormity, and it was probably sailing from Asia Minor back to Rome. The ship was heavily laden with objects, which many researchers believe that the Romans had looted from Pergamon, Ephesus, and Rhodes.
New Technology, New Findings
Several years later, a mechanical engineer and former curator of London’s Science museum named Michael Wright performed his own extensive study of the Antikythera mechanism over a period of twenty-five years. Wright studied Derek de Solla Price’s monograph and ultimately concluded that Price’s reconstruction of the mechanism was fundamentally incorrect. In fact, Wright went so far as to call it bizarre and incomplete, suggesting that Price took some creative liberties to fill in the gaps, and to make the astronomical calculations work out against his gear tooth counts.
But Michael Wright didn’t just throw stones. In addition to writing numerous papers about the mechanism, he collaborated with Australian computer historian Allan Bromley to create a complete reconstruction of the device in bronze and wood, drawing upon his mechanical knowledge and the history of craft techniques. Wright also took his own photographs of the fragments and performed radiography with a device he created to adapt X-ray equipment for this purpose. Together, they created plans for the model by compiling data from hands-on examination and from their own measurements of the delicate fragments.
The front display of Wright’s model was an Earth-centric planetarium with indicators for the Sun, Moon, and the five major planets of ancient Greek astronomy. In creating his reconstruction, Wright attempted to stay as true to the original as the radiographs would prove. He machined gears from thin bronze that measured between one and two millimeters thick, which he proposed was the kind of stock that all of the metallic parts of the mechanism were made from.
A few years later, an international team of scientists with access to much better imaging technology confirmed that the largest gear did indeed bear 223 teeth. This particular gear was crucial to reconciling the 12-month solar year with the 29.5-day lunar month—a cycle of 19 solar years exactly equals 235 lunar months. This number 235, which indicates what the Greeks referred to as the Metonic cycle, is repeated in a series of individual graduations on the back of the mechanism. In Michael Wright’s model, a spiral groove with a resettable arm predicted the dates of solar and lunar eclipses as an output function of the internal gearing.
One of Wright’s most insightful suppositions about the device was that the gearing that drove the display on the back side, where eclipse prediction takes place, appeared to have a pin and slot mechanism. His adapted x-rays revealed a slot and the ghost of a circular piece inside of it. Wright ultimately determined that the pin gear and the slot gear pivot on slightly offset axes. Both are connected to the 223-tooth gear, which keeps track of the Moon’s orbit. This meant that the pin and slot mechanism was a differential gearing solution designed to compensate for the irregular, elliptical orbit of the Moon around the Earth.
Another of Wright’s contributions was his discovery of a fixed boss in the main fragment. This suggests that the Antikythera mechanism was designed to show epicyclical motion with subsystems that moved about a central gear. Wright believed that the Antikythera mechanism had likely been altered, or hacked, if you will at one or more points after it was made. Primarily, he supposes the two spiral output displays on the rear of the device were repurposed from some other piece of equipment and added later, citing the appearance of the enclosure’s remains.
Around the time that Michael Wright was studying the mechanism and creating his reconstruction, a team of scientists, astronomers, and mathematicians had come together in Athens to further research the ancient calendar computer. They worked in conjunction with the Antikythera Mechanism Research Project (AMRP) to continue investigation into the mechanism and published an article in 2006 detailing their findings about the machine.
Shortly after publication, British mathematician and filmmaker Tony Freeth of the AMRP collaborated with Alexander Jones, a professor of the History of Exact Sciences in Antiquity at New York University’s Institute for the Study of the Ancient World. Together, they came up with a computer model of the Antikythera mechanism that incorporates newer knowledge about the device.
In 2005, Tony Freeth engaged scientists from Hewlett-Packard who had created a special technique for creating enhanced images of the surfaces and details of paintings. A dome covered with lamps flashes light on the object in question from various angles while a series of still photos are taken. Freeth convinced them to go to Athens and use this equipment to photograph the tiny inscriptions on the mechanism. The images did wonders for furthering the team’s understanding. They were able to confirm once and for all that the largest gear definitely had 223 teeth. Another inscription directly mentions the number ‘235’ as well as the spiral display on the back with reference to the Metonic cycle.
Freeth and Jones were able to use the month inscriptions to help determine where the Antikythera Mechanism was made. At the time of the shipwreck, each of the Greek states used its own calendar scheme. The month inscriptions on the fragments pointed to Corinth, or a colony of Corinth such as Syracuse on the island of Sicily.
How it Works – the Current Model
Years of study, measurement, photographs, and educated guesswork by several people have provided an increasingly clear picture of the mechanism’s structure. Essentially, it is a collection of gear wheels that was likely contained in a wooden box and operated with a hand crank on the side. As the crank was turned, the indicators on the front would spin around, each modeling the path of one of the major celestial bodies known to the ancient Greeks. There were separate indicators for the Sun, Moon, and five major planets known at the time. The device’s smallest indicator was a tiny sphere, colored half black and half white by those who would later model it. This little ball spun independently of its indicator arm, showing the phases of the Moon as it moved through each day of the solar calendar.
According to Michael Wright, the inner workings contain multiple gear trains for the calendar year, including the true Sun and mean Sun. Two subsystems emerge from this train, one based on the Sun and one on the Moon. The Sun side contains gearing that computes the four-year cycle of the Pan-Hellenic Olympic Games as well as the nineteen-year Metonic cycle, which is a common multiple of both the solar year and the lunar month. It also computes the seventy six-year Callippic cycle, which is four times the length of the Metonic cycle and was proposed by Greek astronomer Callippus around 330BC as an improvement over the Metonic cycle.
The ancient Babylonian astronomers had discovered what Edmund Halley would come to call the Saros cycle, which describes the full cycle of eclipse activity between the Sun and the Moon. The Babylonians found that every 223 synodic (lunar) months, the Sun, Moon, and Earth return to the same relative geometry, resulting in the same type of eclipse.
The lunar gear train connects to a lunar anomaly platform and on to an eclipse gear train that shows the 223-month Saros cycle and its proposed improvement, the 669-month Exeligmos cycle. There are additional epicyclical gearing mechanisms for the five major planetary bodies known to the ancient Greeks: Venus, Mercury, Mars, Jupiter, and Saturn.
These internal gearing systems output their calculations on the back of the device through two spiral grooves. One is divided to show the calendar cycles for the Olympic Games, the Metonic cycle, and the Callippic cycle. The other acts as an eclipse predictor, operating on the 223-month Saros cycle to show the dates of both solar and lunar eclipses. A pointer spans the radius of each ring of the groove, while an attached needle rides in the slot. This design made it possible to reset the output by lifting the pointer as one would lift the arm of a record player.
Who Made the Antikythera Mechanism?
Derek de Solla Price believed there were a few people who could have created this technological wonder. One of them was Andronicus Kyrrhestes, a Macedonian who had built a kind of ancient weather station called the Tower of Winds. His octagonal structure featured a wind vane and a complex sundial on each of its faces. A frieze around the exterior of the tower paid homage to each of the eight prevailing wind gods. Inside the tower was a clepsydra, or water clock, which was driven by water from the Acropolis.
If not Kyrrhestes, Price supposes the Antikythera mechanism was conceived by some Rhodes engineer studying under Posidonios, a renowned philosopher and meteorologist who took a great interest in measuring the distances to the Moon and stars. If the Antikythera mechanism had been the work of Archimedes, Price believes that his name would certainly have been attached to it in historical records, followed closely by a great deal of praise for having invented the differential gear. In his book, De Republica, Cicero described a device he had seen while studying at Rhodes. This was a planetarium constructed by Posidonios. In his writing, Cicero wrote of some novel differences between this new planetarium and an earlier astronomical device he greatly admired, the sphere of Archimedes.
The Future of the Antikythera Mechanism
Until recently, there had only been two officially sanctioned recovery missions of the Antikythera shipwreck: the original dredging, and Jacques Cousteau’s expedition in 1976. But in September and October of 2014, a group of divers, archaeologists, and scientists returned to the site in partnership with the Hellenic navy. With the help of some cutting-edge diving gear, they were able to recover even more objects, ranging from common tableware to treasures of antiquity, such as the giant bronze spear belonging to a life-sized warrior statue.
The group had many goals for this expedition. One of these was to map the full extent of the shipwreck with a 3D digital blueprint. A bright yellow autonomous underwater vehicle (UAV) named Sirius took care of that by providing high-resolution stereo images. Sirius was built by the marine robotics arm of the Australian Centre of Field Robotics at the University of Sydney.
Because the ship’s remains are so far underwater, diving to the site and staying for more than a few minutes is terribly dangerous. The group’s other main goal was testing a new diving suit technology called the Exosuit, which allows for dives down to 1,000 feet. With these suits, the divers could safely stay down at the wreck for over 30 minutes a day.
Both Michael Wright’s physical bronze model and Tony Freeth’s computer model of the mechanism greatly moved the needle of understanding with regard to its inner workings and reason for creation. Wright is not the only craftsman who is moved by the mechanism’s mechanical marvels. In 2010, an Apple engineer named Andrew Carol completed a working replica of the mechanism which he constructed entirely from LEGO Technic pieces.
Carol’s model is much larger than the original device, mostly due to the difference between custom-machining brass gears and modeling the same oddly-numbered cogs with pre-formed ABS gears. It also uses about twice as many gears as the original, mostly because Carol had to reckon with the way the calendar has changed over the last 2,000+ years.
In early 2014, a USC mechanical engineering student modeled the Antikythera mechanism using Solidworks. He based his files on Tony Freeth’s and Alexander Jones’ gearing proposal. He has shared the CAD files through his site, theshamblog.com, noting that they are not quite fit for 3D printing in their current state. In December 2014, he made comment about his plan to release a version intended for lasers and wood.
A Mystery Wrapped in an Enigma
There are many layers to the mystery of the Antikythera mechanism. For instance, it could have been one of a kind, or it may be the only one of many such computers to survive from antiquity.
And what was the Antikythera mechanism doing at the bottom of the Sea of Crete? Was it looted from a Greek colony along with hundreds of works of art and pieces of jewelry, or was it among the Roman shipwreck’s remains by coincidence? In his monograph, Derek de Solla Price discusses the Antikythera mechanism as a historical document, offering the point that much of what remains from ancient Greek society are the sturdier pieces of evidence like architecture, jewelry, and pottery. No Hellenistic artifact had yet been found that was anywhere near as complex as the Antikythera mechanism. Prior to its discovery, the earliest-surviving object of similar complexity dates from 1000A.D—an astrolabe created by a Persian scholar named al-Bīrūnī.
After the fall of the ancient Greek civilization, it is believed that the kind of craftsmanship and technology the mechanism represents moved east through the Byzantine Empire and on to the Arabs after the fall of Constantinople. Complex mechanical clockwork on a smaller scale began to appear in Central Europe around the end of the Middle Ages, and the automata that much of modern technology emerged from in the Victorian Era.
Diagrams Reprinted by Permission
Diagrams reprinted by permission
Gears from the Greeks: The Antikythera Mechanism–A Calendar Computer from ca. 80 B.C.
by Derek De Solla Price (ISBN 9780871696472, published November, 1974)
This article was specifically written for the Hackaday Omnibus vol #02. Order your copy of this limited edition print version of Hackaday.