The Antikythera mechanism ( AN-tik-ih-THEER-ə, US also AN-ty-kih-) is an ancient Greek hand-powered orrery (model of the Solar System). It is the oldest known example of an analogue computer. It could be used to predict astronomical positions and eclipses decades in advance. It could also be used to track the four-year cycle of athletic games similar to an olympiad, the cycle of the ancient Olympic Games.
The artefact was among wreckage retrieved from a shipwreck off the coast of the Greek island Antikythera in 1901. In 1902, during a visit to the National Archaeological Museum in Athens, it was noticed by Greek politician Spyridon Stais as containing a gear, prompting the first study of the fragment by his cousin, Valerios Stais, the museum director. The device, housed in the remains of a wooden-framed case of (uncertain) overall size 34 cm × 18 cm × 9 cm (13.4 in × 7.1 in × 3.5 in), was found as one lump, later separated into three main fragments which are now divided into 82 separate fragments after conservation efforts. Four of these fragments contain gears, while inscriptions are found on many others. The largest gear is about 13 cm (5 in) in diameter and originally had 223 teeth. All these fragments of the mechanism are kept at the National Archaeological Museum, along with reconstructions and replicas, to demonstrate how it may have looked and worked.
In 2005, a team from Cardiff University led by Mike Edmunds used computer X-ray tomography and high resolution scanning to image inside fragments of the crust-encased mechanism and read faint inscriptions that once covered the outer casing. These scans suggest that the mechanism had 37 meshing bronze gears enabling it to follow the movements of the Moon and the Sun through the zodiac, to predict eclipses and to model the irregular orbit of the Moon, where the Moon's velocity is higher in its perigee than in its apogee. This motion was studied in the 2nd century BC by astronomer Hipparchus of Rhodes, and he may have been consulted in the machine's construction. There is speculation that a portion of the mechanism is missing and it calculated the positions of the five classical planets. The inscriptions were further deciphered in 2016, revealing numbers connected with the synodic cycles of Venus and Saturn.
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The instrument is believed to have been designed and constructed by Hellenistic scientists and been variously dated to about 87 BC, between 150 and 100 BC, or 205 BC. It must have been constructed before the shipwreck, which has been dated by multiple lines of evidence to approximately 70–60 BC. In 2022, researchers proposed its initial calibration date, not construction date, could have been 23 December 178 BC. Other experts propose 204 BC as a more likely calibration date. Machines with similar complexity did not appear again until the 14th century in western Europe.
History
Discovery
Captain Dimitrios Kontos (Δημήτριος Κοντός) and a crew of sponge divers from Symi island discovered the Antikythera wreck in early 1900, and recovered artefacts during the first expedition with the Hellenic Royal Navy, in 1900–1901. This wreck of a Roman cargo ship was found at a depth of 45 metres (148 ft) off Point Glyphadia on the Greek island of Antikythera. The team retrieved numerous large objects, including bronze and marble statues, pottery, unique glassware, jewellery, coins, and the mechanism. The mechanism was retrieved from the wreckage in 1901, probably July. It is unknown how the mechanism came to be on the cargo ship.
All of the items retrieved from the wreckage were transferred to the National Museum of Archaeology in Athens for storage and analysis. The mechanism appeared to be a lump of corroded bronze and wood. The bronze had turned into atacamite which cracked and shrank when it was brought up from the shipwreck, changing the dimensions of the pieces. It went unnoticed for two years, while museum staff worked on piecing together more obvious treasures, such as the statues. Upon removal from seawater, the mechanism was not treated, resulting in deformational changes.
On 17 May 1902, archaeologist Valerios Stais, together with his cousin, the Greek politician Spyridon Stais, found one of the pieces of rock had a gear wheel embedded in it. He initially believed that it was an astronomical clock, but most scholars considered the device to be prochronistic, too complex to have been constructed during the same period as the other pieces that had been discovered.
The German philologist Albert Rehm became interested in the device and was the first to propose that it was an astronomical calculator.
Investigations into the object lapsed until British science historian and Yale University professor Derek J. de Solla Price became interested in 1951. In 1971, Price and Greek nuclear physicist Charalampos Karakalos made X-ray and gamma-ray images of the 82 fragments. Price published a paper on their findings in 1974.
Two other searches for items at the Antikythera wreck site in 2012 and 2015 yielded art objects and a second ship which may be connected with the treasure ship on which the mechanism was found. Also found was a bronze disc, embellished with the image of a bull. The disc has four "ears" which have holes in them, and it was thought it may have been part of the Antikythera mechanism, as a cog wheel. There appears to be little evidence that it was part of the mechanism; it is more likely the disc was a bronze decoration on a piece of furniture.
Origin
The Antikythera mechanism is generally referred to as the first known analogue computer. The quality and complexity of the mechanism's manufacture suggests it must have had undiscovered predecessors during the Hellenistic period. Its construction relied on theories of astronomy and mathematics developed by Greek astronomers during the second century BC, and it is estimated to have been built in the late second century BC or the early first century BC.
In 2008, research by the Antikythera Mechanism Research Project suggested the concept for the mechanism may have originated in the colonies of Corinth, since they identified the calendar on the Metonic Spiral as coming from Corinth, or one of its colonies in northwest Greece or Sicily. Syracuse was a colony of Corinth and the home of Archimedes, and the Antikythera Mechanism Research Project argued in 2008 that it might imply a connection with the school of Archimedes. It was demonstrated in 2017 that the calendar on the Metonic Spiral is of the Corinthian type, but cannot be that of Syracuse. Another theory suggests that coins found by Jacques Cousteau at the wreck site in the 1970s date to the time of the device's construction, and posits that its origin may have been from the ancient Greek city of Pergamon, home of the Library of Pergamum. With its many scrolls of art and science, it was second in importance only to the Library of Alexandria during the Hellenistic period.
The ship carrying the device contained vases in the Rhodian style, leading to a hypothesis that it was constructed at an academy founded by Stoic philosopher Posidonius on that Greek island. Rhodes was a busy trading port and centre of astronomy and mechanical engineering, home to astronomer Hipparchus, who was active from about 140–120 BC. The mechanism uses Hipparchus' theory for the motion of the Moon, which suggests he may have designed or at least worked on it. It has been argued the astronomical events on the Parapegma of the mechanism work best for latitudes in the range of 33.3–37.0 degrees north; the island of Rhodes is located between the latitudes of 35.85 and 36.50 degrees north.
In 2014, a study argued for a new dating of approximately 200 BC, based on identifying the start-up date on the Saros Dial, as the astronomical lunar month that began shortly after the new moon of 28 April 205 BC. According to this theory the Babylonian arithmetic style of prediction fits much better with the device's predictive models than the traditional Greek trigonometric style. A study by Iversen in 2017 reasons that the prototype for the device was from Rhodes, but that this particular model was modified for a client from Epirus in northwestern Greece; Iversen argues it was probably constructed no earlier than a generation before the shipwreck, a date supported by Jones in 2017.
Further dives were undertaken in 2014 and 2015, in the hope of discovering more of the mechanism. A five-year programme of investigations began in 2014 and ended in October 2019, with a new five-year session starting in May 2020.
In 2022, researchers proposed the mechanism's initial calibration date, not construction date, could have been 23 December 178 BC. Other experts propose 204 BC as a more likely calibration date. Machines with similar complexity did not appear again until the fourteenth century, with early examples being astronomical clocks of Richard of Wallingford and Giovanni de' Dondi.
Design
The original mechanism apparently came out of the Mediterranean as a single encrusted piece. Soon afterwards it fractured into three major pieces. Other small pieces have broken off in the interim from cleaning and handling, and others were found on the sea floor by the Cousteau expedition. Other fragments may still be in storage, undiscovered since their initial recovery; Fragment F was discovered in that way in 2005. Of the 82 known fragments, seven are mechanically significant and contain the majority of the mechanism and inscriptions. Another 16 smaller parts contain fractional and incomplete inscriptions.
Many of the smaller fragments that have been found contain nothing of apparent value, but a few have inscriptions on them. Fragment 19 contains significant back door inscriptions including one reading "... 76 years ..." which refers to the Callippic cycle. Other inscriptions seem to describe the function of the back dials. In addition to this important minor fragment, 15 further minor fragments have remnants of inscriptions on them.
Information on the specific data obtained from the fragments is detailed in the supplement to the 2006 Nature article from Freeth et al.
Operation
On the front face of the mechanism, there is a fixed ring dial representing the ecliptic, the twelve zodiacal signs marked off with equal 30-degree sectors. This matched with the Babylonian custom of assigning one twelfth of the ecliptic to each zodiac sign equally, even though the constellation boundaries were variable. Outside that dial is another ring which is rotatable, marked off with the months and days of the Sothic Egyptian calendar, twelve months of 30 days plus five intercalary days. The months are marked with the Egyptian names for the months transcribed into the Greek alphabet. The first task is to rotate the Egyptian calendar ring to match the current zodiac points. The Egyptian calendar ignored leap days, so it advanced through a full zodiac sign in about 120 years.
The mechanism was operated by turning a small hand crank (now lost) which was linked via a crown gear to the largest gear, the four-spoked gear visible on the front of fragment A, gear b1. This moved the date pointer on the front dial, which would be set to the correct Egyptian calendar day. The year is not selectable, so it is necessary to know the year currently set, or by looking up the cycles indicated by the various calendar cycle indicators on the back in the Babylonian ephemeris tables for the day of the year currently set, since most of the calendar cycles are not synchronous with the year. The crank moves the date pointer about 78 days per full rotation, so hitting a particular day on the dial would be easily possible if the mechanism were in good working condition. The action of turning the hand crank would also cause all interlocked gears within the mechanism to rotate, resulting in the simultaneous calculation of the position of the Sun and Moon, the moon phase, eclipse, and calendar cycles, and perhaps the locations of planets.
The operator also had to be aware of the position of the spiral dial pointers on the two large dials on the back. The pointer had a "follower" that tracked the spiral incisions in the metal as the dials incorporated four and five full rotations of the pointers. When a pointer reached the terminal month location at either end of the spiral, the pointer's follower had to be manually moved to the other end of the spiral before proceeding further.
Faces
Front face
The front dial has two concentric circular scales. The inner scale marks the Greek signs of the zodiac, with division in degrees. The outer scale, which is a movable ring that sits flush with the surface and runs in a channel, is marked off with what appear to be days and has a series of corresponding holes beneath the ring in the channel.
Since the discovery of the mechanism more than a century ago, this outer ring has been presumed to represent a 365-day Egyptian solar calendar, but research (Budiselic, et al., 2020) challenged this presumption and provided direct statistical evidence there are 354 intervals, suggesting a lunar calendar. Since this initial discovery, two research teams, using different methods, independently calculated the interval count. Woan and Bayley calculate 354–355 intervals using two different methods, confirming with higher accuracy the Budiselic et al. findings and noting that "365 holes is not plausible". Malin and Dickens' best estimate is 352.3±1.5 and concluded that the number of holes (N) "has to be integral and the SE (standard error) of 1.5 indicates that there is less than a 5% probability that N is not one of the six values in the range 350 to 355. The chances of N being as high as 365 are less than 1 in 10,000. While other contenders cannot be ruled out, of the two values that have been proposed for N on astronomical grounds, that of Budiselic et al. (354) is by far the more likely."
If one supports the 365 day presumption, it is recognized the mechanism predates the Julian calendar reform, but the Sothic and Callippic cycles had already pointed to a 365+1/4 day solar year, as seen in Ptolemy III's attempted calendar reform of 238 BC. The dials are not believed to reflect his proposed leap day (Epag. 6), but the outer calendar dial may be moved against the inner dial to compensate for the effect of the extra quarter-day in the solar year by turning the scale backward one day every four years.
If one is in favour of the 354 day evidence, the most likely interpretation is that the ring is a manifestation of a 354-day lunar calendar. Given the era of the mechanism's presumed construction and the presence of Egyptian month names, it is possibly the first example of the Egyptian civil-based lunar calendar proposed by Richard Anthony Parker in 1950. The lunar calendar's purpose was to serve as a day-to-day indicator of successive lunations, and would also have assisted with the interpretation of the lunar phase pointer, and the Metonic and Saros dials. Undiscovered gearing, synchronous with the rest of the Metonic gearing of the mechanism, is implied to drive a pointer around this scale. Movement and registration of the ring relative to the underlying holes served to facilitate both a 1-in-76-year Callippic cycle correction, as well as convenient lunisolar intercalation.
The dial also marks the position of the Sun on the ecliptic, corresponding to the current date in the year. The orbits of the Moon and the five planets known to the Greeks are close enough to the ecliptic to make it a convenient reference for defining their positions as well.
The following three Egyptian months are inscribed in Greek letters on the surviving pieces of the outer ring:
ΠΑΧΩΝ (Pashons)
ΠΑΥΝΙ (Payni)
ΕΠΙΦΙ (Epiphi)
The other months have been reconstructed; some reconstructions of the mechanism omit the five days of the Egyptian intercalary month. The Zodiac dial contains Greek inscriptions of the members of the zodiac, which is believed to be adapted to the tropical month version rather than the sidereal:
ΚΡΙΟΣ (Krios [Ram], Aries)
ΤΑΥΡΟΣ (Tauros [Bull], Taurus)
ΔΙΔΥΜΟΙ (Didymoi [Twins], Gemini)
ΚΑΡΚΙΝΟΣ (Karkinos [Crab], Cancer)
ΛΕΩΝ (Leon [Lion], Leo)
ΠΑΡΘΕΝΟΣ (Parthenos [Maiden], Virgo)
ΧΗΛΑΙ (Chelai [Scorpio's Claw or Zygos], Libra)
ΣΚΟΡΠΙΟΣ (Skorpios [Scorpion], Scorpio)
ΤΟΞΟΤΗΣ (Toxotes [Archer], Sagittarius)
ΑΙΓΟΚΕΡΩΣ (Aigokeros [Goat-horned], Capricorn)
ΥΔΡΟΧΟΟΣ (Hydrokhoos [Water carrier], Aquarius)
ΙΧΘΥΕΣ (Ichthyes [Fish], Pisces)
Also on the zodiac dial are single characters at specific points (see reconstruction at ref). They are keyed to a parapegma, a precursor of the modern day almanac inscribed on the front face above and beneath the dials. They mark the locations of longitudes on the ecliptic for specific stars. The parapegma above the dials reads (square brackets indicate inferred text):
The parapegma beneath the dials reads:
At least two pointers indicated positions of bodies upon the ecliptic. A lunar pointer indicated the position of the Moon, and a mean Sun pointer was shown, perhaps doubling as the current date pointer. The Moon position was not a simple mean Moon indicator which would indicate movement uniformly around a circular orbit; rather, it approximated the acceleration and deceleration of the Moon's elliptical orbit, through the earliest extant use of epicyclic gearing.
It also tracked the precession of the Moon's elliptical orbit around the ecliptic in an 8.88 year cycle. The mean Sun position is, by definition, the current date. It is speculated that since significant effort was taken to ensure the position of the Moon was correct, there was likely to have also been a "true sun" pointer in addition to the mean Sun pointer, to track the elliptical anomaly of the Sun (the orbit of Earth around the Sun), but there is no evidence of it among the fragments found. Similarly, neither is there the evidence of planetary orbit pointers for the five planets known to the Greeks among the fragments. But see Proposed gear schemes below.
Mechanical engineer Michael Wright demonstrated there was a mechanism to supply the lunar phase in addition to the position. The indicator was a small ball embedded in the lunar pointer, half-white and half-black, which rotated to show the phase (new, first quarter, half, third quarter, full, and back). The data to support this function is available given the Sun and Moon positions as angular rotations; essentially, it is the angle between the two, translated into the rotation of the ball. It requires a differential gear, a gearing arrangement that sums or differences two angular inputs.
Rear face
In 2008, scientists reported new findings in Nature showing the mechanism not only tracked the Metonic calendar and predicted solar eclipses, but also calculated the timing of panhellenic athletic games, such as the ancient Olympic Games. Inscriptions on the instrument closely match the names of the months that are used on calendars from Epirus in northwestern Greece and with the island of Corfu, which in antiquity was known as Corcyra.
On the back of the mechanism, there are five dials: the two large displays, the Metonic and the Saros, and three smaller indicators, the so-called Olympiad Dial, which has been renamed the Games dial as it did not track Olympiad years (the four-year cycle it tracks most closely is the Halieiad), the Callippic, and the exeligmos.
