as seen in a photograph of its present state.
2005 National Archaeological Museum in Athens
of the mysterious Antikythera mechanism
that challenges assumptions
about ancient technology...
He was among a group of Greek divers from the Eastern Mediterranean island of Symi who were searching for natural sponges.
They had sheltered from a violent storm near the tiny island of Antikythera, between Crete and mainland Greece. When the storm subsided, they dived for sponges and chanced on a shipwreck full of Greek treasures,
The "dead naked people" were marble sculptures scattered on the seafloor, along with many other artifacts.
Soon after, their discovery prompted the first major underwater archaeological dig in history. One object recovered from the site, a lump the size of a large dictionary, initially escaped notice amid more exciting finds.
Months later, however, at the National Archaeological Museum in Athens, the lump broke apart, revealing bronze precision gearwheels the size of coins.
According to historical knowledge at the time, gears like these should not have appeared in ancient Greece, or anywhere else in the world, until many centuries after the shipwreck.
The find generated huge
The device appears to be a geared astronomical calculation machine of immense complexity.
Today we have a reasonable grasp of some of its workings, but there are still unsolved mysteries.
We know it is at least as
old as the shipwreck it was found in, which has been dated to
between 60 and 70 B.C.E., but other evidence suggests it may have
been made around 200 B.C.E.
The team includes,
Our paper posits a new explanation for the gearing on the front of the mechanism, where the evidence had previously been unresolved.
We now have an even better appreciation for the sophistication of the device - an understanding that challenges many of our preconceptions about the technological capabilities of the ancient Greeks.
They viewed the night sky from a geocentric perspective - every night, as Earth turned on its axis, they saw the dome of stars rotating. The stars' relative positions remained unchanged, so the Greeks called them the "fixed stars."
These early astronomers
also saw bodies moving against the background of stars: the moon
goes through a rotation against the stars every 27.3 days; the sun
takes a year.
They were the deepest problem for astronomy at the time. Scientists wondered what they were and noticed that sometimes the wanderers move in the same direction as the sun - in "prograde" motion - then come to a stop and reverse direction to move in "retrograde."
After a while they reach another stationary point and resume prograde motion again.
These rotations are
called the synodic cycles of the planets - their cycles relative to
the sun. The seemingly strange reversals happen because, as we know
now, the planets orbit the sun - not, as the ancient Greeks
Predicting the positions of the planets along the ecliptic was very difficult for early astronomers. This task, it turns out, was one of the primary functions of the Antikythera mechanism.
Another function was to track the positions of the sun and moon, which also have variable motions against the stars.
Credit: Tony Freeth and Jen Christiansen (graphic),
UCL Antikythera Research Team (model)
Much of the mechanism's design relies on wisdom from earlier Middle Eastern scientists.
Astronomy in particular went through a transformation during the first millennium B.C.E. in Babylon and Uruk (both in modern-day Iraq).
The Babylonians recorded the daily positions of the astronomical bodies on clay tablets, which revealed that the sun, moon and planets moved in repeating cycles - a fact that was critical for making predictions.
The moon, for instance, goes through 254 cycles against the backdrop of the stars every 19 years - an example of a so-called period relation.
mechanism's design uses several of the Babylonian period relations.
He found, for instance, the number 19 inscribed on one of the surviving Antikythera fragments.
This figure was a reference to the 19-year period relation of the moon known as the Metonic cycle, named after Greek astronomer Meton but discovered much earlier by the Babylonians.
On the same fragment, Rehm found the numbers 76, a Greek refinement of the 19-year cycle, and 223, for the number of lunar months in a Babylonian eclipse-prediction cycle called the saros cycle.
astronomical cycles were the driving force behind Babylonian
It referred to remarkable quotations by Roman lawyer, orator and politician Cicero (106-43 B.C.E.).
One of these described a machine made by mathematician and inventor Archimedes (circa 287-212 B.C.E.),
This machine sounds just like the Antikythera mechanism.
The passage suggests that Archimedes, although he lived before we believe the device was built, might have founded the tradition that led to the Antikythera mechanism.
It may well be that the Antikythera mechanism was based on a design by Archimedes.
In the early 1970s they finally got to peek inside. Price worked with Greek radiologist Charalambos Karakalos to obtain x-ray scans of the fragments.
To their astonishment, the researchers found 30 distinct gears:
Karakalos, with his wife, Emily, was able to estimate the tooth counts of the gearwheels for the first time, a critical step in understanding what the mechanism calculated.
The machine was looking
more complicated than anyone had conceived.
Despite these shortcomings, Price identified a gear train - a set of linked gears - that calculated the average position of the moon on any specific date by using its period relation of 254 sidereal rotations in 19 years.
Driven by a prominent feature on the front of the mechanism called the main drive wheel, this gear train starts with a 38-tooth gear (two times 19, as a gear with just 19 teeth would be a bit too small).
It seems that the device could be used to predict the positions of the sun, moon and planets on any specific day in the past or future.
The maker of the machine would have had to calibrate it with the known positions of these bodies. A user could then simply turn a crank to the desired time frame to see astronomical predictions.
The mechanism displayed positions, for instance, on a "zodiac dial" on the front of the mechanism, where the ecliptic was divided into a dozen 30-degree sections representing the constellations of the zodiac.
Based on the x-ray data,
Price developed a complete model of all the gearing on the device.
My first paper, in fact, "Challenging the Classic Research," was a comprehensive demolition of most of Price's proposed gearing structure for the machine.
Nevertheless, Price correctly determined the relative positions of the major fragments and defined the overall architecture of the machine, with date and zodiac dials at the front and two large dial systems at the back.
Price's achievements were
a significant step in decoding the Antikythera mystery.
In collaboration with Australian professor of computer science Alan G. Bromley, Wright carried out a second x-ray study of the mechanism in 1990 using an early 3-D x-ray technique called linear tomography.
Bromley died before this
work bore fruit, but Wright was persistent, making important
advances, for example, in identifying the crucial tooth counts of
the gears and in understanding the upper dial on the back of the
with its precision gears bearing teeth
about a millimeter long,
is completely unlike anything else
from the ancient
X-Tek Systems (now owned by Nikon) developed a prototype x-ray machine to take high-resolution 3-D x-ray images using microfocus x-ray computed tomography (x-ray CT).
Hewlett-Packard used a
brilliant digital imaging technique called polynomial texture
mapping for enhancing surface details.
The new x-rays revealed a large, 223-tooth gear at the rear of the mechanism that turns a pointer around a dial that spirals out, making four turns in total that are divided into 223 sections, for 223 months.
Named after the customary name of the Babylonian eclipse cycle, the saros dial predicts which months will feature eclipses, along with characteristics of each eclipse as described by inscriptions in the mechanism.
The finding revealed an
impressive new feature of the device, but it left a massive problem:
a group of four gears lying within the circumference of the large
gear that appeared to have no function.
These gears turned out to calculate the variable motion of the moon in a very beautiful way. In modern terms, the moon has variable motion because it has an elliptical orbit: when it is farther from Earth, it moves more slowly against the stars; when it is closer, it moves more quickly.
The moon's orbit,
however, is not fixed in space: the whole orbit rotates in a period
of just under nine years. The ancient Greeks did not know about
elliptical orbits, but they explained the moon's subtle motion by
combining two circular motions in what is called an epicyclic
He had studied two of the four mysterious gears at the back of the mechanism. He saw that one of them has a pin on its face that engages with a slot on the other gear. It might seem to be a useless arrangement because the gears will surely just turn together at the same rate.
But Wright noticed that the gears turn on different axes separated by just over a millimeter, meaning that the system generates variable motion. All these details appear in the x-ray CT scan.
The axes of the gears are
not fixed - they are mounted epicyclically on the large 223-tooth
Calculating the epicyclic theory of the moon with epicyclic pin-and-slot gears in this subtle and indirect way was an extraordinary conception by the ancient Greeks.
This ingenuity reinforces the idea that the machine was designed by Archimedes.
This research on the back dials and gearing completed our understanding of the back of the mechanism, reconciling all the evidence to date. My colleagues and I published our findings in 2006 in Nature.
The other side of the device, however, was still very much a mystery.
show inscriptions on the Antikythera mechanism,
including a list of planetary cycles
on the front cover and a user's manual.
Hidden message: X-ray CT scans made in 2005
revealed previously unseen inscriptions
on the Antikythera mechanism, including
a list of planetary cycles on the front cover (shown here)
and a "user's manual" on the back cover.
Credit: © 2005 Nikon X-Tek Systems
It is not a flat disc like most of the other gears:
Following Price, Wright proposed that an extensive epicyclic system - the two-circles idea the Greeks used to explain the odd reversing motions of the planets - had been mounted on the main drive wheel.
Wright even constructed an actual model gearing system in brass to show how it worked.
In 2002 he published a groundbreaking planetarium model for the Antikythera mechanism that displayed all five planets known in the ancient world. (The discovery of Uranus and Neptune in the 18th and 19th centuries, respectively, required the advent of telescopes.)
Wright showed that the
epicyclic theories could be translated into epicyclic gear trains
with pin-and-slot mechanisms to display the planets' variable
It even featured eight coaxial outputs - tubes all centered on a single axis - that brought information to the front display of the device. Was it really plausible that the ancient Greeks could build such an advanced system?
I now believe that Wright's conception of coaxial outputs must be correct, but his gearing system does not match the economy and ingenuity of the known gear trains.
The challenge our UCL
team faced was to reconcile Wright's coaxial outputs with what we
knew about the rest of the device.
In addition to showing the gears in three dimensions, these scans made an unexpected revelation - thousands of new text characters hidden inside the fragments and unread for more than 2,000 years. In his research notes from 1905 to 1906, Rehm proposed that the positions of the sun and planets were displayed in a concentric system of rings.
The mechanism originally had two covers - front and back - that protected the displays and included extensive inscriptions. The back-cover inscription, revealed in the 2005 scans, was a user manual for the device.
In 2016 Alexander
Jones, a professor of the history of astronomy at New York
University, discovered definitive evidence for Rehm's idea within
this inscription: a detailed description of how the sun and planets
were displayed in rings, with marker beads to show their positions.
Yet previous models had failed to incorporate this ring system because of a technical problem that we could not solve.
Wright had discovered that the device used a semi-silvered ball to show the phase of the moon, which it calculated mechanically by subtracting an input for the sun from an input for the moon.
But such a process appeared to be incompatible with a ring system for displaying the planets because the outputs for Mercury and Venus prevented the moon-phase device from accessing the input from the sun gear system.
In 2018 Higgon, one of the graduate students on our UCL team, came up with a surprisingly simple idea that neatly fixed this technical problem and explained a mysterious pierced block on one of the spokes of the main drive wheel. This block could transmit the "mean sun" rotation (as opposed to the variable "true sun" rotation) directly to the moon-phase device.
This setup enabled a ring
system for the front of the Antikythera mechanism that fully
reflected the description in the back-cover inscription.
Earlier research assumed
that they would be based on the planetary period relations derived
by the Babylonians. But in 2016 Jones made a discovery that forced
us to discard that assumption.
These numbers were astonishing...
No previous research had suggested that ancient astronomers knew them. In fact, they represent more accurate period relations than the ones found by the Babylonians.
It seems that the makers of the Antikythera device discovered their own improved period relations for two of the planets:
Jones never figured out how the ancient Greeks derived both these periods. We set out to try ourselves.
Dacanalis, our other UCL graduate student, assembled a comprehensive list of the planetary period relations and their estimated errors from Babylonian astronomy.
Eventually we found a process, developed by philosopher Parmenides of Elea (sixth to fifth century B.C.E.) and reported by Plato (fifth to fourth century B.C.E.), for combining known period relations to get better ones.
Over the years the original mass of the Antikythera mechanism
has split into 82 pieces. Figuring out how they all fit together
has been a challenging puzzle for researchers.
The largest fragment (top left) holds the main drive wheel.
Credit: © 2005 National Archaeological Museum in Athens
The method must be accurate to match the known period relations for Venus and Saturn, and it must be factorizable so the planets could be calculated with gears small enough to fit into the mechanism.
To make the system economical, different planets could share gears if their period relations shared prime factors, reducing the number of gears needed. Such economy is a key feature of the surviving gear trains.
Based on these criteria,
our team derived the periods 462 and 442 using the idea from
Parmenides and employed the same methods to discover the missing
periods for the other planets where the inscriptions were lost or
For Mercury and Venus, we theorized economical five-gear mechanisms with pin-and-slot devices, similar to Wright's mechanisms for these planets.
We found strong supporting evidence for our reconstruction in one four-centimeter-diameter fragment. Inside this piece, the x-ray CT shows a disk attached to a 63-tooth gear, which turns in a d-shaped plate.
The number 63 shares the prime factors 3 and 7 with 462 (the Venus period). A gear train using the 63-tooth gear could be designed to match a bearing on one of the spokes of the main drive wheel. A similar design for Mercury matches the features on the opposite spoke.
These observations gave
us great confidence that we were on the right track for Mercury and
Working independently, Christián C. Carman of the National University of Quilmes in Argentina and I had shown that the subtle indirect gearing system for the variable motion of the moon could be adapted for these planets.
Our UCL team proved that these gearing systems could be extended to incorporate the new period relations for the planets.
This system allowed the
Antikythera makers to mount several gears on the same plate and
design them to precisely match the period relations.
The dimensions of the
available spaces between the plates were exactly right to
accommodate these systems, with some spare capacity and some
evidence still unexplained.
Eclipses happen only when the sun is close to one of these nodes during a full or new moon.
Medieval and renaissance astronomers called a double-ended pointer for the nodes of the moon a "dragon hand."
The epicyclic gearing for this dragon hand also exactly explained a prominent bearing on one of the spokes that had previously appeared to have no function.
We had finally explained all the features on the main drive wheel; we published our findings in March 2021 in Scientific Reports.
The front cover also
displayed the moon's phase, position and age (the number of days
from a new moon), and the dragon hand that showed eclipse years and
This writing is a formulaic list of the synodic events of each planet (such as its conjunctions with the sun and its stationary points) and the intervals in days between them.
Our insight was that the inscriptions on the front could refer to index letters on the planetary rings:
Because the left-hand
side of the inscription, where we would expect these index letters
to be, is missing, we cannot prove the hypothesis - but it is a
They had plumbing and used steam to operate equipment.
But before the discovery of the Antikythera mechanism, ancient Greek gears were thought to be restricted to crude wheels in windmills and water mills.
Aside from this discovery, the first precision-geared mechanism known is a relatively simple - yet impressive for the time - geared sundial and calendar of Byzantine origin dating to about C.E. 600.
It was not until the 14th century that scientists created the first sophisticated astronomical clocks.
The Antikythera mechanism, with its precision gears bearing teeth about a millimeter long, is completely unlike anything else from the ancient world.
We have strong reasons to believe this object can't have been the only model of its kind - there must have been precursors to its development.
But bronze was a very valuable metal, and when an object like this stopped working, it probably would have been melted down for its materials. Shipwrecks may be the best prospects for finding more of them.
As for why the technology
was seemingly lost for so long before being redeveloped, who knows?
There are many gaps in the historical record, and future discoveries
may well surprise us.
We believe our work is a significant advance, but there are still mysteries to be solved. The UCL Antikythera Research Team is not certain that our reconstruction is entirely correct because of the huge loss of evidence. It is very hard to match all of the surviving information.
Regardless, we can now see more clearly than ever what a towering achievement this object represents...