by Derek J. de Solla Price
From June 1959 Scientific American p.60-7
Among the treasures of the Greek National Archaeological
Museum in Athens are the remains of the most complex scientific object that has
been preserved from antiquity. Corroded and crumbling from 2,000 years under the
sea, its dials, gear wheels and inscribed plates present the historian with a
tantalizing problem. Because of them we may have to revise many of our estimates
of Greek science. By studying them we may find vital clues to the true origins
of that high scientific technology which hitherto has seemed peculiar to our
modern civilization, setting it apart from all cultures of the past.
From the evidence of the fragments one can get a good idea of
the appearance of the original object [see
illustration on page 62]. Consisting of a box with dials on the outside and
a very complex assembly of gear wheels mounted within, it must have resembled a
well- made 18ih-century clock. Doors hinged to the box served to protect the
dials, and on all available surfaces of box, doors and dials there were long
Greek inscriptions describing the operation and construction of the instrument.
At least 20 gear wheels of the mechanism have been preserved, including a very
sophisticated assembly of gears that were mounted eccentrically on a turntable
and probably functioned as a sort of epicyclic or differential, gear-system.
Nothing like this instrument is preserved elsewhere. Nothing
comparable to it is known. from any ancient scientific text or literary
allusion. On the contrary, from all that we know of science and technology in
the Hellenistic Age we should have felt that such a device could not exist. Some
historians have suggested that the Greeks were not interested in experiment
because of a contempt-perhaps induced by the existence of the institution of
slavery-for manual labor. On the other hand it has long been recognized that in
abstract mathematics and in mathematical astronomy they were no beginners but
rather "fellows of another college" who reached great heights of sophistication.
Many of the Greek scientific devices known to us from written descriptions show
much mathematical ingenuity, but in all cases the purely mechanical part of the
design seems relatively crude. Gearing was clearly known to the Greeks, but it
was used only in relatively simple applications. They employed pairs of
gears to change angular speed or mechanical ad- vantage, or to apply power
through a right angle, as in the water-driven mill.
Even the
most complex mechanical devices described by the ancient writers Hero of
Alexandria and Vitruvius contained only simple gearing. For example, the
taximeter used by the Greeks to measure the distance travelled by the wheels of
a carriage employed only pairs of gears (or gears and worms) to achieve the
necessary ratio of movement. It could be argued that if the Greeks knew the
principle of gearing, they should have had no difficulty in constructing
mechanisms as complex as epicyclic gears. We now know from the fragments in the
National Museum that the Greeks did make such mechanisms, but the knowledge is
so unexpected that some scholars at first thought that the fragments must belong
to some more modern device.
Can we in fact be sure that the
device is ancient? If we can, what was its purpose? What can it tell us of
the ancient world and of the evolution of modern science?
To
authenticate the dating of the fragments We must. tell the story of their
discovery, which involves the first (though inadvertent) adventure in underwater
archaeology. Just before Easter in 1900 a party of Dodecanese sponge-divers were
driven by storm to anchor near the tiny southern Greek island of Antikythera
(the accent is on the "kyth," pronounced to rhyme with pith). There, at a depth
of some 200 feet, they found the wreck of an ancient ship. With the help of
Greek archaeologists the wreck was explored; several fine bronze and marble
statues and other objects were recovered. The finds created great excitement,
but the difficulties of diving without heavy equipment were immense, and in
September, 1901, the "dig' was abandoned. Eight months later Valerios StaÎs, an
archaeologist at the National Museum, was examining some calcified lumps of
corroded bronze that had been set aside as possible pieces of broken statuary.
Suddenly he recognized among them the fragments of a mechanism.
It is now accepted that the wreck occurred during the first century B.C. Gladys
Weinberg of Athens has been kind enough to report to me the results of several
recent archaeological examinations of the amphorae, pottery and minor objects
from the ship. It appears from her report that one might reason-ably date the
wreck more closely as 65 B.C. ±15 years. Furthermore, since the identifiable
objects come from Rhodes and Cos, it seems that the ship may have. been voyaging
from these islands to Rome, perhaps without calling at the Greek mainland.
The fragment that first caught the eye of StaÎs was one of the
corroded, inscribed plates that is an integral part of the Antikythera
mechanism, as the device later came to be called. StaÎs saw immediately that the
inscription was ancient. In the opinion of the epigrapher Benjamin Dean Meritt,
the forms of the letters are those of the 'first century B.C.; they could hardly
be older than 100 B.C. nor younger than the time of Christ. The dating is
supported by the content of the inscriptions. The words used and their
astronomical sense are all of this period. For example, the most extensive and
complete piece of inscription is part of a parapegma (astronomical
calendar) similar to that written by one Geminos, who is thought to have lived
in Rhodes about 77 B.C. We may thus be reasonably sure that the mechanism did
not find its way into the wreck at some later period. Furthermore, it cannot
have been very old when it was taken aboard the ship as booty or merchandise.
As soon as the fragments had been discovered they were examined
by every available archaeologist; so began the long and difficult process of
identifying the mechanism and determining its function. Some things were clear
from the beginning. The unique importance of the object was obvious, and the
gearing was impressively complex. From the inscriptions and the dials the
mechanism was correctly identified as an astronomical device. The first
conjecture was that it was some kind of navigating instrument – perhaps an
astrolabe (a sort of circular star-finder map also used for simple
observations). Some thought that it might be a small planetarium of the kind
that Archirnedes is said to have made. Unfortunately the fragments were covered
by a thick curtain of calcified material and corrosion products, and these
concealed so much detail that no one could be sure of his conjectures or
reconstructions. There was nothing to do but wait for the slow and delicate work
of the Museum technicians in cleaning away this curtain. Meantime, as the work
proceeded, several scholars published accounts of all that was visible, and
through their labors a general picture of the mechanism began to emerge.
On the basis of new photographs made
for me by the Museum in 1955 I realized that the work of cleaning had reached a
point where it might at last be possible to take the work of identification to a
new level. Last summer, wilt the assistance of a grant from the American
Philosophical Society, I was able to visit Athens and make a minute examination
of the fragments. By good fortune George Stamires, a Greek epigrapher, was there
at the same time; he was able to give me invaluable help by deciphering and
transcribing much more of the inscriptions than had been read before. We are now
in the position of being able to "join" the fragments and to see how they fitted
together in the original machine and when they were brought up from the sea [see illustration's
on these two pages]. The success of this work has been most significant, for
previously it had been supposed that the various dials and plates had been badly
squashed together and distorted. It now appears that most of the pieces are very
nearly in their original places, and that we have a much larger fraction of the
complete device than had been thought. This work also provides a clue to the
puzzle of why the fragments lay unrecognized until StaÎs saw them. When they
were found, the fragments were probably held together in their original
positions by the remains of the wooden frame of the case. In the Museum the
waterlogged wood dried and shrivelled. The fragments then fell apart, revealing
the interior of the mechanism, with its gears and inscribed plates.
As a result of the new examinations we shall in due course be
able to publish a technical account of the fragments and of the construction of
the instrument. In the meantime we can tentatively summarize some of these
results and show how they help to answer the question. What is it?
There are four ways of getting at the answer First, if we knew
the details of the mechanism, we should know what it did. Second, if we could
read the dials, we could tell what they showed. Third, if we could understand
the inscriptions, they might tell us about the mechanism. Fourth, if we knew of
any similar mechanism, analogies might be helpful. All these approaches must be
used, for none of them is complete.
The geared wheels within
the mechanism were mounted on a bronze plate [third from right on
preceding page]. On one side of the plate we can trace all the gear wheels
of the assembly and can determine, at least approximately, how many teeth each
had and how they meshed together. On the other side we can do nearly as well,
but we still lack vital links that would provide a complete picture of the
gearing. The general pattern of the mechanism is nonetheless quite clear. An
input was provided by an axle that came through the side of the casing and
turned a crown-gear wheel. This moved a big, four-spoked driving-wheel that was
connected with two trains of gears that respectively led up and down the plate
and were connected by axles to gears on the other side of the plate. On that
side the gear-trains continued, leading through an epicyclic turntable and
coming eventually to a set of shafts that turned the dial pointers. When the
input axle was turned, the pointers all moved at various speeds around their
dials.
Certain structural features of the mechanism deserve
special attention. All the metal parts of the machine seem to have been cut from
a single sheet of low-tin bronze about two millimeters thick; no parts were cast
or made of another metal. There are indications that the maker may have used a
sheet made much earlier–uniform metal plate of good quality was probably rare
and expensive. All the gear wheels have been made with teeth of just the same
angle (60 degrees) and size, so that any wheel could mesh with any other. There
are signs that the machine was repaired at least twice; a spoke of the driving
wheel
has been mended, and a broken tooth in a small wheel has been
replaced. This indicates that the machine actually worked.
The casing was provided with three
dials, one at the front and two at the back. The fragments of all of them are
still covered with pieces of the doors of the casing and with other debris. Very
little can be read on the dials, but there is hope that they can be cleaned
sufficiently to provide information that might be decisive. The front dial is
just clean enough to say exactly what it did. It has two scales, one of which is
fixed and displays the names of the signs of the zodiac; the other is on a
movable slip ring and shows the months of the year. Both scales are carefully
marked off in degrees. The front dial fitted exactly over the main
driving-wheel, which seems to have turned the pointer by means of an eccentric
drum-assembly. Clearly this dial showed the annual motion of the sun in the
zodiac. By means of key letters inscribed on the zodiac scale, corresponding to
other letters on the parapegma calendar plate, it also showed the main risings
and settings of bright stars and constellations throughout the year.
The back dials are more complex and less legible. The lower one
had three slip rings; the upper, four. Each had a little subsidiary dial
resembling the "seconds" dial of a watch. Each of the large dials is inscribed
with lines about every six degrees, and between the lines there are letters and
numbers. On the lower dial the letters and numbers seem to record "moon, so many
hours; sun, so many hours"; we therefore suggest that this scale indicates the
main lunar phenomena of phases and times of rising and setting. On the upper
dial the inscriptions are much more crowded and might well present information
on the risings and settings, stations and retrogradations of the planets known
to the Greeks (Mercury, Venus, Mars, Jupiter and Saturn).
Some
of the technical details of the dials are especially interesting. The front dial
provides the only known extensive specimen from antiquity of a scientifically
graduated instrument. When we measure the accuracy of the graduations under the
microscope, we find that their average error over the visible 45 degrees is
about a quarter of a degree. The way in which the error varies suggests that the
arc was first geometrically divided and then subdivided by eye only. Even more
important, this dial may give a means of dating the instrument astronomically.
The slip ring is necessary because the old Egyptian calendar, having no leap
years, fell into error by 1/4 day every year; the month scale thus had to be
adjusted by this amount. As they are preserved the two scales of the dial are
out of phase by 13½ degrees. Standard tables show that this amount could
only occur in the year 80 B.C. and (because we do not know the month) at all
years just 120 years (i.e., 30 days divided by 1/4 day per year) before or after
that date. Alternative dates are archaeologically unlikely: 200 B.C. is too
early; 40 A.D. is too late. Hence, if the slip ring has not moved from its last
position, it was set in. 80 B.C. Furthermore, if we are right in supposing that
a fiducial mark near the month scale was put there originally to provide a means
of setting that scale in case of accidental movement, we can tell more. This
mark is exactly 1/2 degree away from the present position of the scale, and this
implies that the mark was made two years before the setting. Thus, although the
evidence is by no means conclusive, we are led to suggest that the instrument
was made about 82 B.C., used for two years (just long enough for the repairs to
have been needed) and then taken onto the ship within the next 30 years.
The fragments show that
the original instrument carried at least four large areas of inscription:
outside the front door, inside the back door, on the plate between the two back
dials and on the parapegma plates near the front dial. As I have noted, there
are also inscriptions around all the dials, and furthermore each part and hole
would seem to have had identifying letters so that the pieces could be put
together in the correct order and position. The main inscriptions are in a sorry
state and only short snatches of them can be read. To provide an idea of their
condition it need only be said that in some cases a plate has completely
disappeared, leaving behind an impression of its letters, standing up in a
mirror image, in relief on the soft corrosion products on the plate below. It is
remarkable that such inscriptions can be read at all.
But even
from the evidence of a few complete words one can get an idea of the subject
matter. The sun is mentioned several times, and the planet Venus once; terms are
used that refer to the stations and retrogradations of planets; the ecliptic is
named. Pointers, apparently those of the dials, are mentioned. A line of one
inscription signfficantly records "76 years, 19 years." This refers to the
well-known Calippic cycle of 76 years, which is four times the Metonic cycle of
19 years, or 235 synodic (lunar) months. The next line includes the number
"223," which refers to the eclipse cycle of 223 lunar months.
Putting together the information gathered so far, it seems reasonable to suppose
that the whole purpose of the Antikythera device was to mechanize just this sort
of cyclical relation, which was a strong feature of ancient astronomy. Using the
cycles that have been mentioned, one could easily design gearing that would
operate from one dial having a wheel that revolved annually, and turn by this
gearing a series of other wheels which would move pointers indicating the
sidereal, synodic and draconitic months. Similar cycles were known for the
planetary phenomena; in fact, this type of arithmetical theory is the central
theme of Seleucid Babylonian astronomy, which was transmitted to the Hellenistic
world in the last few centuries B.C. Such arithmetical schemes are
quite distinct from the geometrical theory of circles and epicycles in
astronomy, which seems to have been essentially Greek. The two types of theory
were unified and brought to their peak in the second century A.D. by Claudius
Ptolemy, whose labors marked the triumph of the new mathematical attitude toward
geometrical models that still characterizes physics today.
The
Antikythera mechanism must therefore be an arithmetical counterpart of the much
more familiar geometrical models of the solar system which were known to Plato
and Archimedes and evolved into the orrery and the planetarium. The mechanism is
like. a great astronomical clock without an escapement, or like a modern
analogue computer which uses mechanical parts to save tedious calculation. It is
a pity that we have no way of knowing whether the device was turned
automatically or by hand. It might have been held in the hand and turned by a
wheel at the side so that it would operate as a computer, possibly for
astrological use. I feel it is more likely that it was permanently mounted,
perhaps set in a statue, and displayed as an exhibition piece. In that case it
might well have been turned by the power from a water clock or some other
device. Perhaps it is just such a wondrous device that was mounted inside the
famous Tower of Winds in Athens. It is certainly very similar to the great
astronomical cathedral clocks that were built all over Europe during the
Renaissance.
It is to the prehistory of the
mechanical I clock that we must look for important analogies the Antikythera
mechanism and for an assessment of its significance. Unlike other mechanical
devices, the clock did not evolve from the simple to the complex. The oldest
clocks of which we are well informed were the most complicated. All the evidence
points to the fact that the clock started as an astronomical showpiece that
happened also to indicate the time. Gradually the timekeeping functions became
more important and the device that showed the marvelous clockwork of the heavens
became subsidiary. Behind the astronomical clocks of the 14th century there
stretches an unbroken sequence of mechanical models of astronomical theory. At
the head of this sequence is the Antikythera mechanism. Following it are
instruments and clocklike computers known from Islam, from China and India and
from the European Middle Ages. The importance of this line is very great,
because it was the tradition of clock- making that preserved most of man's skill
in scientific fine mechanics. During the Renaissance the scientific
instrument-makers evolved from the clockmakers. Thus the Antikythera mechanism
is, in a way, the venerable progenitor of all our present plethora of
scientific hardware.
A significant passage in this story has to
do with the astronomical computers of Islam. Preserved complete at the Museum of
History of Science at Oxford is a 13th-century Islamic geared calendar-computer
that has various periods built into it, so that it shows on dials the various
cycles of the sun and moon. This design can be traced back, with slightly
different periods but a similar arrangement of gears, to a manuscript written by
the astronomer al-Biruni about 1000 A.D. Such instruments am much simpler than
the Antikythera mechanism, but they show so many points of agreement in
technical detail that it seems clear they came from a common tradition. The same
60-degree gear teeth are used; wheels are mounted on square-shanked axles; the
geometrical layout of the gear assembly appears comparable. It was just at this
time that Islam was drawing on Greek knowledge and rediscovering ancient Greek
texts. It seems likely that the Antikythera tradition was part of a large
corpus of knowledge that has since been lost to us but was known to the Arabs.
It was developed and transmitted by them to medieval Europe, where it .became
the foundation for the whole range of subsequent invention in the field of
clockwork.
On the one hand the Islamic devices knit the whole
story together, and demonstrate that it is through ancestry and not mere
coincidence that the Antikythera mechanism resembles a modern clock. On the
other hand they show that the Antikythera mechanism was no flash in the pan but
was a part of an important current in Hellenistic civilization. History has
contrived to keep that current dark to us, and only the accidental underwater
preservation of fragments that would otherwise have crumbled to dust has now
brought it to light. It is a bit frightening to know that just before the fall
of their great civilization the ancient Greeks had come so close to our age, not
only in their thought, but also in their scientific technology.
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