Aug 11, 2012

Nikos Deja Vu - The Antikythera Device (REVISED)

The Antikythera Device

Introduction Video

At the western entrance to the Aegean Sea, midway between the islands of Crete and Kythera, rises little Antikythera. It was off that island in 1900 that a sponge diver found, on the bottom, the wreck of an ancient ship loaded with statues, amphorae, and other objects.

This wreck was the first great underwater find of modern archaeology. It yielded not only a rich hoard of art treasures but an astonishingly sophisticated scientific instrument. But while the statues and pottery were recognized at once as the work of Greek artisans around the time of Christ, the bronze instrument, encrusted with calcareous deposits, lay ignored.

As it gradually dried, the ancient wood casing and internal parts cracked and split into four flat fragments, the inner sides of which revealed parts of geared wheels together with some barely legible inscriptions. As cleaning exposed more gears and inscriptions, scholars affirmed that the device was a navigational tool, an astrolabe, used to determine the altitude of the sun and other celestial bodies. This identification was remarkable enough, considering that only simple implements had previously turned up from the Hellenistic period.

There speculation about the artifact stood until 1951, when Professor Derek de Solla Price of Yale University began what would become the definitive study of the Antikythera Mechanism, as it had come to be called.

Attempting to re-create the device, Price and his Greek co-workers used x-rays and gamma-rays to probe the internal structure. The mechanism, he discovered, contained layer upon layer of variously sized gears. After long calculations of possible gear ratios, Price arrived at an astonishing conclusion: Some ancient Greek inventor had designed a mechanism that mimicked the actual movements of the sun, moon, and planets, past, present, and future. The Antikythera Mechanism was a 2,000-year-old ANALOG COMPUTER



As hypothetically reconstructed by Price, the original mechanism consisted of a rectangular box about twelve by six by three inches, with the bronze machinery contained by wooden sides. The front and back were covered by bronze doors on which the inventor had inscribed detailed instructions. Three dials displayed the device's readout. The first contained two concentric bands, one showing the signs of the zodiac, a sixteen-degree-wide belt straddling the apparent path of the sun, and the other the names of the Greek months. A pointer revealed the position of the sun in the zodiac for every day of the year. A second dial displayed an eighteen-year cycle of solar eclipses, while a third kept track of the different phases of the moon.

Inside the box, some thirty-nine bronze gears were meshed on parallel planes and set in motion by a handle that needed to be turned once a day. These were linked through a kind of toothed turntable that acted as a differential gear train, permitting two shafts to rotate at different speeds. Operating on the same principle that allows the traction wheels on modern automobiles to turn at different rates on curves, the differential gear had long been assumed to be an invention of the seventeenth century.

Fascinated by Price's hypothesis, physicist Allan Bromley, a professor at the University of Sydney, enlisted the help of clockmaker Frank Percival to construct a working modern of the Antikythera Mechanism. Using techniques that would have been available to the ancient Greeks, in 1987 the pair assembled a functional replica that largely verified a majority of Price's theories.

Still, the identity of the machines original creator is lost in time. Some scholars believe that the Antikythera Mechanism originated on the island of Rhodes, where cultural conditions two thousand years ago might have been right for the emergence of the necessary skills. There, without any fanfare, a consummate mechanical artisan who had a strong knowledge of the heavens somehow fashioned the ancient computer of Antikythera, the device that Price called one of "the greatest basis mechanical inventions of all time."

Rising from its water grave after two millennia, the mechanism belied the prevailing view that ancient Greece was a land of brilliant theoreticians who, pampered by slaves, disdained the physical. Our ancestors, it seems had mechanical genius after all...

The documentary clip

A Diagram of the Mechanism

Antikythera Device

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 significantly 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.

Ú In Memoriam Professor N. Mavridis (V 23/12/2003)
UCLA Computer Science

Nikos Deja Vu

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