Two thousand years ago in Greece, scholars created a complex analog computer used to predict lunar and solar eclipses, phases of the moon, and the paths of planets and the sun across the zodiac. The Antikythera mechanism was found in a cargo ship that sank around 70 B.C. off the Greek island of Antikythera. It was created some decades earlier around 150 to 100 B.C., perhaps on the Isle of Rhodes by scholars in that hub of astronomy and engineering.
The instrument was ahead of its time, with intricate components as advanced as some 18th century clocks. The expertise behind its construction was lost for centuries after the device went missing. Its technological complexity would not be seen again for at least a millennium until the creation of medieval clocks in European cathedrals.
The Antikythera mechanism was held in a wooden box with approximate dimensions of 340 x 180 x 90 mm. It was covered with bronze plates on the front and back containing inscriptions describing the geocentric cosmology that the device simulates. The Parapegma (star calendar) inscriptions above and below the central dials listed the dates of arrival and departure of the stars. The center display consisted of little spheres representing the Sun, the Moon, and the five planets known to the Greeks at the time. Their orbital distances were indicated by the spheres' radial positions on pointer pins. The revolving pointers traversed the outer graduated rings representing the zodiac and the months of the Egyptian calendar. In this way, the positions of the heavenly bodies could be determined at specific times of the year.
The entire mechanism was hand-operated by turning a crank located on the side of the box. This input drove the complex gearing system located within. Constructed with an impressive degree of engineering precision, the mechanism was comprised of some 40 gears ranging in thickness from just 1.0 mm to 2.7 mm. Gears were stacked in layers, with the gaps between them measuring only 1.4 mm per gear. This arrangement of gears performed the computations that calculated each of the outputs displayed on the dials.
Planetary and Solar Mechanisms
The surviving physical fragments of the Antikythera mechanism do not represent the instrument in its entirety. To fill in the missing pieces, various models of the device have been developed. In Tony Freeth’s and Alexander Jones’ model, the instrument was comprised of several modules of gears for each of the concepts displayed by the device. These include mechanisms for the lunar anomaly (eclipse), solar anomaly, inferior planets and superior planets.
The planetary and solar motions modeled by the mechanism are based on the ancient Greek deferent and epicycle theories relating two circular motions. Transferring this theory into a physical form can be accomplished by using epicyclic gearing in which the center of one gear revolves around the center of another. Freeth and Jones describe the function of the planetary mechanism as follows:
A gear is turned at the rate of the deferent and a second gear mounted epicyclically on the first gear is turned at the rate of the epicycle. A slotted follower, turning on the deferent axis, follows a pin attached to the epicyclic gear. The follower is connected to a tube and a pointer is attached to the tube. This outputs the variable motion.
The complexity of the Antikythera Mechanism suggests that the device was crafted as part of an engineering tradition developed over decades or centuries. Expertise in both pin-and-slot techniques and epicyclic gearing simulating variable planetary motions was required to make the Antikythera mechanism functional. In addition, an unprecedented level of precision in the construction of the components was necessary. The gears were fabricated down to accuracies of a few tenths of a millimeter by hand with tools such as files, drills, lathes, hammers, pliers, rulers, and dividers.
The engineering knowledge represented in the intricate operations of the Antikythera mechanism is substantial. That this information was lost and lay dormant for centuries is just one peculiarity in the annals of human history. Consider an alternate history in which that knowledge was preserved and expanded upon. What wonders might ancient scholars and engineers have devised centuries ahead of their appearance in the traditional timeline?
If you’d like to see the Antikythera mechanism in person, its remaining fragments and full replicas are on display at the National Archaeological Museum in Athens, Greece.
The Cosmos in the Antikythera Mechanism, ISAW Papers 4 (February, 2012), Tony Freeth and Alexander Jones
Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism, Nature 444, 587-591 (30 November 2006), Freeth et al.