In the year 1900, off the coast of the tiny island of Antikythera in the eastern Mediterranean Sea, a deep-sea diver looking for sponges found evidence of an ancient shipwreck on the sea floor. The diver was surprised because it looked to him like a “heap of dead naked people,” but these were just sculptures that were part of the ship’s cargo. While there was great value in the sculptures, there was one other object found, as yet unique among Greek artifacts, a chunk of corroded metal, that has taken the 120 years since its discovery to comprehend. This is the Antikythera mechanism.
The mechanism is a mechanical computer that, researchers have deduced, was capable of calculating the positions in the sky of the Sun, the Moon (and its phases), the known planets (Mercury, Venus, Mars, Jupiter and Saturn—also taking into account their retrograde motion), and also the timing of eclipses.
The mechanism contains toothed gear wheels made of bronze, 30 of them. Up to the time of its discovery, the thought that the Greeks or any other ancient civilization would have the capability of creating machines with gears was inconceivable. Yet there they are.
The Greeks relied on the previous astronomical achievements of the Babylonians, whose careful observations and recordings catalogued the relations between the periods of motions of the celestial bodies. For example, the Babylonians noted that it took 19 years for the Moon to complete 254 of its cycles. Gears in the mechanism have numbers of teeth that are either equal to the integers in these cycle relations, or are integer multiples of them, which was a hint regarding which celestial body the gear was responsible for.
The only way to study the inside of this ancient artifact without damaging it is through the use of x-rays. This has been the only way to obtain precise tooth counts on the gears. The mechanism has been x-rayed 3 times, most recently in 2005 using a technique that give resolution in three dimensions. Two discoveries were made from this most recent x-ray study: First was that the mechanism could be used to predict eclipses; Second was that the mechanism had a sophisticated way to account for the subtle variations in the motion of the Moon in its path around the Earth. To those who pay any attention to celestial motions, we vaguely know it takes approximately one month to complete one orbit (more precisely 29.5 days). However, if you take very precise measurements of the Moon’s positions as it moves through it’s cycle, you will note that it speeds up during a portion of its orbit, and slows down during another portion. In modern terms, we understand these variations of speed as being due to the fact that the Moon’s orbit is elliptical (rather than circular). When the Moon is closer to the earth, it moves faster, and when it’s further away, it moves more slowly. A more exaggerated example of this effect can be seen in the motion of comets, whose orbits are elliptical in the extreme, and which move very fast when they’re close to the Sun, but very, very slowly when they are flung back out to the furthest reaches of the Solar System.
Now the ancient Greeks held to a geocentric model of the Solar System, and they had no idea about elliptic orbits, but they had apparently performed sufficiently precise measurements to notice the variations in the Moon’s orbital speed. Further, they devised a very clever way to incorporate the variation in the Moon’s speed through the coupling of non-concentric gears in the mechanism. (For further details, again, see the first link above (Scientific American).)
One of the past researchers of the Antikythera mechanism discovered a passage of Cicero, the Roman Senator and author, which described something very similar to it:
One of these described a machine made by mathematician and inventor Archimedes (circa 287–212 B.C.E.) “on which were delineated the motions of the sun and moon and of those five stars which are called wanderers ... (the five planets) ... Archimedes ... had thought out a way to represent accurately by a single device for turning the globe those various and divergent movements with their different rates of speed.” 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.
It would be both amazing and fitting if the Antikythera mechanism were the intellectual child of Archimedes.
Finally, it is assumed that, though the mechanism its the only one that has been discovered, there must have been other mechanisms like it in the ancient world. (If there had been only one, why would you risk taking such a valuable object on a ship where it might sink and never be found again?) So there are likely other ones out there, which may be discovered in, say, the remains of other shipwrecks.
Let me end with a short BBC video about the Antikythera mechanism:
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