The world watched in astonishment last week as NASA delivered its one-ton Curiosity rover to the surface of Mars with astonishing precision, hitting a target area just 12 x 4 miles wide after eight months and 352 million miles in space. While this epic engineering feat was unfolding, I was working on an upcoming NOVA show about another phenomenal achievement involving the precise tracking of objects in space--this one dating back more than 2,000 years. Unlikely as it might seem, a common thread of human ingenuity connects both endeavors.

The Antikythera Mechanism is an intricate ancient Greek astronomical calculating device that has only recently yielded up its secrets. In 1901 AD, a group of sponge divers accidentally discovered it while exploring an ancient shipwreck off the tiny Greek island of Antikythera. All that was left of the Mechanism was an inconspicuous lump of heavily corroded bronze that broke into fragments after it was taken to the National Archaeological Museum in Athens. Traces of carefully cut gearwheels were noted on these fragments, which eventually led to speculation that it was some kind of calculating device. But "decoding" the device has only been possible in the last decade, thanks to state-of-the-art x-ray imaging and digitally enhanced surface photography. In a program scheduled for air on November 21, NOVA presents the unique inside story of how an Anglo-Greek scientific team succeeded in piecing together the exact design and function of all but one of the Mechanism's 30 known bronze gearwheels. Their story is a tour-de-force of scientific detective work.

antikythera.jpg The main fragment of the 2,000 year-old Antikythera Mechanism on display in the National Archaeological Museum, Athens, showing traces of one of the gearwheels.

As reconstructed by the team, the Mechanism was a kind of miniature planetarium, using dials and pointers to show the positions in the sky of the sun, moon, and five major planets. But it was also a computer that predicted the future. By turning a hand crank, the user could read off the date, hour, and even the color of future lunar eclipses, which the Greeks regarded as divine omens. The Athenian navy suffered a calamitous defeat at Syracuse 413 BC when their general interpreted a lunar eclipse as a warning not to put to sea, leading them to be trapped in the harbor by the enemy fleet.

One of the first clues that the Mechanism had something to do with eclipses was when British mathematician Tony Freeth, one of the scientific team, reconstructed a large bronze gearwheel with 223 teeth. That number corresponds to a famous ancient astronomical cycle called the Saros, first recognized by Babylonian sky watchers centuries before the Greeks, and based on a pattern of lunar eclipses that repeats every 223 lunar months. If the eclipse connection seemed obvious, other aspects were baffling, such as an enigmatic pin-and-slot mechanism visible on one of four small gears attached to the big one.

After months of struggling with the problem, Freeth finally realized with a shock that the pin-and-slot mechanism exactly models the ancient Greek theory of the moon's motion, including extremely subtle variations in the moon's position in the sky. By the second century BC, ancient Greek astronomers had calculated these tiny variations with great accuracy, and now Freeth discovered that the Mechanism's engineer had managed to translate them into a complex geared mechanism of equal precision.

The implications are remarkable: the Antikythera Mechanism emerges as the world's first known computer, able to predict eclipses accurately for decades to come. It demonstrates its makers' passion for state-of-the-art astronomical theory and extreme mechanical ingenuity.

Of course, the ancient Greeks didn't get everything right. Since each tooth of the bronze gearwheels had to be cut by hand, the Mechanism's accuracy was limited, while the pattern of eclipses would eventually get out of synch with the Saros cycle. In addition, the Greeks understood the tiny variations they observed in moon's position differently than a modern astronomer. Today, we know these irregularities are due to the moon's complex elliptical orbit around the Earth, while the Greeks explained them with the help of combined circular motions, or "epicycles." It seems likely that the maker of the Mechanism visualized the sun, moon, and planets as revolving on concentric spheres around the fixed Earth.

Yet if that vision of the cosmos was limited, the Antikythera Mechanism is eloquent testimony to qualities the ancient craftsman shared with today's NASA engineers: a drive to impose order on the universe through exact mathematical prediction, reflected in elegant, highly precise, miniaturized design.

Too often, television shows mystify the achievements of ancient technologists by attributing the building of monuments like Stonehenge or the pyramids to lost civilizations or aliens. This denies ancient people their ingenuity and the thread of connection that links our minds to theirs, despite the gulf of thousands of years that separates us.

For more about the Antikythera Mechanism, see:

Edmunds, Mike G., and Freeth, T. 2011. "Using Computation to Decode the First Known Computer," IEEE Computer, July 2011, p. 32.

Freeth, Tony, 2009. "Decoding an Ancient Computer" in Scientific American, December 2009, p. 76.

Marchant, Jo, 2009. Decoding the Heavens, Da Capo Press.

"Ancient Computer" airs on PBS Wednesday, November 21 at 9PM/8C.

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