Message #13418

[Bianca]

Dept. of Archeology
Fragmentary Knowledge
by John Seabrook (page 4)


The National Museum in Athens took no special pains in displaying the lumps of bronze. Item 15087 wasn’t much to look at. When the physicist Richard Feynman visited, in 1980, there was little information explaining what the Mechanism was. In a letter to his family, later published in the book “What Do You Care What Other People Think?,” the physicist wrote that he found the museum “slightly boring because we have seen so much of that stuff before. Except for one thing: among all those art objects there was one thing so entirely different and strange that it is nearly impossible. It was recovered from the sea in 1900 and is some kind of machine with gear trains, very much like the inside of a modern wind-up alarm clock.” When Feynman asked to know more about item 15087, the curators seemed a little disappointed. One said, “Of all the things in the museum, why does he pick out that particular item, what is so special about it?”

For the Greeks, as for other ancient civilizations, astronomy was a vital and practical form of knowledge. The sun and the moon were the basis for calendars by which people marked time. The solar cycle told farmers the best times for sowing and harvesting crops, while the lunar cycle was commonly used as the basis for civic obligations. And, of course, for mariners the stars provided some means of navigating at night.

Xenophon Moussas, one of the two Greek astronomers who are part of the research project, is a compact, soft-spoken man. He grew up in Athens, and as a boy, visiting the museum, he often pondered the Mechanism; now as a professor of astrophysics, he uses it to connect with his undergraduate students, for whom ancient technology is often more compelling than ancient theory.

One evening in January, Moussas led me on a memorable walk around the archeological park in central Athens, which includes both the Greek and the Roman agoras. As a quarter moon shone in the clear night sky, illuminating the ruined temples and markets, Moussas narrated the story of how the ancients slowly learned to recognize patterns and serial events in the movements of the stars, and to use them to tell time and to predict future astronomical events. “It was a way of keeping track not of time as we think of it,” he told me, “but of the movement of the stars—a deeper time.”

For the Greeks, like the Babylonians before them, the year consisted of twelve “lunations,” or new-moon-to-new-moon cycles, each of which lasted an average of twenty-nine and a half days. The problem with a lunar calendar is that twelve lunar cycles takes about eleven days less than one solar cycle. That means that if you don’t make regular adjustments to the calendar the seasons soon slip out of synch with the months, and after eighteen years or so the summer solstice will occur in December. Finding a system that reconciled the lunar year with the solar year was the great challenge of calendar-making.


from the issuecartoon banke-mail thisMost ancient societies readjusted their calendars by adding a thirteenth, “intercalary” month every three years or so, although methods of calculating the length of these months, and when they should be added, were never precise. Babylonian astronomers hit upon an improvement. They discovered that there are two hundred and thirty-five lunar months in nineteen years. In other words, if you observe a full moon on April 4th, there will be another full moon in that same place on April 4th nineteen years later. This cycle, which eventually came to be known as the Metonic cycle, after the Greek astronomer Meton of Athens, was an extremely useful tool for keeping the lunar calendar and the solar calendar in synch. (The Metonic cycle is still used by the Christian Churches to calculate the correct day for celebrating Easter.) The Babylonians also established what would come to be known as the saros cycle, which is a way of predicting the likely occurrence of eclipses. Babylonian astronomers observed that eighteen years, eleven days, and eight hours after an eclipse a nearly identical eclipse will occur. Eclipses were believed by many ancient societies to be omens that, depending on how they were interpreted, could foretell the future of a monarch, for example, or the outcome of a military campaign.

The Greeks, in turn, discovered the Callippic cycle, which consisted of four Metonic cycles minus one day, and was an even more precise way to reconcile the cycles of the sun and the moon. But the Greeks’ real genius was to work out theories to explain these cycles. In particular, they brought the concept of geometry to Babylonian astronomy. As Alexander Jones, a professor of classics at the University of Toronto, put it to me recently, “The Greeks saw the Babylonian formulas in terms of geometry—they saw all these circles all spinning around each other in the sky. And of course this fits in perfectly with the concept of gearworks—the gears are making little orbits.” Some Greek inventor must have realized that it was possible to build a simulation of the movements in the heavens by reproducing the cycles with gears.

But who? Price called the inventor simply “some unknown ingenious mechanic.” Others have speculated that the inventor was Hipparchus, the greatest of all ancient Greek astronomers. Hipparchus, who is also believed to have invented trigonometry, lived on the island of Rhodes from about 140 to 120 B.C. He detailed a theory to explain the anomalous movements of the moon, which appears to change speed during its orbit of the Earth. Hipparchus is also thought to have founded a school on Rhodes that was maintained after his death by Posidonius, with whom Cicero studied in 79 B.C. In one of his letters, Cicero mentions a device “recently constructed by our friend Posidonius,” which sounds very like the Mechanism, and “which at each revolution reproduces the same motions of the sun, the moon, and the five planets that take place in the heavens every day and night.”

“Fragmentary Knowledge” continues