New research reveals more details of ancient calculator

Advanced imaging software and 3-D X-ray tomography have finally let scientists create the most detailed reconstruction yet of a 2,100-year-old astronomical calculator.

The latest research of the Antikythera Mechanism shows it to be a highly sophisticated calculator that could add, subtract and divide by means of a complex and ingenious arrangement of 37 bronze gears. The gear train moved a set of pointers on dials to accurately show the changing positions of the sun and moon (with its phases), and quite possibly of the planets, and to predict solar and lunar eclipses.

Researchers now date its creation slightly earlier than previously thought: between 100 and 150 B.C.

One of the key new findings is the discovery that the device mechanically reproduces the mathematics developed by the great 2nd century B.C. astronomer Hipparchus to account for the irregular movement of the moon through the heavens.

"This is a mechanical reproduction, by means of a clever [pin-and-slot arrangement in the gears] of the so-called first lunar anomaly in Hipparchus' lunar theory," says Francois Charette, a researcher with the Department of the History of Science, Ludwig-Maximilian University, Munich, Germany. Charette authored an assessment of the new research in a story published in the current issue of the journal Nature.

"This means that the pin-and-slot device makes the pointer for the moon on the front dial move at varying speed, following (in modern terms) a mathematical function that resembles more or less a sine curve," Charette says.

The research team speculates that Hipparchus, who lived from about 140 to 120 B.C. in Rhodes, where the mechanism is believed to have been built, or one of his students may have had a hand in its design.

The full research results were announced at a two-day international conference in Athens last week, and published online by Nature.

The mechanism, a clockwork-like collection of bronze gears and dials inscribed with Greek text and numbers, has been slow to yield its secrets since being recovered in 1901 from an ancient wreck off the Greek island of Antikythera.

But critical new details of the gears and their relationships, along with nearly 1,000 never-before-seen Greek characters, doubling the total number identified, were revealed by means of surface imaging software recently developed by scientists at HP Laboratories and by an 8-ton, high-resolution, 3-D X-ray machine from X-Tek Systems, Tring, England.

Those details confirm some previous insights by researcher Michael Wright, based on computer-aided analysis, with the late Allen Bromley of the University of Sydney, Australia, of 700 digitized X-ray plates during the 1990s.

The new research details and conclusions here are drawn from the Nature stories, including the paper by the Antikythera Mechanism Research Project investigators.

Roughly the size of a shoebox, the front of the mechanism shows two concentric circular scales, the inner one showing the Greek zodiac with 360 divisions. The outer, moveable scale is the Egyptian calendar, with 12 30-day months plus five days. The outer dial could be moved to adjust for leap years. Pointers show the relative positions of the sun and moon, based on the Metonic cycle of 235 lunar months (the interval between two identical phases of the moon, such as from one full moon to the next) in 19 solar years, Charette says.

A device showing the moon's phase was probably attached to the moon pointer.

On the back of the device are two main dials, one above the other, both using a spiral design (confirming Wright's earlier proposal), whose pointers show time based on two other astronomical cycles identified by the Babylonians. The upper dial shows the Metonic cycle with a subsidiary dial showing the more accurate Callipic lunar cycle, of 940 lunar months in 76 years (or four Metonic cycles minus one day).

The second dial is for the Saros eclipse repetition cycle, which predicts that a given lunar or solar eclipse will be repeated 223 lunar months later by a similar eclipse.

The researchers now believe the device had 37 gear wheels; seven of those are deduced from the now more visible details of the surviving wheels. And they agree with Wright's speculation that some of the missing gears were likely used to simulate the movement of the known planets, making the Antikythera Mechanism one of the earliest and most complex planetariums.

The Research Project intends to create an online database for continuing study of the Antikythera Mechanism and of the world, and most of all, of the minds that created it.

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John Cox

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