Celestial motion computer dates back to ancient Greece

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Since the beginning of recorded history, people who have looked at the night sky have tried to understand and predict the motions of the stars.

The fixed stars move across the sky every night from east to west, maintaining the same positions relative to each other. But the sun, moon and visible planets change their positions relative to the stars over time.

Today, with modern computers, we can explain and predict these motions far into the future, something we thought not possible for the ancient Greeks until the discovery of the Antikythera mechanism.

In 1900, sponge divers discovered the wreck of a large Roman trading ship off the island of Antikythera in the Mediterranean south of Greece. The ship was carrying one of the largest collections of Greek artifacts ever found. From examination of coins and pottery, the wreck was dated between 80 and 60 BCE.

One of the artifacts was a corroded lump of bronze that split into several pieces a few months after its recovery. The exposed pieces revealed toothed bronze gear wheels. It was the first evidence of metal gear technology in the classical age. The fragments also revealed inscriptions in ancient Greek that indicated it was an astronomical instrument from the second century BCE.

The instrument was in a wood case measuring about 13-by-8-by-4 inches and had one large dial on the front and two smaller dials, one above the other, on the back. Inside were at least 30 gear wheels, though it is estimated that there could have been as many as 60 gears and some might have been lost. Gear trains were present for the sun and moon, and systems had been worked out that could have displayed the five visible planets with just 17 more gears.

The sophistication of the design is extraordinary. For example, the moon’s orbit is elliptical, so it moves faster when it is closer to the Earth and slower when farther away. Through the instrument’s use of slots and pins in offset gears, these differences moon motion were accounted for. The planets are also in elliptical orbits, so this same process could have been used to account for changes in their speeds.

Based on information we have, it is likely that the front dial had pointers for the date, sun, moon and the visible planets, showing their positions in the sky and relative to each other. It would have also given the phase of the moon.

By turning a small hand crank on the side of the case, the date pointer could be moved forward or backward in time. Turning the crank would cause all the interlocking gears to rotate, which moved the sun, moon and planets to the correct position for the indicated date. The movement of additional gears also would adjust the phase of the moon.

The upper back dial has a spiral scale with 235 divisions. This is an important number in Greek astronomy. Nineteen solar years equals almost exactly 235 lunar months, measured from new moon to new moon.

This cycle, known as the Metonic cycle, relates the lunar month to the solar year and allows users to predict dates for lunar phases. Thus, it appears that this dial is a 235-month calendar.

A gear that drove the lower back dial had 223 teeth. This is another important number in Greek astronomy. There are 223 lunar months in the Saros cycle used to predict lunar and solar eclipses. The Saros cycle is based on the observation that 223 lunar months after a solar or lunar eclipse, a similar eclipse is likely to occur, shifted by eight hours.

It is believed that the lower dial is a mechanical representation of the Saros cycle. From the month and year of a known eclipse in the past, this dial could be used to predict lunar months in the future in which an eclipse is possible. Because the cycle repeats, the dial could be used for many decades.

Who created this ingenious design?

This quote from the famous Roman orator Cicero written 150 years later about Archimedes, the Greek mathematician and inventor, may give us the answer:

“Archimedes ... had thought out a way to represent accurately by a single device those various and divergent movements of the 5 planets with their different rates of speed. Thus the same eclipse of the sun would happen on the globe as it would actually happen.”

So it seems Archimedes might have created this device. Then came the Dark Ages, and it would be more than 1,500 years before another metal-geared machines at this level of complexity was seen again in Europe.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu.

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