Celestial computers of ancient Greece

Freeth convinced two companies to volunteer their high tech imaging technologies for the Antikythera Mechanism: X-Tek from England and Hewlett-Packard from the US.

The scientists and engineers who decoded the Antikythera computer concluded that it was the most sophisticated technology in the Mediterranean for more than a millennium. They published their reports in the November 30, 2006 and July 31, 2008 issues of Nature. (These articles and other relevant data can be found on the site of the Antikythera Mechanism Research Project.)

According to the 2006 report, the Antikythera Mechanism “stands as a witness to the extraordinary technological potential of ancient Greece, apparently lost within the Roman Empire”.

The story, however, is more complicated. It was the Christianised Roman Empire that devoured Greece. In all likelihood, the fires of the mint and the blazes of the smelters ate Antikythera Mechanism-like devices, which in the Christian society of Rome lost all utility and meaning.

The celestial Antikythera device provided the names of the Panhellenic games like the Olympics.

The scientists who studied it were right that this “artifact of ancient gearwork” was more than a device of pure astronomy: “exhibiting longitudes of heavenly bodies on the front dial, eclipse predictions on the lower back display, and a calendrical cycle believed to be strictly in the use of astronomers on the upper back display.”

The first inscription on the back of the Antikythera Mechanism reads: “the spiral [ΕΛΙΚΙ] divided into 235 sections.” This meant that one of the back dials was a spiral representing the 19-year Metonic moon and sun calendar of 235 months. Other back dials predicted the eclipses of the sun and the moon. The front dials, on the other hand, were about the months of the year, the zodiac run clockwise around them. The inscriptions on these dials explained which constellations rose and set at any specific time. Moreover, the front dials showed the movement and position of the sun, moon and the planets in the zodiac. They also revealed the date and phase of the moon.

The ideas of Hipparchos, the greatest Greek astronomer, found expression in the Antikythera computer. From about 140 to 120 BCE he had his laboratory in Rhodes. More than other Greek astronomers, he made use of the data of Babylonian astronomers. But like the rest of the Greek astronomers, he employed geometry in the study and understanding of astronomical phenomena. He invented plane trigonometry and made astronomy the predictive mathematical science it is today. In addition, he discovered the “precession of the equinoxes”.

This meant he proved the fixed stars are not really fixed stars but very slow movers that appear to be stationary. He left a list with all his astronomical observations, including the observations he borrowed from the Babylonian and Greek astronomers.

The connection of Hipparchos to the Antikythera Mechanism is in the front bronze plate of the device where pointers displayed the positions and speed of the sun and the moon in the Zodiac.

Hipparchos knew the moon moved around the earth at different speeds. When the moon is close to the earth, it moves faster than when it is farther from the earth when it slows down. This is because the moon’s orbit is elliptical, not the perfect circular movement the Greeks associated with the stars. Hipparchos resolved this difficulty with his epicyclic lunar theory, which superimposed one circular motion of the moon onto another, the second movement having a different centre.

The Antikythera Mechanism modeled the ideas of Hipparchos with one gearwheel sitting on top of another, but located on a different axis. A pin-and-slot mechanism then takes under consideration the non-circular or elliptical orbit of the moon. A pin originating from the bottom wheel enters the slot of the wheel above it. When the bottom wheel turns, it also drives around the top gearwheel. However, the wheels have different centers and, therefore, the pin slides back and forth in the slot, which enables the speed of the top wheel to vary while that of the bottom wheel remains constant.