Ancient Greek Computer Had Surprising Sun Tracker

Ancient Greek Computer Had Surprising Sun Tracker

The world’s oldest astronomical calculator is famous for having intricate gear systems centuries ahead of their time. But new work shows the Antikythera mechanism used pure geometry, as well as flashy gears to track celestial bodies’ motion through the heavens.

The device, a 2,000-year-old assemblage of gears and wheels that matched 19th century clocks in precision and complexity, was salvaged from a shipwreck off the Greek island of Antikythera in 1901.

Called the Antikythera mechanism, the machine gracefully kept track of the day of the year, the positions of the sun and the moon, and perhaps the other planets. It also predicted eclipses and kept track of upcoming Olympic games.

Most of the mechanism’s calculations were driven by a series of 37 interlocking dials, which may have been manipulated by a hand crank. The front of the mechanism had a clock-like face that denoted the calendar date in two concentric circles, one showing the signs of the Greek zodiac, and one carrying the Egyptian months of the year.

Three hands denoting the date and the position of the sun and the moon moved through the zodiac and the months as the gears turned.

“It’s a pretty elaborate piece of machinery,” said science historian James Evans of the University of Puget Sound in a presentation at the University of Washington in Seattle on March 31. “Nobody would ever have guessed that there could be something this complex in the second century [BC].”

Earlier research showed the device also accounted for a subtle weirdness in the motion of the moon. Because the moon’s orbit around the Earth is an ellipse, not a perfect circle, the moon seems to speed up and slow down over the course of a month. In 2006, Tony Freeth of Cardiff University and colleagues showed that a clever configuration of two overlapping gears, with the top gear laid off-center from the bottom gear, could give the moon’s marker its irregular speed.

Because of the Earth’s elliptical orbit around the sun, the sun makes a similarly variable trip across the sky, speeding up and slowing down over the course of the year. But the effect is much more subtle than for the moon.

“It’d be very hard with gears to mimic an effect so small,” Evans said. Also, if there were any extra gears related to the sun’s movement in the original mechanism, none emerged from the shipwreck.

Evans and colleagues suggested a simpler way to make the sun dial appear to change speed: Stretch the zodiac. If the spaces on the front wheel of the mechanism were of different widths, Evans reasoned, then the hand representing the sun would take longer to travel through the part of the year lumped under the zodiac sign of Taurus than through Libra.

The delay would make the sun look like it was moving slower at some times of year and faster at others, even though the gears turning the hand moved at a constant speed.

To check this idea, the researchers examined X-ray images of the Antikythera mechanism, parts of which were badly corroded. In a paper last year in the Journal for the History of Astronomy (which Evans co-edits), the team analyzed 69 degrees of the circle that was preserved well enough for the zodiac names and markings to be read. Since then, the team has extended the study to 88 degrees of the circle.

They found that the two circles representing the zodiac and the Egyptian calendar were divided differently, and in just the right way to account for the sun’s irregular speed.

“It made more sense to do it this way than to try to use an elaborate wheel upon wheel sort of thing, as they did for the lunar orbit,” Evans said.