The world’s oldest mechanical computer used a lunar calendar to study the stars

Researchers may have solved one lingering mystery behind the world’s oldest known mechanical computer using a combination of statistical analysis, techniques used to examine spacetime ripples, and a little bit of holiday season downtime.

The Antikythera mechanism is the world’s oldest known, hand-operated mechanical computer—and still a bit of an enigma. Archeologists recovered deteriorated fragments of the device from a shipwreck off the Aegean island of the same name in 1901, and generations have studied the remnants ever since. Designed and constructed during the second century BCE, the shoebox-sized bronze machine included a set of at least 30 intricate and precisely oriented gears. A user controlled this gearwork using external dials to accurately predict future astronomical calendar events like planetary movements, eclipses, and lunar phases with unparalleled precision—it was even to calculate future Olympics dates. But what type of astronomical calendar—lunar or solar—has remained unclear.

In 2020, X-ray imaging uncovered a series of regularly spaced holes underneath the mechanism’s damaged calendar ring. But because the ring is incomplete, researchers could only hypothesize about the total number of holes at the time—somewhere between 347 and 367. Unfortunately, that range includes the days in both the 354-day lunar and 365-day solar calendars. While on winter holiday break last year, however, researchers at the University of Glasgow found a way to pass the time.

“Towards the end of last year, a colleague pointed to me to data acquired by YouTuber Chris Budiselic, who was looking to make a replica of the calendar ring and was investigating ways to determine just how many holes it contained,” Graham Woan, a professor in the School of Physics & Astronomy, said in a recent statement.

Woan applied Bayesian analysis—a methodology utilized for problems dealing with uncertainty and incomplete data—to count the most likely total number of holes by taking into account the ring’s surviving six fragments and their hole placements. As a result, Woan could confidently state that the calendar ring contained either 354 or 355 holes.

Meanwhile, Joseph Bayley—another faculty member at the university’s Institute for Gravitational Research—decided to incorporate an entirely different statistical analysis approach to help crack the code. In this case, Bayley adapted a technique used to detect and confidently assess uncertainties like the delicate gravitational wave ripples in spacetime generated by gigantic events like black hole collisions. Once again, the results indicated the ring featured either 354 or 355 holes in a circular radius of 77.1mm, with an uncertainty of around 1/3mm. The statistical analysis also showed each hole variated by just 0.028mm—an astounding level of accuracy for when the Antikythera mechanism was built.

“It’s given me a new appreciation for the Antikythera mechanism and the work and care that Greek craftspeople put into making it,” Bayley said in a statement. “The precision of the holes’ positioning would have required highly accurate measurement techniques and an incredibly steady hand to punch them.”

[Related: Human remains found in 2,000-year-old Antikythera shipwreck.]

The team published their combined results in a paper published in the July 2024 issue of Horological Journal. Given that a lunar calendar features 354 days, Woan and Bayley are confident the device focused on this timekeeping methodology instead of the solar-based Julian calendar. 

“Previous studies had suggested that the calendar ring was likely to have tracked the lunar calendar, but the dual techniques we’ve applied in this piece of work greatly increase the likelihood that this was the case,” Bayley added.

For Woan, the team’s methodology behind solving the mystery behind the Antikythera mechanism’s calendar ring feels poetic.

“It’s a neat symmetry that we’ve adapted techniques we use to study the universe today to understand more about a mechanism that helped people keep track of the heavens nearly two millennia ago,” said Woan.