How was the Antikythera mechanism finally read?

X-ray computed tomography, in 2005 and 2006 - a purpose-built eight-tonne scanner shipped to Athens because the fragments could not travel. It saw through corroded bronze that had been fused solid for two thousand years and recovered more than two thousand characters of inscription on internal surfaces, along with the tooth counts of gears nobody could see.

Those tooth counts are what identified the astronomy. Ratios in the gearing correspond to the Metonic cycle of 19 years, the Saros cycle of 223 lunar months for predicting eclipses, and a four-year dial marking the games including Olympia - so the machine's function was read off its own arithmetic. A pin-and-slot arrangement even models the Moon's varying speed around its elliptical orbit.

The inscriptions turned out to be an instruction manual. It describes what the dials show and how to use them, which means the device was meant to be operated by someone who was not its maker. Nothing of comparable mechanical complexity is known again until fourteenth-century European astronomical clocks - a gap of about fifteen hundred years, during which the capability simply did not exist anywhere that anyone has found.