The Antikythera Mechanism

In 1901, sponge divers sheltering from a storm off the Greek island of Antikythera found a first-century BCE shipwreck full of bronze and marble statues. Among the haul was a shoebox-sized lump of corroded bronze that nobody could place. It turned out to contain gears.

What it is

The mechanism is a hand-cranked geared calculator, built somewhere around 150–100 BCE, that modeled the sky. Turning the crank advanced a date and drove pointers showing the positions of the Sun and Moon against the zodiac, the phase of the Moon via a small rotating silver-and-black ball, and the state of several interlocking calendars. Thirty gears survive; reconstructions suggest the original had substantially more.

The eclipse dial

The back of the device carried two spiral dials. One tracked the 19-year Metonic cycle, which reconciles lunar months with solar years; the other tracked the 223-month Saros cycle, after which eclipses repeat in near-identical sequence. Glyphs on the Saros spiral marked which months would carry a solar or lunar eclipse, and inscriptions gave the expected hour and even the color the eclipse was predicted to show.

The clever part

The Moon does not move at a constant rate; it runs faster near perigee. The builders modeled this with a pin-and-slot arrangement — one gear's pin riding in another's slot, the two mounted on slightly offset axes, so the output speed varies sinusoidally over each orbit. The whole assembly is itself mounted on a rotating gear to precess the lunar apogee. It is a mechanical encoding of Hipparchus's lunar theory, and it is the kind of thing that is difficult to arrive at by accident.

How it was read

The fragments are fused and unopenable, so progress has tracked imaging technology. Derek de Solla Price used gamma radiography in the 1970s to establish the gear counts. In 2005 a research team brought an eight-ton microfocus X-ray CT scanner to Athens and resolved the internal tooth counts and thousands of characters of previously invisible inscription — effectively a user manual on the plates. Much of what is now considered settled comes from that scan.

The gap

The uncomfortable part is what comes after. Nothing of comparable gearing complexity survives, or is clearly described, for roughly fourteen centuries, until the astronomical clocks of medieval Europe. Whether that reflects a genuine loss of capability or only a loss of evidence — bronze being eminently recyclable — is unresolved. A single storm and a single shipwreck are why we know about it at all.