Antikythera

What it is

The Antikythera mechanism is the ancient Greek geared astronomical calculator recovered from a Roman-era shipwreck off the island of Antikythera — a bronze box of interlocking wheels that predicted the positions of the Sun and Moon and the dates of eclipses. This is that instrument rebuilt in the browser and made turnable: a full gear train drawn live on a single 2D HTML canvas, with no WebGL, no three.js and no 3D library anywhere in it. Drag the crank on the right-hand side of the case, or the dial face itself, and the whole machine turns with you — the golden Sun pointer sweeping the Egyptian calendar ring and the zodiac, the silver Moon pointer running at its own faster rate, and a half-silvered ball riding the Moon's pointer turning to show the phase, which is just the difference of the two angles.

The gears are real gears, not painted circles. Every wheel's outline is an involute tooth profile generated from a module and a 20-degree pressure angle, with an automatic profile shift standing by for any wheel too small for that angle — none of these needs it, the smallest being 24 teeth against a 17.1-tooth limit — and a tip clamp that stops a tooth closing into a spike, so meshing pairs interleave at the correct centre distance and roll without gaps as you crank. The tooth counts are the mechanism's own: a 48-tooth crown on the crank shaft driving the 223-tooth great wheel, then 64, 38, 48, 24, the famous 127, and 32 through the lunar train — a chain that multiplies out to exactly 254 sidereal months per 19 years, reduced with integers and pinned by unit test rather than eyeballed. Hover any wheel and the page names it and prints its tooth count, and for the six in the lunar train the factor each contributes to that 254/19 ratio.

The interaction the piece exists for is the pin-and-slot lunar anomaly. Two 50-tooth wheels sit on axes offset by just over a tenth of the pin's radius, a pin on one riding a slot in the other, and that small eccentricity turns a constant input into a varying output — the Moon's real speeding-up and slowing-down through its orbit, sized to its 6.29-degree equation of the centre. A toggle stops the pin in its slot, and the Moon's pointer visibly goes metronomic; a magnified inset dial shows the pin sliding and reads out the correction in degrees as you crank. Flip the case over and the back plate carries the two spirals: the Metonic, 19 years across 235 lunar months, and the Saros, 223 months of eclipse repeats whose engraved cells the pointer drops into, with the Exeligmos dial inside the Saros's innermost turn.

Four chips wind the mechanism for you — a full Metonic turn of 19 years, the next cell engraved with a solar glyph, one Saros to the Exeligmos step that adds eight hours, and Reset. The readout in the corner names the Egyptian date, the year within the Metonic cycle, the Sun's zodiac sign and degree, the Moon's phase, and the Saros cell with its eclipse prediction. An X-ray slider fades the engraved front plate away so you can watch the train turning behind it. The whole thing is theme-aware — the bronze plate takes dark engraving on the light page and gold engraving on the dark one, with the case, the verdigris blooms and the pointers re-tinted for each — and reduce-motion drops the crank's coast, the wind animation and the case flip.

How to use it

• Drag the crank at the right of the case, or the dial itself, to wind the mechanism.
• Toggle Pin & slot to stop the lunar anomaly and watch the Moon's pointer go even.
• Tap Next solar eclipse to wind to the next glyph cell on the Saros spiral.
• Pull the X-ray slider to fade the front plate and see the gear train behind it.
• Hover a wheel for its tooth count and, in the lunar train, its factor in 254/19.

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