Moonphase

Before artificial lighting the phase of the Moon decided whether a road could be travelled at night, when a harbour could be entered, and how long field work could go on after dusk. Tides follow the lunar cycle closely enough that a coastal sailor who knew the Moon’s age could estimate high water without tables. An almanac or the sky itself could answer the question, but neither helps under cloud or by day. Putting the count into a clock meant it was always there.

The conventional display puts two identical moons diametrically opposite on a disc, so one revolution covers two lunar months and each moon crosses the aperture in turn. Behind it sits a wheel of fifty-nine teeth, advanced one tooth every twenty-four hours by a finger driven from the hour wheel, with a sprung jumper to hold each position. The aperture is cut with two convex flanks, so that as the disc turns the painted moon is progressively masked and revealed.

That arithmetic is the limit. The mean interval from one new moon to the next is 29 days, 12 hours, 44 minutes and about 3 seconds; fifty-nine teeth for two months models it as exactly 29.5. The display therefore gains about 44 minutes a month and a full day after roughly thirty-three months, after which it needs correcting. Trains built around a 135-tooth wheel reduce the error to a day in something over a century.