Urban living and the ubiquity of accurate calendars that work along unseen principles have largely obviated our need to look up at the sky. It wasn’t always so. For the moon at least, tracing its position in the sky told the months, and its phase determined the day, designating the time to plant and worship. Food for the stomach and the soul, watching the moon provided pretty much everything the human race needed, and this partly explains the development of the moon phase display quite early in the history of watchmaking.

In the 14th century, Giovanni de Dondi’s Astrarium clock showed the time, movements of the planets, stars, moon and moon’s nodes, and an annual calendar. In the 17th century, English watchmaker Simon Bartram made a pendant watch with a rock crystal see-through case, housing a moon phase display as well as indications for calendar, day of the week and the sign of the zodiac.

## Simple Moon Phase

The usual method of displaying the phases of the moon is by using a revolving disc that has two images of the moon on it, moving into and out of a cut-out in the dial, so that, as the moon in the heavens is waxing, the disc on your wrist is creeping into full view; and as the moon wanes, the displayed moon disappears again.

The problem in displaying the phases of the moon is that a watch has a gear train oriented around a 24-hour day, and the full lunar month is not a whole number of days long. The synodic month, or number of days it takes for the moon to return to the same phase, is approximately 29.5 days. If the lunar month were exactly 29 or 30 days in length, matters would be simple — just have a disk with 29 or 30 teeth, advance the disc one tooth per day, and you’d be in business; but 29.5 days presents a problem in that you can’t put half a tooth on a gear.

The traditional solution to the problem, therefore, is to create a disk with 59 teeth on it (29.5 x 2) and have the disk make one full rotation every 2 lunar cycles, advancing one tooth per day. An additional advantage to this is that you can put two moons on the disk opposite each other, so that just as one is disappearing (marking the waning moon), another is about to appear (marking the waxing moon).

Unfortunately, nothing in astronomy is ever quite that straightforward — the lunar month is approximately – not exactly – 29.5 days in length, which means that a conventional 59-tooth moon phase display will gradually drift out of alignment with the actual moon, to the tune of roughly a day every 33 months.

## High Accuracy Moon Phase Displays

The one-day error every couple of years is accurate enough for many people: the moon phase display just isn’t as “mission critical” as it used to be. But while watchmaking isn’t a furious torrent of innovation that characterises the infotech sphere, watchmakers don’t sit very long on past laurels either and it didn’t take long for companies to obliterate this margin of error, at least shrinking it to insignificance.

Two big guns in high accuracy moon phase displays are IWC and A. Lange & Söhne. IWC in their Da Vinci watch devised a gear train of much greater accuracy than the usual 59-toothed wheel. A series of reduction gears allows the watch to rotate the moon phase disk to an accuracy several orders of magnitude greater than a conventional moon phase. The Da Vinci moon phase gearing reflects a synodic month of 29.53125 days and theoretically requires correction for an error of one day in 122 years. This has been improved on in IWC’s Portuguese Perpetual Calendar, in which there is an error of only one day in 577 years.

Similarly, A. Lange & Söhne has implemented several variations on the moon phase complication — the Lange 1 Moonphase watch is accurate to within one day in 122 years, and the Lange 1815 Moonphase goes one better over its sibling by achieving an accuracy of one day’s error in 1,058 years!