Moonphase

A moonphase shows the current phase of the Moon, almost always through a shaped aperture in the dial that reveals part of a rotating disc printed with two moons against a starry ground. The disc is driven by the calendar work and advances one step every 24 hours, so the visible moon waxes and wanes as the days pass. It is the oldest decorative complication still in common production and one of the few that is purely for pleasure.

How it works

The conventional mechanism uses a disc carried on a wheel with 59 teeth, advanced one tooth per day by a finger on the hour wheel's 24 hour drive. Two moons on the disc means one complete lunar cycle every 29.5 days.

The real figure, the synodic month from one new moon to the next, averages about 29 days, 12 hours and 44 minutes. The display therefore runs slow by roughly 44 minutes per lunar month, which accumulates to a full day of error after about two years and seven and a half months. In use you nudge the moon forward one step every couple of years and never notice.

Adding gearing gets much closer to the true period. Designs of this kind, often built around a 135 tooth wheel, are quoted as accurate to within a day over about 122 years, and a handful of specialist mechanisms claim far longer still. The extra accuracy costs a few wheels and, in practice, matters mostly as a demonstration of intent, since the watch will be serviced and reset many times inside a century.

The step is not instantaneous on most calibers. The driving finger engages over an hour or two around midnight and drags the disc across, which is why the moon can look half a step out of place late in the evening. Note also that the crescent you see is made by two circular arcs, the edge of the printed moon and the edge of the aperture, whereas the real terminator on the Moon is an ellipse that appears straight at the quarters. The display approximates a shape rather than rendering one.

Moon indications are much older than the wristwatch, appearing on astronomical clocks and then on pocket watches. On the wrist they became a fixture with the triple calendar watches of the 1940s and 1950s, which paired day, date, month and moon on a single dial.

Types you will see

The aperture at twelve or six o'clock is the standard form. A moon age indicator does the same job with a hand over a scale numbered to 29 and a half days: easier to read precisely, less pretty. Some makers use a rotating sphere rather than a flat disc, and a few print a photographic moon. The double moon, found on IWC's perpetual calendars, shows two moons side by side so Northern and Southern Hemisphere owners each get a correct view.

Mechanically there are two routes. In a module the moonphase sits on top of a plain base caliber, which is how most affordable watches get one: it adds thickness and its own correctors, and at service time it is a separate assembly. In an integrated design the moon work is part of the caliber, as in the ETA 7751, the full calendar version of the Valjoux 7750. Moonphases also travel with calendars: most perpetual calendars include one, which our guide to annual and perpetual calendars covers, and the wider family is in complications explained.

In practice

To set one, find the date of the most recent full moon from an almanac, advance the moon disc until the moon is centered in the aperture, then step it forward one day at a time up to today's date. On a watch with a date, set the date afterwards.

The corrector is usually a recessed pusher in the case band, pressed with the pin tool supplied with the watch; a fingernail will not reach it and a metal point marks the case if it slips. Some watches advance the moon from a crown position instead, and a few have no corrector at all, in which case you run the hands forward through 24 hour cycles.

Use the corrector only outside the hours when the calendar is changing over, typically late evening to early morning, because the driving finger is engaged with the wheels during that window and forcing a correction can bend parts. The same rule governs the date, and is set out in when it is unsafe to change the date. Broader setting order is covered in setting and adjusting your watch.

Common misconceptions

The display shows phase only. It does not track moonrise, distance or eclipses, and the printed moon's orientation is drawn for one hemisphere, so a Southern Hemisphere viewer sees the phases mirrored unless the maker has printed a disc for that market.

A moon that has drifted is not a fault. Owners regularly send watches in over a display that is a day out, when a standard 59 tooth mechanism is behaving exactly as designed. Nor does a 122 year figure describe how long a watch will hold its setting in the real world: it describes a gear ratio, and any service or any stop long enough to need resetting starts the count again.