The Moon-Phase Dial
A painted moon gliding across a small window is one of the oldest complications in clockmaking, and a neat lesson in gear arithmetic.
The cycle a moon dial copies
The moon's phases repeat over what astronomers call the synodic month: the full cycle of phases as seen from Earth, from one new moon to the next. Britannica gives its average length as 29.530588 days, or 29 days, 12 hours, 44 minutes and 3 seconds.
That awkward extra half day is the whole challenge. A gear train can only count in whole teeth, so a clockmaker has to find a ratio close to 29.53 days without making the mechanism impossibly complicated.
There is a further catch. Britannica notes that the moon's orbit is disturbed by other forces, so the length of each real month varies slightly. Any moon dial is really tracking an average.
An idea older than clocks
Showing the moon's phase with gears is ancient. The Antikythera mechanism, a Greek astronomical calculator recovered in 1901 from a shipwreck off the island of Antikythera, had a half-silvered ball on its front for displaying lunar phases, according to Britannica.
Britannica dates the device's construction to about 100 BC, give or take 30 years, which makes the idea of a gear-driven moon display far older than any mechanical clock.
How the 59-tooth disc works
The usual solution is a disc carrying two pictures of the full moon on opposite sides, mounted behind a window cut into the dial, according to the watchmaking encyclopedia Horopedia. Only one moon is in view at a time, while the other waits hidden behind the dial.
In the standard version the disc has 59 teeth, and once every 24 hours a small finger in the movement pushes it forward by one tooth. The disc therefore makes a full turn in 59 days. Because it carries two moons, each one takes 29.5 days to pass across the window, very close to a real lunar cycle.
As the disc advances, a moon slides into view, sits fully visible at full moon and then slips out of sight again, with the empty gap between the two painted moons standing in for the new moon. It needs only one extra wheel, which is why Quill and Pad calls the 59-tooth wheel the standard arrangement.
See also: check the real moon's phase today on OnlineClock.app.
How accurate is it?
The trouble is that 29.5 days is slightly short of 29.53. That small gap builds up until a standard moon dial is a full day out after a little over two and a half years, according to watch magazine Quill and Pad. Horopedia gives a similar figure of about two years and seven or eight months.
The arithmetic is easy to check. The real average month of 29.530588 days, minus the dial's 29.5 days, leaves about 0.03 of a day, roughly 44 minutes, of error each month. It takes a little over 32 lunar months for that to add up to a whole day.
Owners simply nudge the moon forward when it drifts. Many pieces have a separate corrector for this; Horopedia describes one that moves the disc on by a single step with each press, without disturbing the hour and minute hands.
A finer gear shrinks the problem dramatically. Quill and Pad says a 135-tooth wheel lets higher-end watches stretch accuracy from about 2.5 years to about 122.5 years per day of error.
The race for a perfect moon
Some modern watchmakers treat the moon dial as a mathematical sport. In a 2016 ranking, Quill and Pad listed watches rated to stay within a day for more than a thousand years, including models from H. Moser and Cie (1,027 years), A. Lange and Söhne (1,058 years), Patek Philippe (1,073 years) and De Bethune (1,112 years).
The methods vary. Quill and Pad reported that Patek Philippe arrived at its ratios after calculating some 25 trillion gear-train variations, that Ochs und Junior used just five parts to reach about 3,478 years, and that a Jaeger-LeCoultre model was rated at 3,887 years.
The top of the list was a watch by Swiss independent Andreas Strehler, credited with one day of error in about 2,060,757 years. Quill and Pad says that is 145 times better than his own earlier version, achieved with the same number of parts simply by changing the gear ratios and tooth counts. Dutch maker Christiaan van der Klaauw came second with about 11,000 years and a three-dimensional moon.
These are calculated figures. They describe how closely a gear ratio matches the average lunar month, not anything a wearer could ever test, and a watch that is serviced every few years will be corrected long before such tiny errors matter. The appeal is the craft of finding the ratio.
See also: a guide to the parts of a watch dial on OnlineClock.app.
Moon dials on early clocks
Moon displays were common on clocks long before wristwatches. The Fitzwilliam Museum in Cambridge holds a month-going longcase clock by Thomas Tompion, made around 1675 to 1680, whose dial shows the age and phase of the moon and the time of high tide at London Bridge. The museum says the astrolabe dial was added to the movement soon after it was made.
On that clock the hour hand, carrying a gilt sun, turns once every 24 hours, and a third hand bearing a small moon shows lunar time. The movement also has an early anchor escapement.
The Science Museum Group holds an eight-day longcase clock by Christopher Pinchbeck of London from around 1720. The arch of its dial gives the phases, aspects and age of the moon, together with the time of high tide at various English and Continental ports. Two small dials let the owner silence the strike and adjust the pendulum's length.
Pairing the moon with tide times gave these dials a practical job for anyone who depended on the water, since tides follow the moon. For most owners today, though, a moon dial is a reminder of the sky outside rather than a working tool.
Living with a moon dial
Setting a moon dial is usually the last step, after the time and date are right. Look up the moon's current phase in a calendar or almanac, then use the corrector to step the disc forward until the painted moon matches the sky. If your piece came with instructions, follow them, since some makers specify the order and direction.
The display is also a canvas for decoration. Quill and Pad describes an A. Lange and Söhne watch whose back shows the moon orbiting a rotating Earth, while van der Klaauw's sculpted moon makes the phase look almost real.
Questions people ask
How does a moon-phase watch know the phase?
It does not sense the moon. A disc with two painted moons and 59 teeth is advanced one tooth per day, so each moon crosses the window in 29.5 days, close to the real average of about 29.53 days. You set it once to match the real moon.
How often does a moon-phase dial need correcting?
A standard 59-tooth dial drifts about one day in a little over two and a half years. Versions with a 135-tooth wheel stay within a day for about 122 years.
How do I set a moon-phase dial?
Set the time and date first, then look up the moon's current phase and use the moon corrector to step the disc forward until it matches the sky. Check your maker's instructions for any special order or direction.
Why do some old clocks show high tide?
Tides follow the moon, so some 17th and 18th century clocks paired a moon dial with a tide indicator. A Tompion clock at the Fitzwilliam Museum shows high tide at London Bridge.