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THE MOON PHASE

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Moon phase drive mechanism

1. The Moon and watchmaking: a thousand-year relationship

Since the very origins of timekeeping, the Moon has held a central place in humanity’s perception of time. Its regular cycles allowed early civilizations to organize their calendars.

From its beginnings in the 14th century, mechanical watchmaking naturally incorporated the display of the lunar phase. This complication is among the first to be represented on medieval monumental clocks.

It translates into mechanics one of the most visible and most regular astronomical phenomena of the celestial vault. The Moon phase is featured on many calendar watches.

It symbolizes the unbreakable bond between watchmaking and astronomy. Its display constitutes a complication in its own right, requiring a precise mechanism and careful decoration.

2. The lunar cycle: the lunation

The Moon completes a full revolution around the Earth in approximately 27 days, 7 hours and 43 minutes. This duration is called a sidereal month. It corresponds to a complete Lathe relative to the fixed stars.

However, the duration perceived from Earth between two successive moons differs. This duration is called a lunation or synodic month. It averages 29 days, 12 hours, 44 minutes and 2.9 seconds.

This difference is explained by the simultaneous movement of the Earth around the Sun. While the Moon completes its revolution, the Earth has traveled part of its orbit. The Moon must travel an additional arc to return to the initial sun-earth-moon configuration.

The lunation is therefore the reference Period for all watchmaking mechanisms displaying the Moon phase. Its exact, non-whole duration is the source of the mechanical challenges posed by these complications.

3. Moon phases: definition and nomenclature

Moon phases describe the appearance of the illuminated face of the Moon, as observed from Earth. This appearance varies according to the relative position of the Moon, the Earth and the Sun.

Eight main phases are conventionally distinguished. The New moon corresponds to conjunction: the illuminated face of the moon is invisible from Earth. The first crescent marks the beginning of the visible cycle.

The first quarter is reached when the Moon has completed a quarter of its revolution. The waxing gibbous moon precedes the Full moon. The Full moon corresponds to opposition: the visible face is then entirely illuminated.

The waning gibbous moon follows the Full moon. The last quarter and the last crescent complete the cycle before returning to the New moon. In watchmaking, most displays represent this cycle in a continuous, analogue manner.

4. Principle of horological representation

The display of the Moon phase in watchmaking consists in visually reproducing the appearance of the Moon as observed from Earth. The mechanism must match the display element to the actual phase of the lunar cycle.

The most widespread principle relies on a Disc bearing two representations of the full Moon, rotating beneath a Dial pierced with a crescent-shaped Aperture / Window. The Disc completes one full Lathe every two lunations.

Each lunation is represented by one of the two moons featured on the Disc. The portion visible through the Aperture / Window corresponds to the fraction of the Moon illuminated at the given moment.

The Display is therefore analog and continuous. It allows an intuitive reading of the current phase, even without numerical markers. The quality of the representation depends on the drive mechanism and the care given to the Decoration of the Disc.

5. The Moon phase Disc

5.1 Composition and Decoration

The moon phase disc is generally made of metal, most often brass or steel. It bears on its upper face two representations of the full moon, diametrically opposed.

These representations are painted, enamelled, engraved, or made using a combination of these techniques. The space between the two moons is painted midnight blue, representing the night sky. Decorative stars are often added to it.

The quality of the disc’s finish is an important criterion for the aesthetic value of the complication. Major manufacturers create these discs by hand, using grand feu enamel techniques, miniature painting, or by incorporating natural or artificial stones (e.g., lapis lazuli or aventurine glass for the sky).

The diameter of the disc is calculated based on the aperture / window available on the dial and the architecture of the movement. A large-diameter disc allows for a more faithful representation and a more precise reading of the phase.

5.2 The aperture / window and reading

The phase disc moves beneath the dial. Only the visible portion in the aperture indicates the current phase. The aperture is cut into the dial, usually in the shape of an arc.

The moon painted on the disc progresses through this aperture, simulating the crescent, quarter, gibbous, and full moon. When the moon is fully visible in the aperture, it is the full moon. When the moon disappears completely, it is the new moon.

The first and last quarters correspond to intermediate positions, where half of the lunar disc is visible. The starry sky on the disc, visible on either side of the moon, symbolizes the night.

The arc of the aperture gradually masks the moon, reproducing the shadow and light effect of the real lunar cycle. This analogue reading is immediate and requires no prior knowledge of the lunar cycle.

6. Other display modes

6.1 The rotating sphere

An alternative to the phase disc is representation by a rotating sphere. This sphere, half golden and half black or blue, rotates in front of a case back representing the night sky.

Its rotation simulates the gradual change of the moon phase. This display mode offers a more natural and spatial representation of the astronomical phenomenon.

It is mechanically more complex and more costly to produce than a simple flat disc. It is mainly found in grand complications and pieces of very high-end watchmaking.

6.2 The digital display of the lunar day

Some calibers simply indicate the number of the day in the lunar cycle, from 1 to 29 or 30. Most often, this is a hand display, which traces around a dial graduated in days or a dial graphically representing the different states of the moon (new moon, first crescent, first quarter, etc.). This approach sacrifices visual representation in favor of precise and direct information.

It is more common in antique watches with multiple complications having limited space on the dial. Other solutions combine a digital display of the lunar day with a simplified graphic representation.

These digital approaches are easier to read accurately, but they lose the poetic and immediate character of the graphic representation of the lunar cycle.

7. The drive mechanism

7.1 The 135-tooth wheel

The most widespread mechanism for driving the Moon phase disc relies on a 135-tooth wheel. This wheel is driven by a finger that advances it by one tooth per solar day.

In 59 solar days, the finger makes 59 successive advances of one tooth. The disc then completes a movement corresponding to two full lunations. Two lunations represent 59.0221 days on average.

The 135-tooth wheel divides this cycle into 135 mechanical steps, i.e. 59 steps per half-turn of the disc. This ratio presents a slight approximation of the actual duration of the lunation.

This mechanism is simple, robust and energy-efficient. It is suitable for the vast majority of moon phase complications in current production.

7.2 High-precision wheels

To reduce the cumulative error, some calibers use wheels with a higher number of teeth. A wheel with 135 teeth produces an error of one day approximately every two years and eight months.

Mechanisms with multiple gear train stages, combining very precise reduction ratios, make it possible to reduce this error to one day every few centuries. Their design requires a careful calculation of the division ratios.

These high-precision mechanisms are among the grand complications of fine watchmaking.

8. Precision and cumulative error

The precision of the Moon phase display depends directly on the drive mechanism used. The lunation lasts on average 29 days, 12 hours, 44 minutes and 2.9 seconds.

The standard 135-tooth mechanism introduces a cumulative error of about 44 minutes per lunation. This error, small over one cycle, inexorably accumulates over time.

After about two years and seven months, the offset reaches a full day. The user must then manually correct the Display by one step to restore agreement with the actual phase.

High-end watches with precision Moon phase aim for an error of less than one day every hundred years, or even every five hundred years for the most sophisticated pieces.

The precision of the Moon phase is one of the distinctive technical criteria of the quality of a lunar Complication. It reflects the mastery of mechanical calculations and the rigor of manufacturing.

9. Regulation / Adjustment and manual correction

Any Moon phase Display requires an initial Regulation / Adjustment when the watch is put into service. The user must position the Disc to the actual phase of the Moon on the date of the Regulation / Adjustment.

To do this, they generally have a corrector located in the case middle. This corrector advances the disc by one step with each press, without disturbing the hour and minute gear trains.

The correction must be carried out outside the sensitive time range of the drive mechanism, as defined by the manufacturer, but generally offset from midnight and other calendar corrections, in order to minimize energy consumption. Acting during the winding period risks damaging the jumping components.

After several years of use — depending on the precision of the mechanism — a one-step correction is necessary. The frequency of this correction depends on the intrinsic quality of the drive mechanism.

On high-precision haute horlogerie pieces, this correction may only be necessary once per Hobbing. On standard watches with a 135-tooth wheel, it occurs approximately every thirty-one months.

10. The moon phase in high complications

The moon phase naturally integrates into major horological complications. It often appears alongside different types of calendars such as perpetual calendars or the Equation of time.

In these complex pieces, the moon phase is driven by the calendar mechanism or by a dedicated wheel. Its display enriches the Dial with immediately readable astronomical information.

The moon phase is also present in skeleton watches, where the disc becomes a decorative element visible through the openworked dial. Mechanics and aesthetics then merge into a single work of art.

It is sometimes combined with the indication of sunrise and sunset times, or with a tide display linked to the lunar cycle, or even with displays of the celestial vault. These associations recall the medieval astronomical clocks from which it is descended.

11. Moon phase in the southern hemisphere

An often overlooked aspect of the Moon phase display is its visual orientation. In the northern hemisphere, the waxing crescent is illuminated on the right and the waning crescent on the left.

In the southern hemisphere, the perception is reversed. An Australian observer sees the moon wax from left to right, unlike a European observer. Standard phase discs are designed for the northern hemisphere.

For watches intended for the southern hemisphere market, some manufacturers offer reversed phase discs. The Moon then rotates in the opposite direction to a standard disc.

More simply, the moon aperture / window is open on the upper part of the disc for the northern hemisphere and on the lower part of the disc for the southern hemisphere.