Moon-phases

The moon-phase indication is a device serving to visualize on the watch dial the various appearances adopted by the Earth’s satellite in the course of a lunar cycle: it may be waxing, full, waning or entirely hidden by the shadow of the Earth – and thus referred to as a new moon.

These different phases are traditionally represented by a mobile disk bearing two diametrically opposite depictions of the Moon alternately appearing through an aperture. The latter is cut out so as to provide a visual simulation of the Moon’s waning and waxing episodes. As soon as the first interpretation vanishes, the second instantly comes on the scene. Moon phases may also be indicated by means of a hand pointing to various symbols, or by a three-dimensional sphere spinning on its axis, and of which one half is light-colored while the other is dark. This horological complication often accompanies other astronomical indications such as the calendar or the equation of time.

The mechanical difficulty lies in the unusual interval between two full moons. Known as the synodic month or lunar cycle, this interval lasts 29 days, 12 hours, 44 minutes and 2.8 seconds: meaning 29.5305881 days. Watch movements are designed to perform 60-minute, 12-hour and 24-hour rotations. It is impossible with such systems to keep track of a synodical month, apart from by making a gigantic wheel with 295,306 teeth, which would naturally not fit into a watch. That is why the conventional moon-phase display is based on an approximate calculation – which is more or less accurate depending on the mechanism being implemented – and requires a periodic manual correction. The most common solution consists in rounding out the lunar cycle to 29.5 days. The moon-phase indicator is thus driven by a 59-tooth wheel, corresponding to two lunar cycles (2 x 29.5 days). A fingerpiece pushes the wheel one notch forward every 24 hours. This system results in a one-day difference accumulated in 2 years, 7 months and around 20 days.

Another more complex solution consists in using a wheel with 135 teeth. Sometimes referred to as an “astronomical moon”, this system improves the precision of the lunar cycle to 29 days, 12 hours and 45 minutes. The difference between the mechanism and the authentic lunar cycle amounts to one day in 122 years and 46 days.

Certain major manufacturers have decided to push the boundaries of precision yet further. By using wider wheels with more teeth, the IWC Schaffhausen Portuguese Perpetual Calendar model slashed the accumulated difference to one day in 577.5 years. Shortly afterward, A. Lange & Söhne beat this record with its Richard Lange Perpetual Calendar “Terraluna”. With the help of three rotating disks enabling an orbital moon-phase display, the brand pushed the figure to 1,058 years. But a Swiss watchmaker, Andreas Strehler, took the issue into a whole new dimension by presenting his Sauterelle Perpetual Moon model. By means of four wheels and pinions combining external and internal toothing, the independent creator managed to achieve a moon-phase indication with a deviation of one day in 2.045 million years. The principle is based on ensuring the mechanism is constantly driven by the motion-work, contrary to conventional devices that are activated only once a day. Its invention has been entered into the Guinness Book of Records.

All calendars are based on observation of the heavenly bodies. With the appearance of writing from 4200BC., civilizations began organizing social life and religious observances on the basis of movements of the Sun and Moon. Mesopotamians were the first to impose a calendar built on the alternation between day and night, the cycle of the seasons and the phases of the Moon.

Right from the earliest attempts to measure time, attempts were made to reproduce the various appearance of Earth’s satellite. The oldest known mechanism is the Antikythera Machine. Discovered in an advanced state of corrosion by a fisherman off the coast of Crete in 1901, it is dated somewhere between 150 to 100BC. Made of wood and bronze, it is composed of 32 toothed wheels that served to display the Egyptian calendar, the Greek zodiac, the five-planet geocentric cosmos of the Ancient Greeks, a parapegma (ancestor of the modern almanac), the Panhellenic calendar of the Olympiad cycle, as well as four lunisolar calendars serving to predict solar and lunar eclipses.

The modernity of this machine remains a mystery for scientists. It was not until the 11^th century AD and the work of Al-Bîrunî, learned inventor of the mechanical astrolabe, that a gear-train system designed to reproduce the phases of the Moon first appeared, laying the foundations for the Islamic calendar. Prior to this invention, astronomers relied on water-driven instruments, such as the one built by the Spanish Muslim scholar Al-Zarqâlî for his native city of Toledo, also in the 11^th century.

Progressively and thanks to the translation of Arabic texts into Latin, this knowledge – particularly in the field of astronomy – made its way to Europe. Buoyed by renewed economic momentum, Western medieval civilization showed great enthusiasm for progress and mechanization. The late 13^th century saw the appearance of clocks with gear trains, driving weights and regulators.

It was not long afterwards that the indications provided by astrological water clocks began to be reproduced on monumental clocks. The oldest is possibly the one made by Richard of Wallingford, Abbot of St. Albans, for his abbey located to the north of London. Work began on it in 1327 and, once complete, it indicated the positions of the fixed stars, the Sun and the Moon – complete with its phases, age and the line of nodes involved in eclipses – as well as the ebb and flow of the tide at London Bridge

In step with horological progress, the moon-phase indication migrated to a weight-driven wall clock, the spring-driven table clock and then, at the Renaissance, the pocket watch. Too inaccurate to show the minutes, the latter tended to display the hours, the date, the days, the months and often the phases and age of the Moon, as well as Zodiac signs. Much in favor in both Europe and the Near East towards the late 17^th century, this display was mainly intended for devotees of astronomy and astrology.

By the 20^th century, this complication had become essentially a romantic feature. The first wristwatches equipped with a moon-phase indication appeared in the 1920s. Generally accompanying a complete calendar, it became the specialty of brands such as Patek Philippe, Record Watch and Angelus. For many years, the function evolved very little, before once again sparking watchmakers’ interest in 1980, with the work of Kurt Klaus. The head watchmaker at IWC Schaffhausen became the first to make an astronomical moon phase for a wristwatch. Years later, in 2009, Ulysse Nardin launched the Moonstruck. Developed by Ludwig Oechslin, this model reproduces the Moon’s rotation and the visible path of the Sun in the sky as the Earth revolves around it. Moon phases are represented by two superimposed, slowly rotating disks.

That same year, De Bethune became the first Maison to offer a spherical moon. The Digital model made a lasting impression and inspired a number of other creative talents, such as Konstantin Chaykin, a Russian member of the AHCI, who developed a black hood for his Lunokhod that progressively covered the three-dimensional depiction of the Moon. Finally, the most recent milestone development is the above-mentioned Sauterelle Perpetual Moon model presented by Andreas Strehler in 2014.