Watch enthusiasts have long since succumbed to the elaborate and additional mechanical functions, the so-called complications. Classics among them include display of the date or the moon phase, while the eternal calendar represents the superlative.

eternity keeps longest
A masterpiece of mechanics: “Jules Audemars Equation du Temps” by Audemars Piguet is not only an eternal calendar, but also displays the times of sunset and sunrise; automatic clockwork in a golden housing
The secret of the moon: a disc with the moon symbols is hidden beneath the dial of the “Jules Audemars Equation du Temps” by Audemars Piguet
Friendly moon: the chronograph “Pharo” by Limes is a full calendar with display of day, date, month and moon phase. Mechanical clock work in a golden housing
Annual calendar de luxe: yellow gold, diamonds and a mother of pearl dial transform the automatic watch by Patek Philippe into a glamorous companion

Every child learns it in school: a year has 365 days. However, this is not entirely correct. Taken precisely, each year lasts five hours, 48 minutes and 46 seconds longer than the 365 days. After this, the earth has fully orbited the sun Mathematicians and astronomers in ancient times already knew this and were responsible for this kind of calculation. Sosigenes, for example, from Alexandria, whose work formed the basis for the Julian calendar, knew it. It was introduced by the statesman and commander Gaius Julrus Caesar in the year 46 BC. Even at the time, the solar year was divided into twelve months of different lengths and included a leap year with 366 days every four years. However, this was insufficiently precise. In the Middle Ages, Christians noticed that their important Easter celebration was being pushed further and further back into the year. On the orders of Pope Gregory XIII, astronomers replicated the calculations for the length of years and realized that the Julian year lasted eleven minutes and fourteen seconds, or 0.0078 days, too long. Gregory XIII corrected this error, which had by 1582 amounted to a Proud ten days, by removing the leap year every hundred years – apart from in the full centuries, divisible by 400.

From the heavens on high: automatic watch “Icarus” from the collection “Spirit of Moon” by Sothis with full calendar, chronograph and moon phase. Stainless steel housing
Eternal calendar form Saxony: “Senator” by Glashütte Original with a display of the moon phase. Automatic clockwork in a yellow gold housing
Eternity for women: the ladies model “Villeret” by Blancpainis the world’s flattest eternal calendar. Hand wound in a white gold housing

It is fairly complicated, and watches that include a date have their problems with this kind of regulation. Nonetheless, in 1764, a crafty watch maker succeeded in creating an eternal calendar that lent consideration to different lengths of months and also leap years, constantly displaying the correct date in combination with the day of the week and the month. At the start, this feature was reserved to large clocks, but later on, inventors succeeded in miniaturizing this display for pocket and wrist watches. Frequently, the complicated calendar mechanism also has a leap year display, specifying in the precise year of the four year cycle. Nowadays, if one decides to purchase a wrist watch with eternal calendar and lets it run without interruption, it will not need adjustment until the year 2100. The leap year is namely removed in the full century, and the clockwork does not take this into consideration.

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Full calendar by Longines from the “Master Collection” with display of the date, day of the week and month. Chronograph with automatic clockwork and stainless steel housing
A view for the date: dial detail of a classic wristwatch by Longines from the “Master Collection”. Automatic clockwork, stainless steel housing
Eternal calendar: “Master Eight Days Perpetual” by Jaeger-LeCoultre with power reserve and display of day and night time. A watch maker is called on once a century in order to move the four-digit year on the century display forward one number. Hand wound clockwork in a platinum housing
Everything revolves around the sun: the mechanical work “Tellurium” by Hermle with astronomical display. The earth, moon and the sun orbit in the precisely accurate time on its top side

The eternal calendar on the basis of mechanical clockwork is the royal discipline of watch making and astronomical watches. This term relates to all clocks that, in addition to the normal time, also tell the date, day of the week and month, and possibly also the year, the time of sunrise and sunset and also the passage of the moon, planets and fixed stars. In days of old, the full scope of these displays was reserved to monumental public clocks and complicated table or fireside clocks. The German manufacturer of large clocks, Hermle, pays testimony to this with its mechanically operated “Tellurium”, a table clock, on whose top side the heavenly bodies lazily circle – in the precise astronomical time. This means that the earth fully circles the sun in one year and rotates once around its own axis in 24 hours. At the same time, the moon travels around the earth in precisely 29 days and half of one day and also in doing this rotates around its own axis. But that is not enough: the twelve Signs of the Zodiac, the months and the days can also be viewed on a flat display on the top; “Tellurium” also beats time.

Conversely, pocket and wrist watches usually only have reduced displays. Most frequently, we find simple calendars, displaying the date and sometimes the day of the week, and the annual calendar, which also displays the month. Discs or drums take care of this in mechanical watches; they are pushed forwards by the clockwork and display the day of the week or a date in a section of the dial. Another option is to display the date using a hand on a scale on the edge of the dial or in a small, auxiliary dial.

Striking design: “cK challenge chrono” by cK watches with unusual date display at 5 o’clock. Quartz clockwork, stainless steel housing and black silicon strap
Makes you crave for the moon: “L.U.C Lunar One” by Chopard with eternal calendar and moon phase, including a particularly precise display. Automatic clockwork in a platinum housing

A display of moon phases on wrist watches is a particularly charming solution. It portrays the different appearances of the moon over the course of a moon month. This complication was already found in watches during the Renaissance period and is a frequent bedfellow for calendar displays. It takes place entirely realistically using a circular, gold-plated symbol against a dark blue background on a disc, which – coupled with the clock’s date display – rotates. Within this framework, the moon symbol runs across a semicircular section of the dial and shows the individual phases of the ascendant or descendant moon, new moon and full moon. Discerning wristwatches prove particularly precise here: for example, the “L.U.C Lunar One” by Chopard deviates from the moon’s cycle by just 57.2 seconds, or one day every 122 years. Only the most renowned of watch manufacturers manage that.

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However, smaller studios are also producing exciting wristwatches: the practical use of date display, the precision of annual calendars, the miracle of the eternal calendar and the charm of moon phase displays are factors that watch enthusiasts will find in many models.