Beautiful to look at, but of limited modern-day use, moon phases in watches are the just-for-the-gram complication of the watch world. It’s less of a practical and more of an ancestral affectation, a reminder that we once looked up to track our days, months and years and lived in harmony with the lunar and solar orbits rather than relying on scales and calendars.

The origins of the moon phase, which, as the name suggests, tracks the passage from waxing gibbous to waning crescent over the 29.5 days that make up a lunar month, can be found in the second century BC in a mechanism called the Antikythera. This handy analogue computer dating from 205 BC, which was retrieved from the sea off the coast of the Greek island from which it takes its name, had 37 gears that enabled it to follow the movement of the sun, the moon and certain planets such as Venus, as well as, somewhat randomly, being able to track where the next Olympic Games would be held. If you want to know how it works, Hublot kindly made a wrist-sized version in 2011 that you can play with (if you’ve got £254,000 to spare).

How the celestial mechanics at the heart of this remarkable machine were shrunk down to something more wearable is a little more vague. During the Middle Ages astronomical clocks that showed the movement of the planets and the moon, as well as the time and month were developed, but because they represented Earth as the central body with the sun orbiting around it, they fell out of favour when science proved them wrong. The next sighting of this evasive complication was on German long-case clocks at the end of the 16th century, then in England in the 17th century. It was in these clocks that the mechanism by which the moon disc moves today was first seen.

At its heart is a 59-toothed gear wheel that clicks forward once every 24 hours by a metal finger connected to the hour hand. Connected to this gear wheel is a disc with two moons painted on it, one at six o’clock and one at 12. The curves at either edge of the aperture cover parts of the moon, making it appear to wax and wane. It takes 59 days for the disc to complete a single rotation, the equivalent of two lunar cycles.

This system migrated from clocks to pocket watches, where it was first seen sharing dial space with a perpetual calendar by English master watchmaker Mr Thomas Mudge around 1762. The first wristwatch to display a moon phase appeared in 1925, courtesy of Patek Philippe – hardly surprising given the moon phase is one of the Swiss maison’s signature complications.

Although watchmakers have experimented with the aesthetic of the moon phase – from the cherubic visage popular in the 17th century through the winking man of the 18th and 19th centuries to the realistically cratered versions of today – it’s only recently that they have sought to improve its accuracy. “There are, in fact, two accuracies with moon-phase complications,” says Mr Romain Marietta, head of products at Zenith. “There’s the accuracy of the moon phase compared to our real moon and the absolute accuracy of the system itself. Regarding the first point, our moon realises a complete cycle in 29.53 days. The traditional moon-phase complication accuracy is one complete cycle every 29.50 days.”

This standard was considered acceptable for a long time, but in a world of COSC certifications and silicon balances, every fraction of a second counts. Being accurate to one day every two years, seven months and two weeks wasn’t deemed precise enough. The next step, then, was a 135-toothed gear wheel that needed adjusting only every 122 years. Then astronomers discovered that a lunar month was actually 29.530587981 days, which meant watchmakers had eight more decimal places to add into their calculations.

“For my new ultra-precise Moonment, these were the calculations,” says Mr Stepan Sarpaneva, independent watchmaker and moon-phase obsessive. “Based on a lunar month being 29 days, 12 hours, 44 minutes and 2.8 seconds, we ensured that our lunation, or lunar cycle, ran as close to that as possible. Ours is 2.3 seconds rather than 2.8. At this accuracy, we calculated that only after 175,500 lunations would you need to offset a day.” With one day being the equivalent to 0.00000569 lunar months and with 12.5 lunar months in a calendar year, Mr Sarpaneva’s Moonment complication should lose a day every 14,000 years. “Of course, this is simplified,” he says.

Mr Sarpaneva’s moon-phase watches may sound over-engineered, but they’re nothing compared with one made by Mr Andreas Strehler, the Sauterelle À Lune Perpetuelle, which needs to be adjusted by a single day every 2.060 million years, meaning the complication will outlast the rest of the watch. And civilisation.

Most brands, such as Vacheron Constantin and Parmigiani Fleurier, hover around the 122-year mark, while IWC’s Portofino Hand-Wound Moon Phase is good for 577.5 years, which still means you’ll be leaving a reminder in your will for the grandchildren to reset it. By which time, the watch itself will probably be as whimsical an anachronism as moon-phase indicators seem to us now.