At one time the moonphase was as practical as any other complication. It’s easy to forget just how big a difference moonlight made before the invention of artificial lights – this may sound like an obvious observation but nighttime used to be dark, and thanks to the light pollution created by towns and cities, most of us are never going to experience it.

Today, the moonphase is, like mechanical watchmaking itself, an anachronism – an anachronism on an anachronism, as a matter of fact. However, as George Daniels wrote of the remontoire, in Watchmaking, “The fact that the mechanism is quite unnecessary merely adds to its charm.”

The moonphase, logically enough, is expected to show the current phase of the Moon. The amount of time it takes for the Moon to return to a particular phase – the New Moon, for instance – is called a synodic month, which on average is 29 days, 12 hours, 44 minutes, 2.8 seconds (the actual length can vary by as much as seven hours for any given month, thanks to the eccentricity of the Moon’s orbit but the average is precise). That’s 29.530589 days in decimal terms. A standard moonphase complication rounds that off to 29.5 days by using a moonphase disk with 59 teeth that’s advanced once per day, but this means that the display will be off by a full day after about two and a half years.

In practical terms this is probably only relevant if you’re a werewolf, but the pursuit of precision for its own sake is a big part of what makes watchmaking interesting, and for several decades now, watch brands – independent and otherwise – have pursued improvements in the precision of their moonphase displays.

One of the first shots fired was by IWC. The company’s head movement constructor, Kurt Klaus, developed a high-precision moonphase display while working on the perpetual calendar/chronograph movement for the Da Vinci, in the 1980s. He calculated a gear train that would be off by a day every 122 years. Current perpetuals from IWC are accurate to one day every 577.5 years.

You’d expect a high precision moonphase from a German brand and you’d be right – A. Lange & Söhne introduced the Lange 1 Moonphase back in 2002, which is accurate to one day in 122 years. They did better than that, however, in 1999, with the limited edition 1815 Emil Lange moonphase watch. This was made in a series of 150 pieces in platinum, and another 250 in pink gold, and its moonphase indication is accurate to one day in 1058 years, which is really getting up there. The Richard Lange Perpetual Calendar Terraluna is also accurate to one day in 1,058 years; it also has one of the most beautiful examples of a tellurium (an astronomical complication showing the relative positions of the Earth, Moon, and Sun) that I’ve ever seen.

At this point, things start getting more incremental than not – or maybe I should say, they get incremental until they don’t. It’s also at this point that independent watchmakers really start to come into their own – I don’t know why, maybe it’s because they’re a bit more apt to pursue the art for its own sake rather than at least partially to goose the shares of a publicly traded company (though you can’t blame business for running themselves like businesses).

Generally the accuracy of high accuracy moonphase watches seems to plateau a bit in the low four figures – the ochs und junior moonphase, for instance, uses a gear train which calculates a full synodic month at 29.5306122449 days, which is just shy of 29.530589 days for an actual synodic month. That means it will be 3,478.27 years (according to ochs und junior) until the moonphase is off by a day. Not only is this well beyond the realm of practical human life (3400 years ago, Mycenaean Greece was in its prime and Linear A was the way to write if you wanted to be written about) any incremental inaccuracy is so low that it’s well beyond the ability of the human nervous system to resolve it, even over the course of an entire human lifespan (or several human lifespans).

If, by the way, you want something a little less redolent of human perambulations and with a greater air of reassuring permanence, there’s always an Atmos clock. JLC’s Classic Moon Atmos will be off by one day in 3821 years, using a gear train approximating a synodic month to the tune of 29.530568 days. But if you want something that makes a conversation piece both in your living room and when you’re out for a night on the town, there are even more precise moonphase watches out there.

One of the objections raised to a conventional moonphase display, which shows the visible part of the Moon via an aperture in the dial, is that it is not an accurate representation of what you see when you actually look at the Moon over the course of a month. A half-Moon, for instance, has, as seen from Earth, a more or less straight line across its face, because you’re looking at a sphere, viewed from the side, which is lit on one hemisphere and dark on the other. To address this problem, moonphase displays were invented which use a spherical miniature Moon, and the most accurate is from independent watchmaker Christiaan Van Der Klaauw. Van Der Klaaw’s spherical moonphase complication is accurate to one day’s error in 11,000 years.

If, however, you want something which blows the idea of incremental improvement right out of the water,  the man you want to talk to is Andreas Strehler. Strehler is not a name you’re likely to be familiar with unless you’ve been taking a close interest in independent watchmaking, but if you’re looking for a record for moonphase accuracy which is unlikely to ever be beaten – and I do mean ever – you should be familiar with the Sauterelle à lune perpétuelle.

This particular wristwatch is accurate to one day’s deviation in – according to Strehler and confirmed by the good folks at the Guiness Book Of World Records – 2.045 million years. That’s long enough ago that modern humans didn’t exist – as a matter of fact, early members of the genus Homo were just starting to shuffle out of Africa onto the face of an unsuspecting planet. Now that’s a watch that’ll tell you when the next full Moon is for the long haul – a reassuring timepiece indeed, for immortal lycanthropes and the people who love them.