Moondial
Put your watch’s moon back where it belongs.
A moon phase display drifts, and once it has drifted it is genuinely hard to reset by eye: the corrector only goes forwards, the disc carries two identical moons, and half the instructions on the internet are guesswork. Tell this page what your dial is showing and it will tell you exactly how many presses to give it.
Reading the sky…
Why the moon on your watch drifts
The ordinary moon phase complication is a disc with 59 teeth carrying two moons. A finger on the hour wheel pushes it one tooth every 24 hours, so the disc turns once every 59 days and each moon walks through a cycle of 29.5 days.
The Moon takes 29.530589 days on average to get back to the same phase. The disc is therefore running about 44 minutes fast every month. That is invisible at first, then after roughly two years and eight months the display is a full day ahead of the sky, and after five years it is two days out.
High-precision trains attack the same problem with more wheels. A 135-tooth train stays within a day for about 122 years; the astronomical ones run for centuries or millennia. None of them change how you set the watch — one press is still one day.
Why resetting it is awkward
- The corrector only advances. If the display is one day ahead you cannot take a day off it; you have to push it 28 or 29 days forward, all the way round.
- The two moons look identical. There is no way to tell which of them is in the window, and they sit half a day apart, which is where the last bit of error usually hides.
- A crescent is hard to judge. Nobody can look at a sliver of moon in a small window and name the day. Full moon, dead centre in the window, is the one position you can identify exactly, which is why setting from full moon is the reliable method.
How this page works it out
Phase instants come from Jean Meeus, Astronomical Algorithms, chapter 49, which places new and full moons within a few seconds of the published almanac values, and the illuminated fraction comes from the Moon and Sun positions of chapters 47 and 25. The Moon does not run to a metronome: individual lunations range from about 29.27 to 29.83 days, so a calculation built on an average month can be half a day out on its own. The arithmetic runs entirely in your browser.