How accurate is a moon phase watch?

A standard 59-tooth moon phase is out by a day after about 2 years and 8 months. Precision versions with larger gear trains stay within a day for more than a century.

How do I set a moon phase?

Look up the date of the last full moon, advance the disc until the full moon is centered in the window, then advance it once for each day since. Avoid using the corrector while the date is changing, usually between 9 p.m. and 3 a.m.

Calendar complication

What is a moon phase watch?

A disc that shows the current phase of the moon through a window in the dial, advancing a little every day.

The moon today

Full moon

100% lit · 15.4 days old

Next full moon November 24, 2026; next new moon November 9, 2026. Set a standard moon phase watch today and it will be a full day out by about June 18, 2029.

How it works

A moon phase display is a disc printed with two moons, turning behind a window shaped so that the moon appears to wax and wane. The disc advances once a day, driven by the calendar mechanism.

The time from one new moon to the next is 29.53 days. Most moon phase watches use a 59-tooth disc, which covers two lunar cycles in 59 days, or 29.5 days each. That's close, but it loses about 0.03 of a day every month, so a standard moon phase drifts a full day out every 2 years and 8 months or so.

High-precision moon phases use a larger gear train, often a 135-tooth wheel, and stay within a day for over 100 years. To set any moon phase, advance it to the most recent full moon and then forward the right number of days.

History

Moon phase indicators appeared on astronomical clocks centuries before wristwatches, when the moon mattered for travel, tides and farming. On wristwatches they became a classic companion to triple and perpetual calendars from the 1920s onwards, and today they're mostly chosen for their looks.

Before you buy

Moon phases are one of the most affordable complications: you can find them on quartz watches for a few hundred dollars and on Swiss automatics from around $1,500. If you won't wear the watch often, remember that an automatic stops and the moon phase needs resetting.

Moon phase watches we've measured

Frederique Constant Classics Moneta Moonphase 37mm FC-206N3S6
37 mm case · — lug-to-lug · $1,495
Omega Speedmaster Moonphase Meteorite 304.30.43.52.01.001
43 mm case · 48.6 mm lug-to-lug · $19,300

Questions

How accurate is a moon phase watch?: A standard 59-tooth moon phase is out by a day after about 2 years and 8 months. Precision versions with larger gear trains stay within a day for more than a century.
How do I set a moon phase?: Look up the date of the last full moon, advance the disc until the full moon is centered in the window, then advance it once for each day since. Avoid using the corrector while the date is changing, usually between 9 p.m. and 3 a.m.