Set a moonphase

How the disc moves

The disc usually carries two moons and advances one step a day. The curved “humps” at each side of the window hide the moon as it enters and leaves, imitating the waxing and waning moon. Press the practice corrector to see it move.

The general process

• 11. Find a reference date

  Look up the date of the most recent full moon (or new moon) in a published almanac or a reliable astronomy source. Use the guide above only as a rough check.

• 22. Check the time and safe window in your manual

  Many manuals say not to adjust the moonphase or date during a certain period, often in the evening and overnight. Your manual will say whether you should move the hands to a particular position first.

• 33. Bring the disc to the reference phase

  Using the method your watch uses — a recessed corrector pressed with the supplied tool, or a crown position — move the disc until a moon is at the reference phase. For full moon, that's usually a moon sitting centrally in the window.

• 44. Advance by the days since then

  Count the days from the reference date to today and advance the disc that many steps. Then set the date and time as normal.

Why instructions vary between watches

• Corrector type: some use a pusher in the case side that needs a soft tool; others set the moon through the crown, sometimes linked to the date.
• Gearing: a common 59-tooth disc drifts about a day every 2½–3 years; more precise mechanisms drift far less, so their step size and setting method differ.
• Linked calendars: on annual and perpetual calendars, the moon, date, day and month are connected, and the setting order is critical.
• Hemisphere: some dials show the moon as seen from the southern hemisphere, which looks mirrored.
• Quartz moonphases often set automatically from the date, or through a mode in the crown or buttons.

If you're unsure, a watchmaker or the manufacturer's service centre can set it for you.