How to read a moon phase window
Read it by where the painted moon sits. Hidden behind a hump means new moon and centred in the arch means full moon. Coming out on the left is waxing, slipping behind the right hump is waning. On most mechanical watches, each click of the moon corrector moves the disc by one day.
The parts of the window
The window is an arch. Along its bottom edge are two rounded bumps, often called humps or bosses. Behind the arch sits a disc printed with two moons on opposite sides. Only one moon is ever in the arch. The humps slowly cover and uncover it, so a round painted moon looks like it is waxing and waning.
- Moon hidden behind a hump: new moon.
- Moon peeking out from the left hump: waxing, growing towards full.
- Moon centred in the arch: full moon.
- Moon sliding behind the right hump: waning, shrinking towards new.
day 0
day 4
day 7.4
day 14.77
day 22.1
day 26.5
The real sky over the same month, as seen from the northern hemisphere:
Why there are two moons
A full lunar cycle (new moon to new moon) averages about 29.53 days. The common mechanism rounds this to 29.5 days and uses a wheel with 59 teeth. That is two cycles of 29.5 days, so the disc needs two moons: each half-turn of the wheel is one lunation. The date mechanism pushes the wheel one tooth per day, so one click of the corrector is one day. That's why setting instructions say to centre the full moon, then add one click for each day since the last full moon.
Jumping vs. creeping discs
On most mechanical watches the disc jumps once a day, around midnight, when the date changes. Between jumps it doesn't move, so it can only ever be right to the day. Some quartz watches move the disc in smaller steps. The MoonSwatch Mission to the Moonphase is one: you can nudge its disc in small increments. Either way, read the window as "about which day of the cycle".
Other layouts you may meet
- Moon in a sub-dial: the same arch-and-disc idea, made smaller. Omega's current Speedmaster Moonphase (calibre 9904) shows it at 6 o'clock.
- Lunar age scale: some dials print 0–29½ around the window, so you can read the moon's age in days.
- Pointer or indicator style: some quartz models (Citizen's calibre 8730, for example) show the phase with a moving indicator. Citizen's manual describes the indicator as hidden at new moon and straight up at full moon.
- Look-alikes: not every moon on a dial is a moon phase. Some watches use a sun-and-moon disc for AM/PM, or a moon icon for the day of the week. Check before you try to set it.
Southern hemisphere: the sky is mirrored, your watch isn't wrong
From south of the equator the moon appears flipped left-to-right. A waxing crescent is lit on the left rather than the right. Nearly all moon discs are drawn the northern way, so the shape on your watch may look like a mirror image of the sky. The timing is still correct: new, first quarter, full and last quarter happen at the same moment everywhere.