Collection: Moon Phase Watches

Moon phase watches, and how they work

A moon phase display shows where the moon is in its monthly cycle, from new moon to full moon and back. Citizen offers it two ways on this page: the Calendrier, a calendar watch whose moon you set by hand, and the Tsuki-yomi A-T, which works out the moon’s phase on its own and corrects its time by radio.

Reading the display

A small disc shows the moon growing and shrinking through the month. Citizen counts it in days of lunar age: day 0 is the new moon, when the moon is hidden, first quarter comes at about day 7, the full moon at about day 15 and the last quarter at about day 22. The display is only a guide to the phase and won’t match the moon’s exact shape, and Citizen suggests setting or checking it on a new or full moon, when the phase is easiest to read.

Calendrier: set by hand

The Calendrier puts the whole calendar on one dial. On the men’s model the date has its own hand, the month and day of the week sit in windows, and the moon phase has its own display. You set the moon once with the crown, matching it to the real moon, and from then on it moves by itself. The date follows a 31-day cycle, so after a 30-day month or February you move it on by hand. The men’s Calendrier BU0057-54E is 44 mm in grey steel with a black dial; the women’s Calendrier is 37 mm with a mother-of-pearl dial set with diamonds. Both run on Eco-Drive, and the men’s keeps going for about six months on a full charge. The Citizen Calendrier page has the whole line.

Tsuki-yomi A-T: set by radio

The Tsuki-yomi A-T was the first moon phase watch with atomic timekeeping. It receives time signals from stations in Japan, the United States, Germany and China, and calculates the moon’s phase from its own time and date, so there’s nothing to set. A setting switches the display for the southern hemisphere, where the waxing and waning look different, and you can adjust it by up to two days either way if it ever disagrees with the sky. It also shows world time in 24 zones and keeps a perpetual calendar until 28 February 2100. The 43 mm case and the bracelet are Super Titanium, water resistant to 100 m. The Tsuki-yomi A-T BY1010-57L has a blue dial, and Citizen also makes it in purple, red and grey. There’s more on the Tsuki-yomi A-T page.

Questions

Is the Bulova moon watch a moon phase watch? No. The watch people call the Bulova moon watch is the Lunar Pilot, a chronograph based on the Bulova worn on the moon in 1971. It has no moon phase; you’ll find it on the Bulova Lunar Pilot page.

Will I need to reset the moon phase? On the Tsuki-yomi A-T, no. On the Calendrier, only if the watch runs down and stops, so keep it in the light when you’re not wearing it.

Which one suits a smaller wrist? The women’s Calendrier at 37 mm. The men’s models run from about 42 to 44 mm.