Collection: Citizen Tsuki-yomi A-T Watches

Citizen Tsuki-yomi A-T

Tsuki-yomi A-T was the first moon phase watch with atomic timekeeping, according to Citizen: it sets itself from radio time signals and shows the current phase of the moon in a window at 6 o’clock. It is a 43mm Super Titanium Eco-Drive with world time in 24 zones and a perpetual calendar, under sapphire crystal.

What it does

Calibre H874 is an Eco-Drive movement, charged by light, that brings together several functions most watches keep apart. Atomic timekeeping corrects the time and date from radio signals sent by transmitters in Japan, China, Germany and the United States, late at night when there is less radio noise. The perpetual calendar handles every month’s length on its own, leap years included, so the date never needs moving forward by hand. World time covers 24 time zones, and the day and date share the dial with the moon phase. Citizen’s other self-setting models are in our atomic timekeeping watches.

The case and the dials

Case and bracelet are Citizen’s Super Titanium, which Citizen says is five times harder and 40% lighter than stainless steel. The whole watch weighs about 105 g, light for a 43mm case, and at 10.87mm it is far slimmer than the Skyhawk A-T, at around 14mm. Citizen styles the dial on the surface of the moon, in deep colours: blue, red, grey and a purple version, the Tsuki-yomi A-T BY1010-81Z. The crystal is sapphire, the bracelet closes with a fold-over clasp and push buttons, and the watch is water resistant to 100 metres. For a wider choice of lighter watches, see our titanium watches.

Living with it

Give it light and it looks after itself. Daylight by a window charges an Eco-Drive fastest, indoor light works more slowly, and a fully charged one keeps running for months without light. Our guide on how Eco-Drive works explains the charge and what the watch does when it runs low. For reception, Citizen notes that buildings, terrain and weather can get in the way, so leaving the watch by a window overnight helps. Rinse it in fresh water after the sea, and keep it off a hot dashboard.

Questions

Does it need a battery? No. Eco-Drive stores the light it collects in a rechargeable cell designed to last for many years.

What if it can’t pick up a signal where I live? It keeps running on its quartz movement, which is accurate on its own, and corrects itself the next time it receives a signal. You can always set it by hand. If moon phase is what you’re after and you don’t need radio control, compare our moon phase watches.

Can I swim with it? Yes. At 100 metres, Citizen’s chart rates it for swimming and snorkelling, not scuba diving.

How does it compare with the Citizen Calendrier? Both show the moon phase on Eco-Drive. The Citizen Calendrier is a steel watch with the month, day and date on hands and no radio control. The Tsuki-yomi adds atomic timekeeping, world time and a perpetual calendar, in titanium.