Citizen Tsuki-Yomi Moonphase BY101 Review – A Moon Powered by the Sun

Tags: Review | Citizen | Classic | Traditional

13.11.2025 | 5 MIN

The watch world is wonderfully diverse and never ceases to surprise with both innovation and design. Japan’s giant Citizen has hardly been standing still lately, turning out pieces whose price-to-performance ratio is tough to beat. Let’s take a look at the latest radio-controlled release from the Land of the Rising Sun, the Citizen Moonphase Tsuki-Yomi BY101.

Introduction

The new model arrives in four core colorways: red, blue, black, and grey, the latter being the one in front of me. The watch is powered by Citizen’s Eco-Drive system—a light-powered quartz movement. Everything is wrapped in titanium with a matching bracelet in the same metal.

What does Tsuki-Yomi mean?

I have a deep appreciation for Japanese culture and the mindset behind their manufacturing. It’s rarely just about a straightforward product; there’s often a nod to a rich history. So let’s, we Europeans included, dive into a bit of Nippon lore.

Tsuki-Yomi was the Japanese god of the Moon. You can infer it from the components Tsuki (Moon) and Yomi (counting).

Tsukuyomi — traditional depiction of the Japanese moon god. Source: mythopedia.com

Tsuki-Yomi was once sent by his wife Amaterasu to a feast hosted by Ukemochi, the goddess of food. Ukemochi created the meal by gazing at the ocean and spitting out fish, looking at the forest and spitting out game, and finally, looking at the fields and spitting out rice. Tsuki-Yomi was so disgusted by the way such fine food had been produced that he killed the goddess. When Amaterasu, the sun goddess, learned what had happened, she was so enraged she never wanted to see Tsuki-Yomi again. She moved to the other side of the sky, and day and night were never together again.

Amaterasu — traditional depiction of the Japanese sun goddess. Source: mythopedia.com

Another tale says Tsuki-Yomi went to greet Lucifer, who was on a journey to find his lost wings. The symbolism is beautiful here as well: Lucifer, the morning star and bearer of light, meets the god of the Moon.

A reunion of sun and moon

Moonphase watches have been coming thick and fast lately. From the excellent reissue of the Bulova Lunar Pilot chronographs, to Certina’s Moon Phase line, and the entire Orient M45 collection.

Citizen has joined the fray with a moonphase as well, and true to form, they’ve added another first. Enter atomic-precision timekeeping combined with a moonphase. Thanks to radio control, the technology automatically updates the watch’s time, date, and moonphase based on time signals from atomic clocks with a deviation of one second every 100,000 years.

The time signals are received from transmitters or atomic clocks in four regions: Europe, the United States, China, and Japan.

Even without signal reception, the manufacturer quotes a maximum deviation of ±15 seconds per month. In other words, it’s a highly accurate movement—ideally aided by information from atomic clocks.

The moonphase sits at six o’clock, alongside a reminder that you’re wearing titanium—specifically, Super Titanium, developed in-house and more scratch-resistant than standard titanium (personally tested on the NY0100; apart from a single hairline on the clasp, I haven’t noticed a flaw).

If not for the inscription calling it out, you could easily mistake these for steel. Polished bevels alternate with brushed surfaces, and it’s astonishing what Citizen can do with titanium. At approximately 100 g, they’re ideal daily wearers, and worries about cutting off circulation all but vanish (I can confirm: the only watches that let me enjoy a metal bracelet without hesitation are these Super Titanium ones).

Caliber H874 features a perpetual calendar, which ensures accurate indications in the date window at four o’clock and the day display at twelve. As a bonus, you also get a charge-level indicator here; fully charged, the watch will run for up to 2.5 years without exposure to light. In everyday use, that makes for essentially maintenance-free ownership.

Solar power paired with this set of functions is about as good as it gets. At nine o’clock you can also check whether it’s AM or PM—handy if the night ran long. The new caliber computes the moonphase daily based on the radio signal, and the moonphase display auto-adjusts at six o’clock. Thanks to hemisphere recognition, the phase can be observed correctly from both the Northern and Southern Hemispheres.

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The dial, as you’d expect, has a lunar vibe and is beautifully executed. Citizen’s mastery of dials is no secret—they’ve delivered top-notch work, innovative designs, and Japanese thoroughness time and again.

Sword-shaped polished hands and conical indices are filled with green luminescent material, casting a subtle glow over this lunar scene. An ergonomic 43 mm case with a thickness of 10.9 mm, sapphire crystal included, ensures a perfect fit on the wrist. Lug width is my favorite 22 mm, opening the door to plenty of strap-change options—though who’d swap out such an excellent titanium bracelet?

And amid all this beauty I almost forgot a hidden party trick. Why hidden? Because you won’t notice anything at first glance—until you flip the watch over. Time zones are engraved on the caseback, making it easy to calculate the time at your destination. What a refined touch.

Good times ahead

I can’t speak for rival Seiko, but Citizen has quality control down pat. Another Citizen, another great piece through my hands. I’m genuinely excited to see what Citizen brings next. Even setting aside the innovations and firsts, you always get excellent quality with a healthy dose of durability—without needing to take out another bank loan.

This brand is knocking confidently on the door of the higher-end segment and is deservedly winning a new wave of interest among watch enthusiasts. What else do they have up their sleeve? Only Citizen knows, and we can only wait to see when the next glove lands in the ring. Just look at Seiko and its elevated Grand Seiko line. And Citizen? Who knows—let’s keep being surprised…

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