The Citizen Tsuki-Yomi A-T Moonphase Perpetual, And How Radio Atomic Time Really Works

More than meets the eye.

There is a tendency, which I’ve fallen victim to myself over the years, to think of radio controlled watches and clocks as fundamentally uninteresting and even writing them off as parasites, unworthy of a serious enthusiast’s interest. The latest Eco-Drive release from Citizen prompted me to look a little harder at what radio controlled watches really are, and how they really work, and there is as it turns out (and this is so often the case in both life and watchmaking) that if you approach the subject will an open mind, you will have room to fill your teacup with as it were, the decoction of wisdom 😀.

The Tsuki-Yomi AT is a somewhat bewildering watch to look at, at least at first, but there is a logic to its design which you can understand better when you remind yourself of its functions. It is a moonphase watch, showing the moonphase in both the Northern and Southern hemispheres; it is also a perpetual calendar (and we all know what that is: A watch that correctly switches to the first of the month at the end of every month of both 31 and 30 days, as well as switching on the 28th of February, or the 29th in a Leap Year). It is also a world time watch, capable of switching to the correct local time automatically, and it can be set to show either summer time DST all the time by default, standard time all the time by default, or to automatically switch between the two depending on the time signal. There is also the option to manually connect with a radio time signal, or to let the watch update itself automatically, which it will do between 2AM and 4AM local time, every day.

With this all in mind, most if not all of the indications above are intuitive, or close to it with a little explanation. Moving around the dial proper, starting from 9:00, we have an AM/PM indicator, and then a subdial which shows the day of the week by default, but which can also show the remaining power reserve (full/empty; you can check the power reserve by pushing the lower of the two pushers)) as well as whether or not the moonphase is set for the Northern or Southern hemisphere. The date is at 4:00, and the moonphase at 6:00.

On the rehaut, starting at the top we have a scale marked from -10 to +10 which is used to set the current time zone – basically the numbers show whole hour offsets from GMT, Next, moving clockwise, is an indication for automatic or manual summer time, and directly opposite, an indication for showing whether the watch is set for automatic or manual standard time. Finally, at the bottom, is an indicator showing whether or not the last attempt to calibrate the watch to a radio time standard was successful; yes means yes, no means no, and “RX” indicates that you’re attempting to connect the watch manually.

The various setting maneuvers are complicated enough that you need both the crown and the two pushers to set the watch up but once you have done that, the watch requires little if any human intervention, unless you want to change one of the settings. The time and date will update automatically if you change time zones as long as the watch can receive a radio signal (note that both airplanes and automobiles are Faraday enclosures and will block radio signals – the time and date therefore will not update when you fly, for instance, until you exit the aircraft. Come to think of it, elevators are Faraday enclosures as well (which is why cell phones tend to cut out inside them) so if you happen to be inside one between 2AM and 4AM I guess you’d have to calibrate the watch to the radio time signal manually that day … or just wait a day, which should have no effect on precision.

One thing to bear in mind is that when you travel, you will need to reset the time zone manually since the radio stations the watch is set up to receive, broadcast across multiple time zones (station WWVB, at Fort Collins, Colorado, broadcasts UTC time so the watch needs to be set to the appropriate time zone in order to derive the correct local time from the radio signal).

The watch, like all Eco-Drive watches, has a substantial power reserve and if you keep the watch in total darkness, it will keep time for up to six months (although the hands will stop moving in order conserve power).

Setting the correct time zone when you are setting up the watch requires you to know the offset of your time zone from GMT but Citizen has conveniently engraved the information on the back of the case.

Water resistance is 10BAR/100 meters and while the watch is rather large, at 43mm, it’s a relatively flat 10.9mm thick and that, plus the Duratect-treated Super Titanium case and bracelet, mean it should be pretty light on the wrist. Super Titanium is about five times harder than 316L stainless steel (Duratect is the case hardening technology, and the final product is Super Titanium).

And then there is the name – Tsukuyomi-no-Mikoto is the Shinto god of the Moon; to this day, the Japanese enjoy the tradition of celebrating the mid-autumn Moon Festival, which marks the beginning of the harvest season.

How does the moonphase work? Well, the shadow of the waning Moon appears to move from right to left if seen from the Northern hemisphere, but from left to right from the Southern; the arrows adjacent to the moonphase display show which direction the lunar disk is moving. NASA has a couple of great animations showing how the Moon’s shadow moves above and below the Equator. (Accompanied by, I should warn you, some rather frisky music. I would have gone for something a little more baroque or earlier – Hildegard of Bingen, maybe. If anyone knows if the position of Senior Vice President of Music for NASA is open, let me know).

One final note on the dial – it is meant to be evocative of the surface of the Moon as seen under different conditions; the red dialed version, for instance, represents the so-called “blood moon.” I would go for that one if for no other reason than that my entire collection seems to consist, with few exceptions, of black dialed watches.

Let’s look at radio controlled Atomic Time, which Citizen calls A-T. The Man Behind The Curtain, as it were.

Time, Time, Atomic Time

It is probably natural to think of radio controlled watches as a simplistic and unromantic solution to the problem of precision timekeeping – to consider them little more than zombies animated by a distant, imperiously uncaring master. In fact, of course, things are much more interesting than that and the radio controlled watch is reliant on a transmission and timekeeping network as full of human intelligence and ingenuity, and indeed, real passion for precision, as you could wish.

Radio controlled watches are relative newcomers, inasmuch as they require a network of radio stations dedicated to broadcasting a radio time signal. The very first was the Junghans Mega 1, from 1990. The Mega 1 received a time signal from the station in Frankfurt (via an antenna which was concealed inside the strap, so don’t go swapping in a NATO) which was controlled by the national atomic clock at Braunschweig. In 1993, Citizen launched the first multi-band radio controlled watch, the reference 7400, which got around the problem of needing a large antenna through the simple and rather daring expedient of making the antenna coil the center of the design.

Your watch has an antenna? My antenna has a watch.

Today, Citizen’s mult-band A-T watches can set themselves to a time signal from any of five radio stations around the world. The stations are all in the Northern hemisphere but they all have reception ranges, as given by Citizen, varying from 1500 to 3000 kilometers although these are just approximations; the actual reception range varies (sometimes a lot) according to atmospheric conditions. The longest range transmitter is located in Fort Collins, Colorado, and the radio station has the call sign, WWBV. The time signal service is courtesy the folks at NIST, the National Institute of Standards and Technology. WWVB began transmitting as radio station KK2XEI, in Boulder, in 1956 but in 1962, NIST began the process of moving the station to its present location in Fort Collins. Part of the reason for relocating was that Fort Collins is further from the mountains and is therefore a better location for broadcasting an omnidirectional signal.

The time signal itself is an amplitude modulated (AM) signal and each 60 second broadcast contains information about the time and date, as well as an indication for the day of the year, the offset of Universal Time, which is defined by the Earth’s rotation, from Coordinated Universal Time (UTC) which is defined by atomic clocks (which are, sorry Mother Earth, more stable than the Earth’s orbit).

The time signal is ultimately derived from the NIST-F2 cesium atomic clock, which is located in Boulder and which has a precision equal to about one second every 300 million years.

Citizen’s figure for the precision of the Tsuki-Yomi is one second every 100,000 years. If NIST-F2 is precise to one second every 300 million years, why isn’t the watch equally precise?

A radio controlled watch, is a stand-alone quartz watch (in this case, precise to ±15 seconds per month) but which has been fitted with an antenna and software for decoding the radio time signal. NIST-F2 transmits its time signal to Fort Collins, where it is used to control the frequency of a suite of smaller, on-site atomic clocks whose ensemble produces the time signal. The signal is then broadcast at one bit per second over a one minute period, on a 60kHZ carrier wave. (The antenna for the Tsuki-Yomi is located at 9:00 under the dial and the manual advises aiming the antenna at the transmitter, if possible, when manually synchronizing).

Every step of the transmission process has the potential to introduce some frequency instability, and the biggest single source as far as I can tell from poring through NIST’s extremely extensive literature on WWVB, is variation in transmission time. I would imagine that the time signal processing system in the watch must also account for the fact that the information takes up a one minute window and is not received instantaneously. The drift in the watch’s quartz oscillator is corrected once per day, which may introduce further frequency instability but the upshot is still that you would have to wait a hundred thousand years for the watch to be off by one second. You’ll notice, by the way, that the time signal does not include information about the moonphase – that data is derived from the date information, using the first-of-its-kind “Luna Program” system from Citizen . Setting the year for the moonphase is a separate operation, as the time of the month for each full and new Moon changes over an 18 year cycle, known as the Metonic cycle.

If you’re interested in the full operational details of the watch I recommend checking out the very detailed manual for module H874, right here.

The watches are all priced at $850 which is a wild value proposition no matter how you slice it; the more I thought about it the more unbelievable it seemed, but here we are. While the setting procedure is a bit arduous, the watch is essentially care-free once the initial setup is completed (that said, I’d hang onto that manual, just in case). You get a technically sophisticated piece that implements an underappreciated but surprisingly fascinating technology, a beautiful implementation of the moonphase display, and a watch that travels just as easily as it stays at home (although I suppose given the locations of the transmitters you’ll struggle to get a time signal as you go further south of the Equator).

All this and a sapphire crystal too.

Edited 10/4/2023 to clarify that time zone needs to be manually re-set when changing time zones.

Find out more at CitizenWatch.com.