Will an atomic watch work where I live?

Two answers circulate. One says a radio-controlled watch works anywhere in the United States, or anywhere in Europe. The other says it only works within a few hundred miles of the transmitter and is a gimmick everywhere else. Both come from real manuals: the makers print different distances for the same transmitters. We set the reception ranges from eleven Casio, Seiko and Citizen manuals beside what the five agencies that run the stations publish, then measured the distance from 32 cities to their nearest station. The result is a table you can read your own city from, and a rule for the cities in between.

The six stations a watch can hear

A radio-controlled watch is a quartz watch with a small longwave receiver. The transmitters it listens to are run by national time agencies, and each publishes its own figures. The agencies' numbers below come from their own pages; where an agency gives a power but no range, the table says so.

All six sit between latitude 33 and 55 degrees north. There is no station a mainstream watch can receive in the Southern Hemisphere, in India, or anywhere in Southeast Asia south of China.

Not every watch hears all six. Casio's Multi Band 6 models, described on the Casio page, receive all of them. Seiko's 8B63 and 7B62 calibers receive all six; its 1B21 and 1B25 calibers receive JJY, BPC, WWVB and DCF77 but not MSF. The Citizen manuals we read for the H149, H800, H820 and H874 calibers list five stations in four regions, with no MSF, and the older E650 lists only WWVB and DCF77. In Britain or Ireland, a watch without MSF relies on DCF77 from Germany.

What the makers print, station by station

Each figure is the manual's approximate radius of reception. Casio prints the same figures in all four manuals we read (modules 3537, 3541, 5673, 5713), as does Seiko (8B63, 7B62, 1B21, 1B25). Citizen's manuals disagree with each other, so they get two columns.

The spread is widest where most buyers live. For the US signal, the published radius runs from 1,500 km to 3,000 km: a circle that stops short of Seattle, or one that takes in New York and Miami. For MSF it runs from 500 km to 1,500 km. Citizen's E650 manual prints 3,000 km for WWVB and 1,500 km for DCF77, the same as its H874; only the H149 manual is more cautious.

No maker presents its figure as a hard edge. Seiko says the watch "may be able to receive radio signals outside a reception range if the receiving conditions are favorable"; Casio lists what can stop it inside: "geographic contours, weather, the season, the time of day, wireless noise."

Which figure is right, and why both persist

The larger figures are closer to what the operators say, and the reason they differ from the smaller ones is the time of day. PTB, which runs DCF77, explains that the signal reaches a receiver two ways: as a ground wave along the Earth's surface, and as a sky wave reflected off the lower ionosphere. The single-hop sky wave reaches about 1,900 km by day and about 2,100 km at night, and PTB states that the field strength normally achieved at 2,000 km is enough for commercial receivers. NIST's simulated coverage maps for WWVB show the same pattern: the area where a clock should synchronize "is largest at night, and smallest in the daytime."

So the cautious figures are closer to reception at any hour, and the generous ones to reception at night, which is when the watches try. Casio's 5713 attempts between midnight and 5:00 a.m. and stops once one succeeds; Citizen's H800 tries at 2:00 a.m., then 3:00 a.m.; Seiko says to leave the watch by a window at bedtime. NIST gives the reason: the signal "is easiest to receive when it is dark at both WWVB and at the site where the clock is located."

Both answers persist because both match some owners' experience. NIST says it has heard from "many owners" whose clocks fail inside the mapped coverage, probably because of local interference, and from Alaska, outside the maps, where clocks "work fine," probably because a sparsely populated area has little radio noise. Distance sets the odds. The room the watch spends the night in decides the result.

City by city: distance to the nearest station

We measured the great-circle distance from each city center to each station, using the station coordinates published by NIST, PTB, NPL and NICT (and, for BPC, the commonly listed site near Shangqiu), and rounded to 10 km. The verdict column applies the makers' figures above.

The US East Coast is where the brands split: Casio and most Citizen manuals include it, Seiko does not. Lisbon, Athens, Istanbul and Helsinki are outside every maker's circle but inside the 2,000 km PTB gives for DCF77, so night reception there is possible but not promised.

The watch must also know where it is

Distance is not the only condition. These watches choose which station to listen for from the home city or time zone you set. Seiko's 8B63 manual says the station "will be automatically changed accordingly to the selected time zone"; Citizen's H800 says "when the time difference is corrected, the signal station is switched"; and Casio's 5713 will not receive at all "while the Home City area setting is one where time signal reception is not possible." A traveler who leaves the home city on New York while in London gets no correction. Set the home city first, then judge the reception. Our page on world time watches covers city codes, and what a GMT watch is for covers the traveler's other option.

The hour shown comes from the zone and summer-time rule stored in the watch. NIST says a clock off by an hour "probably has to do with a time zone setting", and Casio's 5713 manual warns that summer time will be wrong if the authorities change their rules.

How to get a sync where reception is marginal

1. Leave it by a window overnight. All three makers say so. Casio wants the 12 o'clock side toward the window; Seiko and Citizen put the antenna at 9 o'clock and want that side facing the station.
2. Keep it still and alone. Do not move it or press a button during reception, which cancels it on Seiko watches. Keep it away from metal, phones in use, computers and appliances. NIST suggests 1 to 2 meters from a computer monitor.
3. Charge it. A solar watch in its low-charge state will not receive; on Citizen's H800 that is the seconds hand jumping every two seconds. Our page on charging a solar watch gives the times, and the Eco-Drive entry explains the cell.
4. Give it time. Casio's 5713 takes about 2 to 10 minutes, and up to 20. Citizen's H800 on-demand reception takes 2 to 30 minutes.
5. Change the room before the watch. Reinforced concrete, metal roofs and steel siding block the signal. Citizen and Casio both list vehicles, high-voltage lines and mountains.

What the watch does without the signal

A radio-controlled watch that never syncs is a quartz watch. Casio's 5713 and Citizen's H800 both rate the movement at ±15 seconds a month without reception, which is the standard figure in our answer on how accurate a quartz watch is. At that rate, a watch that syncs once a week is never more than about 4 seconds out, and one that syncs once a month stays within about 15. Cold makes quartz lose a little, as our page on cold and accuracy explains, and a sync wipes that out.

So on the edge of the range, occasional reception still delivers most of the benefit. Check rather than guess: Casio and Citizen watches show whether the last reception succeeded, and our guide to testing a watch's accuracy at home shows how to time it against a reference.

If you live outside every range

Buy for the other features, or for a different correction method. Casio's 5713 module corrects from a paired phone and tries radio only if it could not connect the previous day; phone-linked watches are covered in our smartwatch comparison, and the Citizen and Seiko pages cover their satellite models. A high-accuracy quartz watch needs no correction at all for most purposes: Citizen's caliber 0100 is rated at ±1 second a year with no outside signal, as set out in our Eco-Drive and Seiko solar comparison.

When this answer stops applying

• Your manual gives different figures. Your caliber's own manual beats this page; even Citizen's manuals differ from each other.
• The watch hears fewer stations. Citizen's E650 receives only WWVB and DCF77; check your manual's station list before reading the city table.
• A station is off the air. NIST publishes a log of WWVB outages. A watch that fails for a few nights running may be waiting for the transmitter.

For the wider picture of how accuracy is rated and certified, see our guide to watch accuracy standards.