How Does an Atomic Clock Know What Time It Is?

Atomic clocks are famous for keeping incredibly accurate time. But if you've ever owned a SkyScan atomic clock, you may have wondered something:

If my clock runs on AA batteries, how can it possibly be an atomic clock?

The answer is both simple and fascinating. Your SkyScan clock doesn't contain an atomic clock—it listens to one.

What Is an Atomic Clock?

Unlike ordinary clocks that measure time with gears, quartz crystals, or electronic oscillators, an atomic clock measures the natural vibrations of atoms.

The international definition of one second is based on the vibration of a cesium-133 atom. Specifically, one second is defined as exactly 9,192,631,770 vibrations of that atom.

Because those vibrations are extraordinarily consistent, atomic clocks are the most accurate timekeepers ever created.

Your SkyScan Clock Isn't Actually Atomic

This surprises many people.

Your SkyScan clock contains a high-quality quartz clock that keeps time just like most digital clocks. What makes it "atomic" is its ability to automatically synchronize with the official U.S. time signal broadcast by the National Institute of Standards and Technology (NIST).

In other words, your clock regularly checks its time against one of the most accurate clocks on Earth and corrects itself whenever necessary.

Meet WWVB

The official U.S. time signal is transmitted by radio station WWVB, located near Fort Collins, Colorado.

WWVB broadcasts a continuous low-frequency radio signal at 60 kHz that carries the current date, time, daylight saving status, and other information.

Millions of atomic clocks across North America quietly listen for this signal every day.

Why Does My Clock Usually Set Itself at Night?

Many owners notice that their clock seems to update overnight.

That's because radio reception at 60 kHz is usually much better after sunset. During the day, sunlight energizes Earth's upper atmosphere, making long-range radio reception more difficult.

At night, those atmospheric effects decrease, allowing the WWVB signal to travel much farther with less interference.

For this reason, most radio-controlled clocks attempt synchronization during the late evening or early morning hours.

What If My Clock Doesn't Receive the Signal?

Don't worry.

Your clock continues running on its internal quartz movement. It simply waits for the next successful synchronization attempt.

Even without receiving WWVB for several days, a quality quartz movement is typically accurate enough that you'll never notice the difference.

Why Doesn't My Clock Always Synchronize?

Several factors can affect reception:

Distance from the WWVB transmitter
Building construction materials
Nearby electronics
Metal window frames
Weather and atmospheric conditions

Simply moving the clock a few feet—or placing it near a window overnight—can often improve reception.

One Clock Helps Keep an Entire Nation on Time

Every day, millions of clocks, watches, weather stations, and other devices quietly synchronize with WWVB.

It's one of those remarkable technologies that most people never notice, because it simply works.

Behind that simple convenience is a network of atomic clocks whose accuracy is measured in billionths of a second.