Keeping Time By Rubidium At The Naval Observatory You know when you dial a number, and a man reads you the exact time at the tone? That precise timekeeping starts at the Naval Observatory in Washington, D.C. NPR's Linda Wertheimer takes a tour.

Keeping Time By Rubidium At The Naval Observatory

Keeping Time By Rubidium At The Naval Observatory

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You know when you dial a number, and a man reads you the exact time at the tone? That precise timekeeping starts at the Naval Observatory in Washington, D.C. NPR's Linda Wertheimer takes a tour.


We're spending part of our summer stargazing. And this week, as part of our series looking at the heavens, we went to the U.S. Naval Observatory, which sits on high ground, overlooking most of Washington. You're probably already familiar with what it does.


FRED COVINGTON: U.S. Naval Observatory Master Clock. At the tone, the Eastern daylight time - 14 hours, 20 minutes exactly. Universal time...

WERTHEIMER: The Naval Observatory is the home of our national timekeeper, but it may be best known now as the official residence of the vice presidents. The observatory was completed in 1893. And basically, it is still doing its original job, still looking at the stars.

GEOFF CHESTER: This is the biggest telescope that we still have here in Washington. And I'm going to take you in there and show you how it works 'cause it's pretty cool. As far as we know, this is oldest continuously operating telescope that is still used for gathering professional data in the world.

WERTHEIMER: Our tour guide is Geoff Chester, who is himself an astronomer. He told us, the telescopes original 1873 lens is still in place, but everything else is state-of-the-art 1890s. This handsome relic of the 19th century now teams up with powerful cameras and computers not dreamed of when it was built. It began by helping ships to navigate. Now it helps steer satellites. Geoff Chester took us inside the white dome where the telescope towers high above, rising to point at its window, the slit in the dome. I can see the eye piece for the telescope, but it's way up there.

CHESTER: Now, we do have ladders and that sort of thing, but that was a very - you know, that's so 19th century.

WERTHEIMER: Oh, my goodness. The whole deck of the round floor of the dome is going up.

CHESTER: Indeed.

WERTHEIMER: Like we're on an elevator that's the entire floor.

CHESTER: We think this is the largest elevator in Washington.

WERTHEIMER: Son of a gun. That was really scary when it started.

Elevating the astronomer to the eyepiece is a bit of a parlor trick these days since it is a camera that captures 30 frames a second that is really looking through the telescope at the stars. So what is the relationship between stargazing and timekeeping? At its most basic, for us earthlings, we observe the rotation of our planet as it moves around the sun, our star, as a 24-hour day. We divide it into hours and into seconds. We relate time to events in our world. But really, Geoff Chester says, we invented time.

CHESTER: As far as we know, we are the only creatures that actually use time as a tool. And the other...

WERTHEIMER: So what happens if you don't keep time?

CHESTER: With the technology that we have today, you would wind up back in the Stone Age, essentially.

WERTHEIMER: Why would you go back to the Stone Age?

CHESTER: Well, think, for instance, about how dependent we are now on things like GPS, cell phones, computer networks - that kind of stuff. You make a phone call with your cell phone. And if there is no precise time synchronization then all of the analog-to-digital conversion, the transmission of the digital data, the decoding of that transmitted data back into an analog signal that goes to the phone and the person that the other and that you are talking to - if you didn't have precise time, that wouldn't happen, period.

WERTHEIMER: It is in the evolution of technology, Chester says, that is driving the requirement for more precise time. And Chester took us to see the precision instruments which now measure time - the atomic clocks in their climate controlled glass cells. These clocks are far beyond anything so basic as noticing when the sun rises or sets. Atomic clocks measure defined seconds. So what is the second?

CHESTER: Now, I only thought it was one Mississippi, but then I started working here.

WERTHEIMER: Geoff Chester tells us, atomic clocks count the oscillations of atoms bombarded by microwaves.

WERTHEIMER: You make a decision that so many billion oscillations of this little atom are going to be one second.

CHESTER: Correct. And that decision was made by a large committee of physicists...

WERTHEIMER: (Laughing) Of course.

CHESTER: ...Back in 1967. And since 1967, that has been what the definition of one second is.

WERTHEIMER: I don't suppose you have that number by heart.

CHESTER: Oh, yes.

WERTHEIMER: Oh, you do?

CHESTER: Oh, yes. So the second today is defined as the interval of 9,192,631,770 transitions of the hyperfine levels in the ground state of the cesium 133 atom.


CHESTER: That's one second.

WERTHEIMER: The observatory also uses the even more precise rubidium atom. And then, as they have always done, they consider a number of clocks and deliver a mean. And that's the time our digital devices, our satellites and our weapon systems use, but they also deliver a time that anyone can use.


COVINGTON: U.S. Naval Observatory Master Clock. At the tone, Eastern daylight time - 11 hours, 13 minutes exactly.

CHESTER: So anyway, this is Fred Covington, and he passed away on July 28 of 1993. He recorded our original voice announcer back in the 1970s. And you can see - we had the little plaque up here that's dedicated to his memory. He's been serving us very well over the years.

WERTHEIMER: But our guide Geoff Chester says, we all use the much more precise and sophisticated measurements of time, as well.

CHESTER: If you have a wireless router at your house - just about everybody does these days - once a day, that will ping one of our NTP servers to get the correct time. And I always tell people, if you use a GPS, cell phone or the Internet, then the Naval Observatory is reaching out and touching you with its time.

WERTHEIMER: This is WEEKEND EDITION from NPR News. I'm Linda Wertheimer.

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