# Thread: How accurate is atomic?

1. ## How accurate is atomic?

An interesting question, which has been playing at the forefront of my mind given my recent, frenzied spate of G-Shock purchases. These sync each night via radio so that they’re deadly accurate – but how accurate is accurate?

Luckily I didn’t have to work too hard to find the answer, as the hugely clever Mike Lombardi from the National Institute of Standards and Technology (NIST – the US standards agency responsible for time keeping) has written a paper answering my question.

However his paper has not just answered that initial, rather simplistic question. Instead he’s answered, in a beautifully clear and logical manner, all the other questions which us sad watch types would inevitably ask given the subject matter. It’s a tour de force and you can download it here.

Just in case you’re not convinced that you should read Mike’s paper for yourself – you should – I’ve set myself the task of summarising his four pages as succinctly as I can.

• The “atomic” clocks that you and I can buy are not actually atomic, they are ordinary quartz watches or clocks.
• They do however synchronise with a world-wide network of radio time signals.
• The radio signals in turn are synchronised with Universal Time (UTC) which is generated by averaging many caesium and hydrogen atomic clocks around the world.
• The difference between the radio signal and UTC is never more than 35 nanoseconds, put another way that’s a drift of 1 second every 30 million years!
• It takes about 10 milliseconds for the radio waves to reach the watch, remember they travel at the speed of light.
• It may then take a further 10 milliseconds for your watch to process the signal and synchronise.
• Finally you need to take into account the accuracy of the quartz crystal, most are rated at +/- half a second a day.

So the answer to the question is, if your watch syncs daily it is always going to be better than half second off, since each sync brings it to within 50 milliseconds of UTC, or less than one second in 20 million years.

That’s impressive, although there’s always something new around the corner, in this case an atomic clock using a single ion of aluminium. It’s allegedly 100,000 times more accurate than current caesium clocks. Why would we need such accuracy? GPS technology relies on extremely accurate time keeping to determine locations precisely and improved accuracy could assist with space exploration and determining the effects of gravity.

2. ## Re: How accurate is atomic?

Never thought of it this way. Information that's good to know!

3. ## Re: How accurate is atomic?

Ubelievably, incredibly accurate.

4. ## Re: How accurate is atomic?

thanks for sharing, great info!

5. ## Re: How accurate is atomic?

I never thought of it but I did after reading the title of your post and my result was +/- 0.5 seconds with exactly the same reasoning as Mr. Lombardi. The only difference is that I didn't know the 35 nanosecond and 10 millisecond figures but I estimated that it would be negligible. So the only real deviation was the watches accuracy. Most Gs actually run better than their spec. Rather less than +/-10spm instead of 15. So a half second deviation per day is the worst case scenario.

Thanks for posting the link, though. Sticky?

Till

6. ## Re: How accurate is atomic?

Facinating... still fact remains that even an automatic watch that does not deviate more than +/- 10 SECONDS per day is more accurate than I really ever need.

Cheers

7. ## Re: How accurate is atomic?

Casio (and I think other manufactures too) mention, that due to internal calculations in the radio controlled watch, it might be 1 second off.

So if a Radio Controlled quartz wtach is specified @ +/- 15 sec. pr. month, it can be up to 1,5 sec. of each day.

And then there of course is the basic question: Does your radio controlled watch sync regularly?

Which in turns can lead to some kind of obsession: Did my watch sync last night? Where can I place it so it syncs better? Did I configure it correctly? Oh, is that little indicator turned on on my display?

Personally, I like atomic watches. But they are not always as good, as advertised...

8. ## Re: How accurate is atomic?

Originally Posted by duke4c
facinating... Still fact remains that even an automatic watch that does not deviate more than +/- 10 seconds per day is more accurate than i really ever need.

Cheers
+1

9. ## Re: How accurate is atomic?

too acurate..................................

10. ## Re: How accurate is atomic?

That is a fantastic pdf from NIST. I've read it before, and it is a showpiece of science, technical writing, and frankly openness, that epitomizes what's good about such an organization as they are. I was thinking, once my Paypal fees crusade is done, about changing my sig to links to several of their docs, even.

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