# How Do They Measure Clock Accuracy?

**URL:** <https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278>\
**Category:** Factual Questions\
**Created:** [February 9, 2010, 3:50pm UTC](https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278 "2010-02-09T15:50:45Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\
**Post date:** [February 10, 2010, 7:39am UTC](https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278/21 "2010-02-10T07:39:35Z")

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> [@Leo\_Bloom](#):
>
> Wasn’t there some proof of the accuracy of atomic clocks and/or time dilation by flying one in one direction around the earth and the other in the opposite direction, and comparing those to each other and to the stable click time on Earth?

There have been a number of experiments using super-accurate clocks to measure time dilation. In one case they flew them on planes and showed they disagreed after landing. In another they put one atop a water tower and one at the base (light moves slower the deeper in a gravity well thus time moves differently).

I suppose it is a nod to the accuracy of these clocks that they can discern this difference which, in these cases, is quite small.

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**Author:** ![Terry\_Kennedy](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@Terry\_Kennedy](https://boards.straightdope.com/u/Terry_Kennedy)\
**Post date:** [February 10, 2010, 7:39am UTC](https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278/22 "2010-02-10T07:39:37Z")

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> [@Leo\_Bloom](#):
>
> Wasn’t there some proof of the accuracy of atomic clocks and/or time dilation by flying one in one direction around the earth and the other in the opposite direction, and comparing those to each other and to the stable click time on Earth?

Are you thinking of [this](http://leapsecond.com/great2005/) amateur experiment?

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**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\
**Post date:** [February 10, 2010, 1:38pm UTC](https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278/23 "2010-02-10T13:38:24Z")

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I’m telling you, you gotta go look at [leapsecond.com](http://leapsecond.com).

At [Project GREAT: General Relativity Einstein/Essen Anniversary Test](http://leapsecond.com/great2005/index.htm) you can read about the time dilation experiment this guy took his kids along for.

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**Author:** ![groo](https://avatars.discourse-cdn.com/v4/letter/g/ecd19e/32.png) [@groo](https://boards.straightdope.com/u/groo)\
**Post date:** [February 10, 2010, 4:26pm UTC](https://boards.straightdope.com/t/how-do-they-measure-clock-accuracy/528278/24 "2010-02-10T16:26:51Z")

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(Sorry; I completely mis-read the title).

But as long as I’m here, I’d like to add that we trust these clocks because Quantum Mechanics has held up remarkably well as a theory; I’m tempted to state it as a fact, but I’m wishy washy.

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