# Is it possible to measure the speed of light without sophisticated equipment?

**URL:** <https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919>\
**Category:** Factual Questions\
**Created:** [November 7, 2003, 5:05pm UTC](https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919 "2003-11-07T17:05:50Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Engywook](https://avatars.discourse-cdn.com/v4/letter/e/bc8723/32.png) [@Engywook](https://boards.straightdope.com/u/Engywook)\
**Post date:** [November 7, 2003, 5:05pm UTC](https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919/1 "2003-11-07T17:05:50Z")

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Using little more than clever trigonometry, it’s possible to estimate:

- the circumference of the earth
- the ratio of distance earth to the sun : earth to the moon
- the ratio of size of the sun and the moon.  
and for that matter, all of these stats were known in (IIRC) ancient Greece.

Has it been proven that the same thing could have been done with measuring the speed of light (a little high school math + some measuring tools available BCE)?

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**Author:** ![Jpeg\_Jones](https://avatars.discourse-cdn.com/v4/letter/j/4bbf92/32.png) [@Jpeg\_Jones](https://boards.straightdope.com/u/Jpeg_Jones)\
**Post date:** [November 7, 2003, 5:25pm UTC](https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919/2 "2003-11-07T17:25:59Z")

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Actually, I think that’s how the first close approximation of the speed of light was calculated.

Something involving two light sources located on adjacent mountains and a rapidly spinning triangular mirror. Seriously. I don’t remember exactly how it worked, but there wasn’t anything really sophisticated about the equipment.

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**Author:** ![Jpeg\_Jones](https://avatars.discourse-cdn.com/v4/letter/j/4bbf92/32.png) [@Jpeg\_Jones](https://boards.straightdope.com/u/Jpeg_Jones)\
**Post date:** [November 7, 2003, 5:49pm UTC](https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919/3 "2003-11-07T17:49:16Z")

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Aha. [Here](http://www.nhn.ou.edu/~johnson/Education/Juniorlab/C_Speed/Historyof_c_F2002.ppt) is a PowerPoint presentation file that is a good history of measurement of the speed of light.

I was thinking of Michelson’s 1878 rotating mirror experiment. There were, of course, previous attempts.

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**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\
**Post date:** [November 7, 2003, 6:19pm UTC](https://boards.straightdope.com/t/is-it-possible-to-measure-the-speed-of-light-without-sophisticated-equipment/211919/4 "2003-11-07T18:19:27Z")

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> [@](#):
>
> \*Originally posted by Engywook \*  
> Using little more than clever trigonometry, it’s possible to estimate:
> 
> - the circumference of the earth
> - the ratio of distance earth to the sun : earth to the moon
> - the ratio of size of the sun and the moon.  
> and for that matter, all of these stats were known in (IIRC) ancient Greece.

In order to measure the circumference of the Earth, you have to be willing to travel at least a couple hundred kilometers. But I agree, this can be done, and was done.

While one astronomer _did_ make an estimate of the ratio of the sun and moon’s distances, to do this the way he did with any sort of accuracy at all, you need pretty precise measurements. I don’t think you could do it without a telescope. IIRC, the result was off by a factor of 20. And I’m assuming that your third measurement is based on this second one.