# Speed of Light in a Vacuum is Variable

**URL:** <https://boards.straightdope.com/t/speed-of-light-in-a-vacuum-is-variable/653946>\
**Category:** Factual Questions\
**Created:** [March 25, 2013, 8:44pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum-is-variable/653946 "2013-03-25T20:44:38Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\
**Post date:** [March 30, 2013, 2:55pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum-is-variable/653946/21 "2013-03-30T14:55:42Z")

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> [@davidm](#):
>
> This is probably a very dumb question. The variation is tiny. Is it so tiny that it isn’t meaningful even at large distances? Could it affect, for example, calculations of the size of the universe or the distance of very distant astronomical objects?

An attosecond is 10[sup]-18[/sup] seconds, So you’re talking a difference of seconds across the width of the observable universe. Plus since astronomical distances are calculated using the measured speed of light c, it probably doesn’t make any difference.

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**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\
**Post date:** [March 30, 2013, 3:32pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum-is-variable/653946/22 "2013-03-30T15:32:44Z")

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How is C (as opposed to c) determined?

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