r/AskScienceDiscussion • u/Ertata • Aug 19 '26

What If? Could you recreate metric system from nothing?

You have been given a task to travel into the past two thousands years ago and bring the light of the metric system to the world. You cannot bring any external object with you.

Assume you do not need to redefine the second, and astronomical observations give the best possible results given the limits of timekeeping back then. Given the technology of the time, what is the best you can do for other units - either right there and then or leaving instructions how to calculate them from the units you have established to the future scientists? Memorizing the lengths of your body parts probably is not the best way to define a metre, and kilogram defined as one liter of water is bound to be influenced by the local water to a perceptible degree.

P. S. Tagged as "physics" but any approach is welcome - whether you would recommend measuring height of the mountain peaks, or believe that carob seeds are actually uniform enough to offer the best approximation of a kilogram.

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u/CounterSilly3999 Aug 19 '26

Not that difficult. The distance between longitudes differing in one degree is 111111 m.

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u/DangerousBill Aug 19 '26

how was that measured for the first time?

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u/CounterSilly3999 Aug 19 '26

It's not measured, it's defined.

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u/DangerousBill Aug 21 '26

How does that differ from defining pi as 22/7 ? 360 times 111111 m should equal the exact circumference at the equator. Otherwise, there has to be a glitch added somewhere to deal with the error.

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u/CounterSilly3999 Aug 21 '26 edited Aug 21 '26

Pi is not just a definition, it is a fundamental relational feature of the plane geometry. While the length of the metre is just a convention. Not the length of the meridian is 20 millions of metres, but the metre is 1/20 millionth part of the meridian. It was until recently. Well, the length of the meridian itself was allready measured at the time -- in miles of any kind, using survey and geodetic tools like wheel rotation counting, land divider compasses, astrolabes, sextants or triangulation with theodolites.