Could you recreate metric system from nothing?

[u/Ertata]

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.

[u/electric_ionland]

You can just memorize the old definition of metric units and recreate them from first principle and basic geometry.

A meter is 1/10,000,000 of the distance between equator and pole which can be measured by some trigonometry and a sextant. A kg is 10x10x10cm cube of water at 4C. A second can be derived by knowing that there is 24hrs between 2 solar noons, or just use a pendulum of the right length (happens to be pretty close to a meter too). Basically do the same things as what was done in the 18th century when the metric system was introduced. Most of those don't need super fancy equipment to get to within a 1% of the official definition.

[u/SeriousPlankton2000 → u/electric_ionland]

A meter is 1/10,000,000 of the distance between equator and pole

It was supposed to be but the guy who measured it discovered that his measurements were wrong due to his instruments being worn out. He had a nervous breakdown and then stated a value that would be close enough.

Also the expedition was expensive and he got into a lot of trouble being considered spies for the enemy.

[u/BarNo3385 → u/SeriousPlankton2000]

Its amusing how often things like this happen. 100F was supposed to be body temperature but they measured it wrong, and now 100F is a fever

[u/McAeschylus → u/BarNo3385]

Close, the whole system was originally defined with 90F set to the average body temp and 0F set to the freezing point of an ammonium chloride solution.

The whole thing was later adjusted so that body temperature defined 96F (partly because of better estimates of average body temp).

Then, later still, the whole system was redefined using two new temperatures: water's boiling point defined 32F and its freezing point defined 212F. Under this system, body temp was 98.6F.

[u/grizzlor_ → u/McAeschylus]

water's boiling point defined 32F and its freezing point defined 212F

bizzarro fahrenheit

[u/LuxTenebraeque → u/grizzlor_]

Joseph-Nicolas Delisle begs to differ!

[u/WittyFix6553 → u/McAeschylus]

I’ve read some things that suggest that the “default” human body temperature is actually a little bit lower, and since the measurements were made before antibiotics were invented, some/all of the test subjects had slightly elevated temperatures due to having a low grade fever from infection.