Why is a second equal to the duration of 9,192,631,770 of radiations? (2016)

Not quite - don't forget your elevation | air pressure.

Of course not even elevation above mean sea level is straightforward .. what is the MSL of Australia, for example, and how does it relate to the Geoid, the long tidal run through the long narrow straits to the north of Australia, the WGS ellipsoid, etc.

(That's assuming you're at sea level, as another commenter noted - if you're at 5,000 feet, the boiling point of water is something like 95°C.)

Anyways.. the counter example was the old definition of the Ampere:

"the constant current which, if maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length."

Perfectly unrealizable within this Universe.

A mole of electrons has a charge of about 96487 coulombs. 100000 would have been nicer.

Metric measurements are as intuitive to me as American customary units are to you. I can just as easily use my handspan or the lengths of my fingers to estimate lengths in centimetres as you can in inches. I know without thinking what clothes I should wear when I see the weather forecast in Celsius. I know roughly what temperature I should set the oven to in Celsius based on what I’m cooking.

Having grown up in a metric country, American or imperial units mean nothing to me. If I’d grown up in America I’m sure American units would be intuitive for me. The only difference, as you say, is metric is far easier and more intuitive as soon as you want to do anything complicated with those measurements.

> By contrast, the "foot", which is literally named after the device used to approximate it. The median human being has two of these.

It seems odd to criticize the metre for a lack of accuracy against historical basis when the modern definition is accurate to within 0.02% of its historical basis, and yet praise a unit that, if measured by its historical basis, would be off on its mean by around 20% and vary by 40-50% amongst the population.

If you're talking about Canada, none of that mess comes from a plethora of choice, it's just the result of American cultural and economic influence, in addition to history. Yes Canadians are well versed in both systems but it's not like we particularly like having to deal with US units in our lives.

It was objectively better system and that’s why it ultimately prevailed.

Which foot: the international foot (0.3048 meters) or the US survey foot (1200/3937 meters)?

* https://oceanservice.noaa.gov/geodesy/international-foot.htm...

* https://www.nist.gov/pml/us-surveyfoot

Here's another one: which weighs more, a pound of gold or a pound of feathers? It's a pound found of feathers that weighs more.

This is because gold (and other precious metals) are measured in the Troy system, where one pound is the equivalent of 373g:

* https://en.wikipedia.org/wiki/Troy_weight

Whereas most other things use the Avoirdupois system, where one pound is the equivalent of 454g:

* https://en.wikipedia.org/wiki/Avoirdupois

Now, which weighs more: one ounce of gold or one ounce of feathers? In this case the gold, because the Troy system has 12 ounces per pound, so you'd have 31g, while the Avoirdupois has 16 ounce per pound, so you'd have 28g of feathers.

And remember that there are British/Imperial units and US customary units, so things like cups are different between countries:

* https://en.wikipedia.org/wiki/United_States_customary_units

So when you say "We should have used unit X as the basis of Y", whose unit of X should have been used?

Also worth noting that the US has used the metric system as the foundation of the customary system since 1893:

> Under the Mendenhall Order of 1893, metric standards, developed through international cooperation under the auspices of BIPM, were officially adopted as the fundamental standards for length and mass in the United States, though some metric standards were used in practice before then. The definitions of United States customary units, such as the foot and pound, have been based on metric units since then.

* https://en.wikipedia.org/wiki/Metrication_in_the_United_Stat...

The U.S. automotive industry certainly disagrees. They've largely (80~90%) gone metric quite a while ago.

You don't need to sail around the world for that, you can get it by shadow measurements, but you need measurements from two places on earth at the same time of year/position of the sun. cf. [ https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_Ea... ]. The result you get from that is knowing what fraction of the earth's circumference is between the two places, which with the 1/40000 definition you can convert to meters. It's certainly doable, but you need the places to be reasonably far apart to get any accuracy, so… more of a "summer road trip" project, but could be fun with kids? ;)

To be fair, the entire idea of metrology is that a few people do the work to get primary references and you distribute from there. There's a reason entities like NIST and PTB gets tax dollars.