For those who don't know: The original definition of a metre is 1/40 000 000th of a full circle of longitude of Earth. That was considered not accurate enough, so it was redefined as the length of a certain metallic bar kept in Paris. Then it was defined in terms of wavelengths of krypton light. Finally, it was redefined in relation to the speed of light.
But each time the metre was redefined, the new definition was within the error bounds of the previous definition and the instruments that could be used within the previous definition - this ensured that backwards compatibility was retained. That's how we end up with these weird-looking numbers; it's not for fun and games.
Meanwhile, backwards compatibility was absolutely broken many times in traditional and imperial measurement systems. Heck, we have a break even in recent history: The survey foot has been discontinued in terms of the international foot, but they differ by 2 parts per million. That might not sound like much, but if you're measuring a whole continent, then being wrong by 2 ppm over 3000 km means having a discrepancy of 6 m, which is more than enough to fit an extra house in.
> For those who don't know: The original definition of a metre is 1/40 000 000th of a full circle of longitude of Earth.
That’s incorrect. It’s “one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris” (https://en.wikipedia.org/wiki/Metre). Different fraction, and much better defined (different circles of longitude may have different lengths)
> That was considered not accurate enough, so it was redefined as the length of a certain metallic bar kept in Paris.
I can’t find a reference, but I think it at least partially was a matter of practicality, not of accuracy. It’s not simple to measure that 10,000 km distance (https://en.wikipedia.org/wiki/Arc_measurement_of_Delambre_an...: The arc measurement of Delambre and Méchain was a geodetic survey carried out by Jean-Baptiste Delambre and Pierre Méchain in 1792–1798 to measure an arc section of the Paris meridian between Dunkirk and Barcelona. This arc measurement served as the basis for the original definition of the metre.)
More on the backward compatibility of the metre, this video released 7 hours ago explains it nicely: https://youtu.be/PWbfVcDcfFw?t=1039 Joseph Newton - "Cursed Units 3: The British Empire Strikes Back".
Other parts of the video highlight the insanity of inconsistent and shifting definitions of imperial units such as the mile, gallon, fluid ounce, pound-force/pound-mass, etc. It's one big damning essay against pre-metric units.
Handily enough the speed of light is about a foot per nanosecond, give or take. A nanosecond doesn't sound like much, unless you start to work out, for example, how many bits are inside your 40Gbps USB-C cable at the moment (they do travel at less than the speed of light).
nayuki|1 month ago
But each time the metre was redefined, the new definition was within the error bounds of the previous definition and the instruments that could be used within the previous definition - this ensured that backwards compatibility was retained. That's how we end up with these weird-looking numbers; it's not for fun and games.
Meanwhile, backwards compatibility was absolutely broken many times in traditional and imperial measurement systems. Heck, we have a break even in recent history: The survey foot has been discontinued in terms of the international foot, but they differ by 2 parts per million. That might not sound like much, but if you're measuring a whole continent, then being wrong by 2 ppm over 3000 km means having a discrepancy of 6 m, which is more than enough to fit an extra house in.
Someone|1 month ago
That’s incorrect. It’s “one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris” (https://en.wikipedia.org/wiki/Metre). Different fraction, and much better defined (different circles of longitude may have different lengths)
> That was considered not accurate enough, so it was redefined as the length of a certain metallic bar kept in Paris.
I can’t find a reference, but I think it at least partially was a matter of practicality, not of accuracy. It’s not simple to measure that 10,000 km distance (https://en.wikipedia.org/wiki/Arc_measurement_of_Delambre_an...: The arc measurement of Delambre and Méchain was a geodetic survey carried out by Jean-Baptiste Delambre and Pierre Méchain in 1792–1798 to measure an arc section of the Paris meridian between Dunkirk and Barcelona. This arc measurement served as the basis for the original definition of the metre.)
nayuki|1 month ago
Other parts of the video highlight the insanity of inconsistent and shifting definitions of imperial units such as the mile, gallon, fluid ounce, pound-force/pound-mass, etc. It's one big damning essay against pre-metric units.
MarkusWandel|1 month ago
MarkusWandel|1 month ago
mark-r|1 month ago