A second is the period of a pendulum with a length of one meter. :-)
(But not really; that was originally considered as a way to standardize the meter, IIRC, but the period of a pendulum varies too much even over the area of France to be used as a concrete standard.
But the relationship is remarkably close for a coincidence, like the way a rod is almost exactly five meters.)
The calculation of a second has been updated in 2019
The second is defined by taking the fixed numerical value of the cesium
frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of
the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which
is equal to s−1.
The conference where this was made official was quite a beautiful, upbeat demonstration of people from around the world coming together to agree upon a change:
Technically, yes. But measurements of weight, volume, and temperature are also part of the metric system, and those didn’t derive from the meter.
Seconds are also part of the metric system, but one of the few not based on decimal/base-10.
Again, I’m just speculating that the author used “metric” because it often represents decimal/base-10 measurements. Not really arguing whether they were technically correct in doing so.
This isn't totally true. Mass and volume measurements were indeed derived from the meter. A gram is a cubic centimeter of water. A liter is 1/1000 of a cubic meter. Apparently Celsius is derived from Kelvin (really just translated so 0 is the freezing point of water), which is derived using metric units in a formula that is a bit beyond me but available here:
> But measurements of weight, volume, […] are also part of the metric system, and those didn’t derive from the meter.
For volume this is obvious nonsense since the metric system expresses volumes in … cubic meter! And even weight, a kilogram is “the weight of a liter of water” (that is a thousands of a cubic meter of water).
Weight and volume were defined based on the meter. Volume is just expressed in m³ or litre, which is just 1/1000 of 1 m³. Mass was originally defined such that 1 kg of water is the mass of one litre of pure water at sea level.
And based on a 1,000 (kilo) of something so 1 metre 1,000g (kilometre), 1 gram 1,000g (kilogram). You can have centimetres but that not in the spirit of the metric system really.
Maybe we need a cron for an hour or a day? 1,000 crons for an hour sounds better than millicrons by breaking up a day if it was 1 cron.
> You can have centimetres but that not in the spirit of the metric system really.
"Centi" is an SI prefix just as much as "kilo" is, and has been part of the metric system since 1795 just like "kilo". I don't see how it's "not in the spirit" of the metric system. It also fits in neatly in that 1cm^3 of water is roughly 1 gram (the original provisional definition of gram was 1cm^3 of water at the melting point of ice; the current definition is more precise), and so 10cm^3 of water is roughly 1 liter.
The SI system has prefixes going up and down one power of 10 up to 10^3 and down to 10^-3, and then in steps of 10^3. They're all equally part of the system; some are just more common in some contexts that others (e.g. we use hectograms but rarely hectolitres, and decilitres but rarely decimetres, and centimetres and centilitres but rarely centigrams) depending on what happens to be convenient.
E.g. kilo(10^3), hecto(10^2), deca(10^1), deci(10^-1), centi(10^-2), milli(10^3), but then mega (10^6) and micro(10^-6) are the next steps.
