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> From 1983, a metre was considered the distance that light travels in a vacuum in 1/299,792,458 of a second (because light travels 299,792,458 metres per second).

So the speed of light was calculated using a previous definition of metre, and that magic number was used to upgrade its own definition? That's a tautology, sounds wrong to me.

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There's a consistent effort to make all the SI metric units based off of discreet measurements of physical constants. The speed of light is a constant, and the SI second is "defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s^–1"

It's a way of expressing the backwards compatibility of the new definition, set to match the old one so that everybody's distance measures remain usable when the definition changes.

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