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Comment by jabl | Hacker News Reader

If you do positioning in degrees, minutes, and seconds (or decimal minutes which I guess is more common nowadays), a 'natural' length unit for navigation is a minute of longitude, which is the original definition of the nautical mile. And then we get velocity as nautical miles per hour, or knots.

Blame the French for reinventing the wheel instead of, say, using a 1/1000th of a nautical mile as the definition of the metre. Alas, that's all water under the bridge by now.

If you want to do navigation in SI units, the UTM projections (see sibling thread) typically do positioning in metres inside each grid square.

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Funnily enough the metre is defined by the globe as well: the arc from the equator to North pole is defined as 10,000 km. I think this might be where the alternative angle metrics came from, which went with the metre:

full circle is 400 grad, so a right angle (10,000 km) is 100 grad.

Then minutes and seconds are factors of 100, so a centesimal minute is 1 km and 1 centesimal second is 10 m.

Originally it was defined like that, but it was fairly quickly redefined as the distance between marks on a prototype platinum bar. It's had a couple of other definitions since then, but it's currently defined "by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit ms⁻¹, where the second is defined in terms of the caesium frequency ΔνCs".

https://www.bipm.org/en/si-base-units/metre

But that isn't a change to the definition of the meter. What do you think the motivation for the constant 299,792,458 is?

> that isn't a change to the definition of the meter

How is it not? it's not a change to the actual distance (well, it is for a sufficient precision), but a change to the definition of the metre is exactly what it is. It's defined relative to the speed of light rather than a random platinum bar length, with the constant picked so that the actual measurement stay the same (up to a certain level of precision).

Yes technically speaking there is a different physical reference now but that's where it came from. A bit like how 1 ft is now 304.8mm but that's because it's the size of a human foot (with some historic details no doubt).

Etymology vs dictionary, I suppose.

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