Comments by "EebstertheGreat" (@EebstertheGreat) on "Joe Scott"
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Over the span of a few years, all French units were replaced with decimal units. Not just the SI units, or the time of day and day of week, but even measure of angle. A right angle of 90° was to be replaced with a right angle of 100 grad. As with other units, this could get metric prefixes, the most notable of which was the centigrade, which was 0.01 grad, equal to 0.54'. This explains the choice to make a meter 1/20,000,000th of the meridian through Paris. That would make the circumference of the Earth 40,000 km, so each centigrade would be 1 km. Therefore, the km would fill the role usually played by the nautical mile, where 1 nm is the distance along the Earth's surface subtending 1' of arc.
(BTW, earlier measurements like those by Gabriel Mouton, though impressively accurate, were a little too optimistic about the sphericity of the Earth. They of course knew it was not a perfect sphere due to topography, but they thought it was pretty close. They did not appreciate that the Earth's rotation caused its equator to be wider, since most still did not believe that the Earth rotated at all. Still, Mouton's measurement eventually influenced the French definition of the nm, and is exactly its value today, neglecting very slight changes in the definition in the meter.)
Also, a Kibble balance doesn't really "figure out how many joules are in a kilogram." Despite its name, a Kibble balance is just a scale, so all it really measures is the net force. This can be converted to mass by measuring the local gravitational field, which uses gravimeters that are in the most general sense similar to accelerometers, and thus don't depend on the mass of the measuring device. The definition of the kilogram comes from fixing the Planck constant. So from that definition, you could realize the kilogram by measuring the energy of a photon of known frequency. But in actual practice, that is not how we realize the kilogram, and that is not how a Kibble balance works. Although gravity does not appear in the definition of the kilogram at all, it is still how we realize it.
And to get even more nitpicky, the earth in fact takes 365.256363004 days to orbit the sun (as calculated for J2000.0). The tropical year is only 365.242190402 days, which is the period of the seasonal cycle. The equinoxes precess as the earth orbits the sun, so that each orbit does not quite trace out the same ellipse as the last, and you get a pattern like a spirograph. This means our reference frame for the seasons is constantly changing, so what appears to be a complete cycle from the reference frame of the Earth is slightly less than one complete orbit from the reference frame of the stars. This means that the Earth takes more than 1 Julian year to complete an orbit about the Sun but less than 1 Julian year to complete once cycle of the seasons.
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