Problem is "the planet" is part of the requirements in that sentence too, since the original meter definition was 1/40000 of the circumference of the earth. Yo

Problem is "the planet" is part of the requirements in that sentence too, since the original meter definition was 1/40000 of the circumference of the earth. You don't need to sail around the world for that, you can get it by shadow measurements, but you need measurements from two places on earth at the same time of year/position of the sun. cf. [ https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_Earth's_circumference https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_Ea... ]. The result you get from that is knowing what fraction of the earth's circumference is between the two places, which with the 1/40000 definition you can convert to meters. It's certainly doable, but you need the places to be reasonably far apart to get any accuracy, so… more of a "summer road trip" project, but could be fun with kids? ;) To be fair, the entire idea of metrology is that a few people do the work to get primary references and you distribute from there. There's a reason entities like NIST and PTB gets tax dollars.