why should the distance from the pole to the equator be used as the baseline for a measurement used by humans?
For both philosophical and scientific reasons, they wanted a definition that could be recreated any where in the world, certainly not the thumb or foot of any one man. With the “distance from pole to equator” definition, you just need to measure a long north-south axis (say a few hundreds km), and knowledge about the figure of the Earth helps you scale that to the full definition. You can do that anywhere on Earth.
Another considered definition was the length of a pendulum whose half period is 1s, but even at the time the effect of local gravity was deemed to be too much
For both philosophical and scientific reasons, they wanted a definition that could be recreated any where in the world, certainly not the thumb or foot of any one man. With the “distance from pole to equator” definition, you just need to measure a long north-south axis (say a few hundreds km), and knowledge about the figure of the Earth helps you scale that to the full definition. You can do that anywhere on Earth.
Another considered definition was the length of a pendulum whose half period is 1s, but even at the time the effect of local gravity was deemed to be too much