The metre was a slice of the planet
The first definition of the metre, in 1791, was one ten-millionth of the distance from the North Pole to the equator, along the meridian through Paris.
A commission of the French Academy — Borda, Lagrange, Laplace, Condorcet, Monge — recommended basing the new unit on the Earth rather than on a pendulum, so that it would not depend on local gravity. Delambre and Méchain spent six years surveying the meridian from Dunkirk to Barcelona during a revolution. The metre was cut from their result.
Their Earth was slightly wrong. The metre they made does not exactly match one ten-millionth of the modern meridian. The error is small and historical. The metre has since been redefined, first by a bar of metal, then by a wavelength, and now by the speed of light. The planet was the first authority and is no longer one.
Caveats
- The survey measured a portion of the meridian and extrapolated. It did not walk to the pole.
- The modern metre, defined by the speed of light, is not legally tied to the Earth's size anymore. The historical definition is the claim.
Sources
- 1Alder K.. The Measure of All Things. Free Press, 2002.