What was originally defined as one 10-millionth of the distance from the equator to the North Pole? The answer is the metre (or meter), the metric unit of length.
In 1791, during the French Revolution, France’s National Assembly adopted the metre as a supposedly universal standard based on Earth itself. It represented one ten-millionth of the meridian distance from the equator to the North Pole, measured along a great circle passing through Paris. This made the quarter of Earth’s polar circumference approximately 10 million metres, or 10,000 kilometres.
Surveyors Jean-Baptiste Joseph Delambre and Pierre Méchain later measured a meridian arc between Dunkirk and Barcelona to establish a practical standard. Their work led to a physical platinum metre bar, the mètre des Archives, in 1799. The original Earth-based calculation was slightly inaccurate because Earth is not a perfect sphere, but the unit remained close to the intended length.
The metre’s definition has since changed several times: first to metal prototypes, then to wavelengths of krypton-86 light, and finally to the distance light travels in a vacuum during a precisely defined fraction of a second. Today, “metre” is standard in British English, while “meter” is the usual American spelling.