How the "meter" came to be exactly one meter long

"Measurement standards are needed for knowing "how much" exists. Early distance standards, like "cubits" or "feet," were based on body parts. A single "pace" was often used: around one yard/meter. The idea of a "standard meter" came from pendulum observations. A swinging pendulum's period is determined by two factors: length and gravity. A seconds pendulum, where each half-swing lasts one second, requires a pendulum one meter long. Because gravity varies by ~0.2% across Earth, any pendulum-based "length" isn't universal."

"In 1790, the meter was defined as 1/10,000,000th the distance from the North Pole to the equator. That distance was then cast into a platinum bar. After correcting an early error of 0.2 millimeters, these bars became distance standards for decades. Platinum-iridium alloys, with X-shapes to better resist distortions, replaced those originals. In the 1920s, atomic interferometry - based on light's wavelength - superseded the "bar" standard. The right number of wavelengths of light defined the 20th century's meter."

"Platinum-iridium alloys, with X-shapes to better resist distortions, replaced those originals. In the 1920s, atomic interferometry - based on light's wavelength - superseded the "bar" standard. The right number of wavelengths of light defined the 20th century's meter. First cadmium, then mercury, and next krypton atoms defined the meter. Finally, in 1983, a new standard was adopted: the distance light travels in 1/299,792,458th of a second. Because the speed of light in a vacuum is always constant, this definition is universal."

Early length measures used body parts such as cubits and feet, and a common pace approximated one yard or meter. Pendulum observations suggested a seconds pendulum of one meter, but gravity's ~0.2% variation across Earth made pendulum length non-universal. In 1790 the meter was defined as 1/10,000,000th the meridian distance from the North Pole to the equator and embodied in a platinum bar, later corrected by 0.2 mm and replaced by platinum‑iridium X‑shaped bars. Atomic interferometry using light wavelengths in the 1920s replaced bars; later the meter was defined by light travel in 1/299,792,458th of a second.

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