How has the definition of the meter evolved throughout history, and what does this tell us about scientific progress?

-Karthik Gurumurthy

I’ve always been fascinated by how our standards of measurement evolved throughout history. The story of the meter is particularly interesting because it shows how our definitions of fundamental units change as our technology and precision advance.

The origins of the meter date back to the eighteenth century, when two competing proposals emerged. Astronomer Christian Huygens suggested defining it by the length of a pendulum with a one-second period. Others proposed defining it as one ten-millionth of the earth’s meridian along a quadrant (essentially one-fourth of Earth’s circumference).

In 1791, after the French Revolution, the French Academy of Sciences endorsed the meridian definition. They rejected the pendulum approach because gravity varies slightly across Earth’s surface, which would affect the pendulum’s period and create inconsistencies. French researchers measured the arc from Dunkirk to Barcelona, and in 1799, the French Academy officially adopted this standard, recording it on a platinum bar.

Ironically, they miscalculated Earth’s flattening due to rotation, making their meter bar about 0.2 millimeters shorter than one ten-millionth of Earth’s quadrant. Despite this flaw, the definition persisted. The Treaty of the Meter was signed in 1875, and by 1889, a platinum-iridium bar was established as the International Prototype Meter.

This physical standard served well but became cumbersome and error-prone over time. After 71 years, science advanced enough to create a more precise definition. In 1960, the General Conference on Weights and Measures redefined the meter in terms of light wavelengths from krypton-86 atoms.

But precision continued to improve, and by 1983, even this definition was outdated. The Conference discarded the krypton standard and redefined the meter in terms of the speed of light – a theoretical definition where one meter equals 1/299,792,458 of the distance traveled by light in a vacuum in one second.

What I find most remarkable about this evolution is how it reflects progress in science and technology. Today, with commercially available lasers, anyone can create a reference meter with accuracy far beyond what the greatest scientists could achieve a century ago. As our precision improves, our fundamental standards must evolve accordingly.