The History of the Metre: From the Earth's Quadrant to the Speed of Light

The metre is the foundation of the international system of units. Almost everything else — the litre, the newton, the joule, the watt — is built on top of it. That foundation has been rebuilt four times, and each rebuild tells you something about what scientists of the era could actually measure.

1793: a fraction of the Earth

Revolutionary France wanted a unit derived from nature rather than from a king's body part. The French Academy of Sciences proposed one ten-millionth of the distance from the North Pole to the equator, measured along the meridian through Paris.

Two astronomers, Delambre and Méchain, spent seven years surveying the meridian arc between Dunkirk and Barcelona, working through a revolution and a war. The result was a platinum bar deposited in the French archives in 1799.

The survey contained a small error. The Earth is slightly more oblate than their calculations assumed, so the metre came out roughly 0.2 millimetres short of its own definition. Rather than correct it, the bar itself became the standard. The definition had quietly changed from "a fraction of the Earth" to "the length of this object".

1889: the international prototype

The Metre Convention of 1875 created a permanent international body and commissioned a new standard: a bar of 90% platinum, 10% iridium, in an X-shaped cross-section chosen for rigidity. Thirty copies were made and distributed to member states.

Platinum-iridium is dense, hard, and resists corrosion and oxidation. Even so, an artefact standard has an unavoidable flaw: it can be damaged, it can drift, and there is no way to check it except against other copies that might themselves have drifted. If the prototype changed, the metre changed with it, and nobody could prove otherwise.

1960: counting wavelengths of krypton

Atomic physics offered an escape. Atoms of a given isotope emit light at frequencies fixed by nature and identical everywhere in the universe. A definition built on atomic emission cannot be dropped or scratched.

In 1960 the metre was redefined as 1,650,763.73 wavelengths of the orange-red emission line of krypton-86 in a vacuum. Any properly equipped laboratory could now realise the metre independently, without borrowing a bar from Paris. Precision improved roughly hundredfold.

1983: fixing the speed of light

By the 1970s lasers could measure the speed of light so precisely that the limiting factor was the metre itself. The uncertainty in c had become an uncertainty in length, not in speed.

The solution inverted the problem. Rather than measure the speed of light in metres, the definition fixed the speed of light and derived the metre from it:

The metre is the length travelled by light in vacuum in 1/299,792,458 of a second.

The speed of light is now exactly 299,792,458 metres per second — not because we measured it that precisely, but because the metre is defined to make it so. Measuring c more accurately is no longer possible; any improvement simply refines our realisation of the metre.

Why the number is so awkward

A tidy 300,000,000 m/s would have been possible, but it would have changed the length of the metre by about 0.07% — enough to invalidate every calibrated instrument, engineering drawing and survey on Earth. The committee chose continuity over elegance, and picked the value that best matched the existing krypton metre.

That principle runs through the whole history of SI: redefinitions change how a unit is realised, never how long it is. Your tape measure was as valid in 1888 as it is today.

The modern definition depends on the second

Because the metre is derived from the speed of light and the second, its accuracy is bounded by the accuracy of the second — defined since 1967 by the caesium-133 hyperfine transition, and realised to about one part in 10¹⁶.

This matters practically. GPS positioning is fundamentally a timing calculation: satellites broadcast the time, receivers compute distance from signal delay. A timing error of one nanosecond becomes a position error of about 30 centimetres. Your phone finding your street is the 1983 definition of the metre doing its job.