Why a Foot Is a Foot: The Strange History of Measurement

Every measurement you make is the end of a long argument about what counts as standard. For most of history, the answer was: whatever is attached to the person doing the measuring.

Measured in bodies

The oldest units are anatomical. A foot was a foot. A cubit was elbow to fingertip. A yard was roughly an arm's length, an inch about a thumb's width, and a fathom the span of outstretched arms — which is why it survives in seafaring, where a rope was measured by pulling it across the body.

The obvious flaw is that people vary. Attempts to fix this produced some memorable legislation: English statute defined the inch in terms of barleycorns laid end to end, and various rulers simply declared their own body the reference. A horse is still measured in "hands" — now standardised at four inches, because actual hands would not do.

The metric solution

Revolutionary France decided in the 1790s that units should come from nature rather than from a monarch. The metre was defined as one ten-millionth of the distance from the equator to the North Pole along a meridian through Paris — a definition requiring an extraordinary multi-year survey conducted partly through a war zone.

From there the system built itself: a litre as a cube of one-tenth of a metre a side, a kilogram as the mass of that volume of water, and decimal steps throughout so that conversion needed no arithmetic beyond moving a point. The same revolution also tried decimal time — ten-hour days, hundred-minute hours — which lasted about eighteen months and remains the great cautionary tale about over-reforming.

The last physical object

For over a century the kilogram was defined by an actual metal cylinder kept in a vault near Paris. This was a problem: the reference kilogram and its official copies slowly drifted apart in mass by tiny amounts, meaning the unit itself was quietly changing.

In 2019 the kilogram was redefined in terms of a fundamental physical constant, retiring the cylinder. Every SI base unit is now defined by constants of nature rather than by an object, so any sufficiently equipped laboratory anywhere — or on any planet — can reproduce them from first principles.

When units go wrong

The stakes are not academic. In 1999 NASA lost the Mars Climate Orbiter because one team supplied data in imperial units while the spacecraft's software expected metric. The probe approached the planet at the wrong altitude and was destroyed — a very expensive footnote about conversion.

A similar near-miss on the ground gave aviation the "Gimli Glider": an airliner fuelled using the wrong unit conversion ran out of fuel mid-flight and was landed, without engines, on a decommissioned runway. Everyone survived, which is the only reason it is remembered as a good story.

Measurement looks like the most boring topic available right up to the moment it is wrong.