1. Why Was "One Foot" Different in Every Town?
Imagine ordering a wooden board asking for "one pied" (foot). According to one book review, the length of one pied was about 32.5 cm in Paris, about 34.25 cm in Lyon, and about 51 cm in the Piedmont region [9]. That is a difference of nearly 19 cm between just two places.
Before the French Revolution, even if units had the same name, their actual size varied by city [2]. Units also differed depending on the region or the profession, so lengths and weights did not match up consistently [2].
In 1789, petitions collected from across France show that three social groups—clergy, nobility, and commoners—asked for unified standards for weight and length [8]. While it is not accurate to say everyone demanded this, the records suggest many people found the confusion inconvenient.
So, what could be used as a ruler that was fair to everyone and showed the same length everywhere?
2. Why Did They Choose the Earth as Their Ruler?
In March 1790, Talleyrand proposed a unified, simple system of units based on unchanging natural standards [3]. His first idea was to use the length of a pendulum that swings once per second at a latitude of 45 degrees [3]. By May of that same year, the National Assembly had adopted principles for unifying weight and length [3].
However, in March 1791, a different plan was chosen. The new definition set one metre as one ten-millionth of the distance from the North Pole to the Equator along the meridian (one-quarter of the Earth's circumference) [3]. Working backward, this total distance is about 10,000 km.
Here is the answer to our question: A plan was selected to use the size of the Earth as a standard. It was thought that this would create a ruler not influenced by local town rules [3].
But if you use the Earth as a ruler, you have to actually measure the Earth. Two surveyors took on this job.
3. How Did They Measure from Dunkirk to Barcelona?
The section they measured ran from the northern port city of Dunkirk in France to Barcelona in Spain. They used a method called triangulation to measure part of the meridian [3]. This method involves measuring angles between distant landmarks to form triangles, which allows you to calculate the distance between two far-apart points.
Delambre was responsible for the northern section from Dunkirk to Rodez, while Méchain handled the southern section from Rodez to Barcelona [4]. Méchain left Paris on June 28, 1792, heading toward Barcelona [4].
The survey was interrupted in 1793 and finally completed around 1798 [3]. In 1798, Delambre went to help finish Méchain’s triangulation work [4]. The survey took about six years from the start.
During this long survey, Méchain noticed something strange in the southern section.
4. What Small Error Did Méchain Notice in His Barcelona Measurements?
By March 1794, Méchain realized that the latitude values he measured in Barcelona did not match up [4]. He could not go back to check them again, and it is said that he did not tell anyone about this discrepancy [4].
Later, Delambre found his records [4]. The effect on the metre was small: about 0.23 mm per metre. That is roughly the thickness of two sheets of copy paper, so it was a very tiny error for such a large-scale measurement.
We do not know from the records why Méchain stayed silent. However, the records show that someone on the front lines of the survey noticed a small mismatch in the numbers.
Based on the survey results, the length of one metre was established between 1796 and 1797 using conversions from the old unit called the toise [3]. So, what did this new length look like?
5. How Did the Earth's Length Become a Platinum Metre Bar?
In 1795, the metric system using platinum as the base unit for the metre was officially established by law [3]. Then, in 1799, a platinum metre standard and a kilogram standard were made and placed in the National Archives in Paris. These are considered the first examples of showing units in physical form [1]. They were deposited in June and approved by the parliament on December 10 [1][7].
The weight unit, the kilogram, is connected to water. A cube with sides of 10 cm has a volume of 1000 cm³, which is 1 litre. The weight of the water that fits in that space is almost 1 kg [5]. Length, volume, and weight were connected through the decimal system.
The length measured from the Earth thus became a bar you could hold in your hand. Did everyone in France start using the metre right away?
Safety Note: The temperature conditions for water are not specified in the sources, so it is safer to say "almost" 1 kg rather than exactly.
6. Why Did It Take Over 40 Years for the Metre to Be Used Everywhere?
The answer is no. In 1812, Napoleon I allowed traditional units close to old customs for retail use [6]. These were abolished by a law in 1837, and from January 1, 1840, only the metric system was used again [6]. Counting from the 1795 law, this took 45 years.
In 1875, the Metre Convention was signed in Paris by representatives from 17 countries [1][2]. Japan joined in 1885 (Meiji 18) and received metre standard No. 22 and kilogram standard No. 6 in 1890 [2]. These two standards were designated as Important Cultural Properties in 2012 [2].
The length that was supposed to be determined by measuring the Earth arrived in Japan in the form of metal bars. So, how is the metre defined today?
7. How Is the Metre Defined Today?
Around 1869, the length of the archive standard bar itself was considered the definition of one metre [7]. The definition based on the size of the Earth changed to a form using a metal bar as the standard.
In 1983, a definition based on the speed of light was adopted. Today, one metre is defined as "the distance light travels in vacuum in 1/299,792,458 of a second" [7]. Light travels about 300 million meters in one second. If you calculate the Earth's circumference from the metre definition (about 40,000 km), light circles the Earth about 7.5 times in one second.
The Earth, a metal bar, and light. The standard has changed, but the goal of finding a length that is the same for everyone remains connected.
8. How Can You Try Measuring Like People in the Past?
If you have a kitchen scale, try weighing 1 litre of water. If you subtract the weight of the container, it should be close to 1 kg [5]. Be careful not to spill the water; do this on a flat surface.
Try one more thing: use your own shoe length as a ruler to measure the length of a hallway. Ask someone else in your house to measure the same spot and compare the numbers. If the numbers are different, you are experiencing a small version of the problem people had when they used body parts like feet as rulers.
If you want to learn more, articles in Newsweek Japan [2] and Gendai Business [7] discuss the metre standards. For Méchain’s survey records, the MacTutor biography (in English) is a good clue [4].