The Metre

Look at a ruler, a tape measure or a door frame marked with a child's height. In most of the world, the numbers on them count metres and parts of a metre. But who decided how long a metre is? And why that length?

A country with too many units

In the Your Foot, My Foot lesson we saw that towns used to have their own foot, their own cubit and their own pound, all a little different. France in the 1700s was one of the worst places for this. A unit could change from one town to the next, and traders and buyers argued about it all the time.

In 1789 a revolution began in France. The people who took charge wanted to change many old things, and the units were one of them. They wanted new units that did not belong to any king or any town, but to everyone.

Taken from the Earth

What does everyone share? The Earth. So in 1791 French scientists chose a length taken from the Earth's own size:

start at the North Pole;
go straight down the Earth to the equator (the line around the middle of the Earth), passing through Paris;
split that distance into ten million equal parts. One part is one metre.

A quarter of the Earth, from the North Pole down through Paris to the equator. That distance was split into ten million equal parts, and each part was called one metre

Nobody could walk from the North Pole to the equator. So two astronomers, Jean-Baptiste Delambre and Pierre Méchain, measured one piece of that line very carefully, from Dunkirk in the north of France to Barcelona in Spain. It took them about six years. From their piece, they worked out the whole.

Two men in 1790s coats and three-cornered hats on a grassy hilltop. One looks through a telescope on a brass measuring wheel set on a wooden tripod, towards a church tower on a distant hill; the other kneels and writes in a notebook. A helper waits with a horse, and fields and villages spread out below

Picture: an illustration of surveyors measuring France in the 1790s.

A portrait of Jean-Baptiste Delambre, one of the two astronomers who measured the line from Dunkirk to Barcelona, engraved in his lifetime

Picture: Jean-Baptiste Delambre, an engraving made in his lifetime (Rijksmuseum, Amsterdam, CC0).

In 1799 the result was made into a bar of platinum metal, kept in Paris: the new standard for the metre. Before that, in 1796, marble copies of the metre had been set into walls around Paris, so anyone could check their own ruler. Two of them can still be seen today.

A man in a three-cornered hat holds his wooden ruler against a marble plaque set into a Paris wall. The plaque is marked "MÈTRE" and has a groove exactly one metre long between two metal stops. A woman with a basket of vegetables watches.

Picture: an illustration of a Parisian checking his ruler against a marble metre, around 1796.

How long is a metre?

The word comes from the Greek metron, "a measure". Its short form, or symbol, is m: "3 m" means "3 metres".

A metre is about:

the length of a guitar;
the height of a 4-year-old child;
a big step for an adult.

A metre stick standing next to things about one metre long: a guitar and a 4-year-old child

Today the metre is no longer measured from the Earth or from a metal bar. It is fixed using light, which is the same everywhere in the universe. We will see how in a later course.

Try it at home

Find a tape measure or a metre stick. Lay out one metre on the floor.
Take one big step next to it. Is your step longer or shorter than a metre?
Find three things in your home that are about one metre long.

Words to know

metre - the main unit of length in most of the world; its symbol is m
equator - the line around the middle of the Earth, halfway between the North and South Poles
symbol - the short way to write a unit, like m for metre

Big idea

The metre was made in France in the 1790s to belong to everyone. It was taken from the Earth: the distance from the North Pole to the equator, split into ten million parts. A metre is about the length of a guitar.