The meter (or meter in US spelling; symbol: m) is the basic unit for measuring length in the International System of Units (SI). Since 2019, the meter has been defined as the distance light travels in a vacuum during a time interval of 1/299,792,458 of a second. The second is defined by a specific frequency of caesium atoms.

In 1791, the French National Assembly first defined the meter as one ten-millionth of the distance from the equator to the North Pole along a circle that passes through Paris. This made the Earth's polar circumference equal to 10,000 kilometers.

In 1799, the meter was redefined using a physical bar as a reference. This bar was replaced in 1889. In 1960, the meter was redefined based on the number of wavelengths of a specific light emission from krypton-86. The current definition was adopted in 1983 and slightly updated in 2002 to clarify that the meter measures proper length. From 1983 until 2019, the meter was defined as the distance light travels in a vacuum during 1/299,792,458 of a second. After the 2019 revision of the SI system, the definition was updated to include the caesium frequency ΔνCs. These changes did not significantly alter the size of the meter. Modern measurements show the Earth's polar circumference is approximately 40,007.863 kilometers.

Spelling

The metric unit for length is usually spelled "metre" in most English-speaking countries. However, the United States and the Philippines use the spelling "meter" instead. Tools that measure things, like ammeters and speedometers, are always spelled with "-meter" in all forms of English. The ending "-meter" comes from the same Greek word as the unit of length, "metre."

Etymology

The word "metre" comes from the Greek verb μετρέω (metreo), meaning "to measure, count, or compare," and the noun μέτρον (metron), meaning "a measure." These words were used to describe physical measurements, the rhythm in poetry, and also to mean keeping things balanced or avoiding extremes, as in "be measured in your response." Similar meanings appear in Latin (metior, mensura), French (mètre, mesure), English, and other languages. The Greek word is linked to the Proto-Indo-European root *meh₁-, which also means "to measure." In English, the word "metre" was first used to describe the French unit "mètre" as early as 1797.

History of definition

During the French Revolution, the old ways of measuring length were changed to use new, consistent measures based on natural events. Scientists had once suggested using a pendulum that swings once every second as a standard for length, but this idea was later rejected because the pendulum’s length changed depending on where it was measured due to differences in gravity. Instead, the meter was created as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming Earth’s shape was flattened by 1/334.

After measurements taken by Delambre and Méchain, the first official French standard of the meter was made as the Mètre des Archives, a platinum bar stored in Paris. Scientists had also planned to define the meter by counting the number of swings of a one-meter-long pendulum at a latitude of 45° over one day. However, this method proved less reliable than using physical objects like the Mètre des Archives.

In the mid-1800s, after the American Revolution and the independence of many countries in the Americas, the meter became widely used in scientific work there. It was officially recognized as an international unit of measurement in 1875 through the Metre Convention, which happened at the start of the Second Industrial Revolution.

The Mètre des Archives and its copies, like the Committee Meter, were replaced in 1889 with a new standard made of platinum-iridium. Twenty-nine bars, each matched to this new standard, were sent to different countries. This improved standardization required new tools for measuring and a temperature scale that could be consistently reproduced.

Advances in science eventually allowed the meter to be defined without a physical object. In 1960, the meter was redefined as a specific number of wavelengths of light from a particular energy change in krypton-86. In 1983, the definition was updated to use the speed of light. This definition was revised in 2019:

The meter, symbol m, is the SI unit of length. It is defined by fixing the numerical value of the speed of light in a vacuum, c, to be 299,792,458 meters per second, where the second is defined based on the frequency of caesium atoms.

Older units of length, such as the yard, are now defined in terms of the meter. For example, the yard has been officially defined as exactly 0.9144 meters since 1959.

SI prefixed forms of metre

SI prefixes are used to show smaller and larger units of the metre, as shown in the table below. Long distances are often measured in kilometers (km), astronomical units (149,597,871 km), light-years (63,000 au; 9.5 trillion km), or parsecs (210,000 au; 31 trillion km) instead of using megameters (Mm) or larger units. The phrases "30 cm," "30 m," and "300 m" are more commonly used than "3 dm," "3 dam," and "3 hm," respectively.

The terms "micron" and "millimicron" were sometimes used instead of "micrometre" (μm) and "nanometre" (nm), but this practice is no longer encouraged.

Equivalents in other units

In this table, the words "inch" and "yard" refer to the "international inch" and "international yard." The approximate conversions listed in the left column apply to both international and survey units.

One metre is exactly equal to 5,000⁄127 inches and to 1,250⁄1,143 yards.

A simple memory aid to help with conversions is "three 3s": 1 metre is nearly equal to 3 feet 3 and 3⁄8 inches. This gives a slightly higher value by 0.125 mm.

The ancient Egyptian cubit measured about 0.5 metres (rods found today measure between 523 and 529 mm). The Scottish ell, which is equal to 2 cubits, measured 941 mm (0.941 m), while the English ell measured 1,143 mm (1.143 m). The ancient Parisian toise (a type of fathom) was slightly shorter than 2 metres. It was later set exactly to 2 metres in the mesures usuelles system, meaning 1 metre was exactly 1⁄2 toise. The Russian verst measured 1.0668 km. The Swedish mil measured 10.688 km, but it was later changed to 10 km when Sweden adopted metric units.