Unit of length

A unit of length is a special measuring tool we use to find out how long something is. We pick one thing to be our guide, like a ruler, and then we compare everything else to that guide. Today, most of the world uses the metric units. These units are used in every country around the globe.

A ruler, depicting two customary units of length, the centimeter and the inch

In the United States, people often use different measurements called U.S. customary units. In the United Kingdom and a few other places, some people still use British Imperial units for certain things. The metric system can be split into two groups: SI units and units that are not part of SI. These different units help us measure everything from tiny objects to huge distances.

History

Units of length are tools we use to measure how long things are. Long ago, people picked special objects or marks to help them measure things, like the length of a hand or a foot. These early ways of measuring were not always the same everywhere.

Today, most of the world uses the metric system, which includes units like the meter. This system is used in every country to make sure everyone measures things the same way. In the United States, people often use different units called U.S. customary units, such as the inch or the foot.

Metric system

Main article: Metric system

Main article: International System of Units

Comparison of the three principal measures of geographic length.

Name	Symbol	SI value
fermi	fm	1 femtometer
ångström	Å	100 picometers
micron	μm	1 micrometer
Norwegian/Swedish mil or myriameter		10,000 meters
x unit	xu	0.1 picometer

Imperial/U.S.

Main articles: Imperial units, United States customary units, and English units of measurement § Length

The main unit of length in the imperial and U.S. systems is the yard, which is exactly 0.9144 meters. Common units for measuring length include the thou or mil, the inch, the foot, the yard, the mile, and the league.

Marine

Sailors use special units to measure distances at sea. One unit is called a fathom, used for measuring depth in countries that do not use the metric system. Another unit is the nautical mile, which is based on the Earth's shape and equals exactly 1852 meters.

Aviation

Air traffic control uses feet to measure altitude worldwide, except in a few countries like Russia, China, North Korea, and many CIS countries. Distance in the sky is measured in nautical miles.

Surveying

Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service

Surveyors in the United States still use special tools to measure land. These include a chain, which is about 22 yards long, and a rod (also called a pole or perch), which is about 5 yards long.

In Australia, builders started using the metric system in 1966. They mainly use meters (m) and millimeters (mm) for measuring length. They avoid using centimeters (cm) because it can cause confusion. For example, a length of two and a half meters is written as 2500 mm or 2.5 m, not 250 cm.

American surveyors have a special system created by Edmund Gunter in 1620. The main unit is Gunter's chain, which is 66 feet long. It is divided into smaller parts called rods and links.

Science

Astronomy

Main article: Astronomical system of units

Astronomy uses special units to measure very large distances. These include the Earth radius, which is about 6,371 kilometers. Another unit is the lunar distance, the average distance between Earth and the Moon, about 384,402 kilometers. The astronomical unit is the distance between Earth and the Sun, defined as exactly 149,597,870,700 meters. A light-year is the distance light travels in one year, about 9,460,730,472,580.8 kilometers, measured through a vacuum over a Julian year. The parsec is another unit, about 30,856,775,814,671.9 kilometers or roughly 3.26156 light-years. Finally, the Hubble length is about 14.4 billion light-years or 4.55 gigaparsecs.

Physics

In physics, especially when studying atoms and the universe, scientists often use units based on fundamental constants. These natural units help describe the size and behavior of very small particles.

Atomic property	Symbol	Length, in meters
The classical electron radius	r_e	2.817940285(31)×10⁻¹⁵
The Compton wavelength of the electron	λ_C	2.426310215(18)×10⁻¹²
The reduced Compton wavelength of the electron	λ_C	3.8615926764(18)×10⁻¹³
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle	λ_x
The Bohr radius of the hydrogen atom (Atomic unit of length)	a₀	5.291772083(19)×10⁻¹¹
The reduced wavelength of hydrogen radiation	1 / R_∞	9.112670505509(83)×10⁻⁸
The Planck length	𝓁_P	1.616199(97)×10⁻³⁵
Stoney unit of length	l_S	1.381×10⁻³⁵
Quantum chromodynamics (QCD) unit of length	l_QCD	2.103×10⁻¹⁶
Natural units based on the electronvolt	1 eV⁻¹	1.97×10⁻⁷

Archaic

Informal

Sometimes, people use everyday objects to talk about how long something is. For example, people might say something is as long as a double-decker bus, which is about 9.5 to 11 meters. Another common example is an American football field, which is 100 yards long. Even the thickness of a single hair can be used to describe very small lengths, being around 80 micrometers.

Other

Horse racing and other activities with horses still use some special ways to measure distance.

A furlong is a distance equal to one-eighth of a mile, which is about 201 meters. A horse length is roughly 8 feet, or about 2.4 meters.