SafekipediaUnit of length
Adapted from Wikipedia · Adventurer experience
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.
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 help us measure how long things are. Long ago, people used things like a hand or a foot to measure. These ways were not the same everywhere.
Today, most of the world uses the metric system with units like the meter. This helps everyone measure the same way. In the United States, people often use units like the inch or the foot.
Metric system
Main article: Metric system
Main article: International System of Units
See also: Orders of magnitude (length)
The main unit of length in the International System of Units (SI) is the metre. It is the distance light travels in a vacuum in a very short time. Other units like millimetres and kilometres are made by adding prefixes to the metre, creating smaller or larger measurements.
| 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, 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. It helps measure depth in places that do not use the metric system. Another unit is the nautical mile. This unit is based on the shape of the Earth and equals 1852 meters.
Aviation
Air traffic control uses feet to measure how high planes fly. Most of the world uses feet, but some places like Russia, China, North Korea, and many CIS countries use different units. Distances in the sky are measured in nautical miles.
Surveying
Surveyors in the United States 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. A light-year is the distance light travels in one year, measured through a vacuum over a Julian year. The parsec is another unit. Finally, the Hubble length is about 14.4 billion light-years.
Physics
In physics, scientists 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 | re | 2.817940285(31)×10−15 |
| The Compton wavelength of the electron | λC | 2.426310215(18)×10−12 |
| The reduced Compton wavelength of the electron | λC | 3.8615926764(18)×10−13 |
| The Compton wavelength (or reduced Compton wavelength) of any fundamental particle | λx | |
| 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 | 𝓁P | 1.616199(97)×10−35 |
| Stoney unit of length | lS | 1.381×10−35 |
| Quantum chromodynamics (QCD) unit of length | lQCD | 2.103×10−16 |
| Natural units based on the electronvolt | 1 eV−1 | 1.97×10−7 |
Archaic
See also: English units of length
Archaic units of distance include:
- cana
- cubit
- rope
- league
- li (China)
- pace (the "double pace" of about 5 feet used in Ancient Rome)
- verst (Russia)
Informal
Sometimes, people use everyday things 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.
See also: List of humorous units of measurement and List of unusual units of measurement
Other
Horse racing and other activities with horses use some special ways to measure distance.
A furlong is a distance equal to one-eighth of a mile.
A horse length is roughly 8 feet.
Related articles
This article is a child-friendly adaptation of the Wikipedia article on Unit of length, available under CC BY-SA 4.0.
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