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1

The megalithic yard (sometimes abbreviated to MY) is a theoretical unit of prehistoric measurement first suggested by the Scottish engineer, Alexander Thom in 1955.
Alexander Thom was a lecturer in engineering who undertook an initial statistical analysis of 46 of the Neolithic and Bronze Age stone circle in the British Isles. By plotting his measurements on a graph, he noted that many of the diameters of the stone circles came clumped together in groups, there were several examples with close to 22 foot (6.7 m) diameters, another group measuring c. 44 feet (13.4 m) across and another measuring c. 55 feet (16.8 m). A best fit for these results implied a common factor of 5.43 feet (1.65 m) which he believed could have served as a manageable unit for measuring out figures on the ground. Thom went on to survey more than 300 sites, becoming increasingly convinced of the yard's existence.
Thom halved his original best fit to 2.72 feet (82.96 cm) as he argued that the circle builders would have set out their monuments using a radius from a central point rather than using a diametrical measurement.
Analytical methods employed by the British statistician S.R. Broadbent and D.G.

6.33

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**Measurement System:**English unit

The shaftment is a unit of length, 6 - 6½ inches or 2 palms, i.e. 15.24 cm. A shaftment is the width of the fist and outstretched thumb. The lengths of poles, staves, etc. can be easily measured by grasping the bottom of the staff with thumb extended and repeating such hand over hand grips along the length of the staff.
It occurs in Anglo-Saxon written records as early as 910 and in English as late as 1474. After the modern foot came into use in the twelfth century, the shaftment was reinterpreted as exactly 1/2 foot or 6 inches (15.24 cm).
Other spellings include schaftmond and scaeftemunde, and shathmont. It is derived from Old English sceaft, in turn from Germanic skaftaz (shaft) and OE mund, from the Germanic mund, in turn from Indo-European root man (hand.)
Two shaftments make a pes manualis, literally Latin for "a foot fitted to the hand."
This unit has mostly fallen out of use, as have others based on the human arm: digit (1/8 shaftment), finger (7/48 shaftment), palm (1/2 shaftment) hand (2/3 shaftment), span (1.5 shaftments), cubit (3 shaftments) and ell (7.5 shaftments).
University of Nord Carolina at Chapel Hill - How Viele? - A Dictionary of Units of Measurement

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3

**Measurement System:**Planck units

In physics, the Planck length, denoted ℓP, is a unit of length, equal to 1.616199(97)×10 metres. It is a base unit in the system of Planck units. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, Planck's constant, and the gravitational constant.
The Planck length is defined as
where is the speed of light in a vacuum, is the gravitational constant, and is the reduced Planck constant. The two digits enclosed by parentheses are the estimated standard error associated with the reported numerical value.
The Planck length is about 10 times the diameter of a proton, and thus is extremely small.
The physical significance of the Planck length is a topic of research. Because the Planck length is so many orders of magnitude smaller than any current instrument could possibly measure, there is currently no way of probing this length scale directly. Research on the Planck length is therefore mostly theoretical. According to the generalized uncertainty principle, the Planck length is in principle, within a factor of order unity, the shortest measurable length - and no improvements in measurement instruments could change that.
In some

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**Measurement System:**Imperial unit system

A cable length or cable's length is a nautical unit of measure equal to one tenth of a nautical mile or 100 fathoms, or sometimes 120 fathoms. The unit is named after the length of a ship's anchor cable in the age of sail. The definition varies:

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5

**Measurement System:**International System of Units

A hectometre (American spelling: hectometer, symbol hm) is a somewhat uncommonly used unit of length in the metric system, equal to one hundred metres. It derives from the Greek word "ekato", meaning "hundred". A regulation football or soccer field is approximately 1 hectometre in length.

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6

**Measurement System:**Chinese units of measurement

The cun (Chinese: 寸; pinyin: cùn; Wade–Giles: ts'un; Japanese: sun; Korean: chon) is a traditional Chinese unit of length. Its traditional measure is the width of a person's thumb at the knuckle, whereas the width of the two forefingers denotes 1.5 cun and the width of all fingers side-by-side is three cuns. In this sense it continues to be used to chart acupuncture points on the human body in various uses of traditional Chinese medicine.
The cun was part of a larger system, and represented one-tenth of a chi ("Chinese foot"). In time the lengths were standardized, although to different values in different jurisdictions. (See chi (unit) for details).
In Hong Kong, using the traditional standard, it measures ~3.715 cm (~1.463 in) and is called a "tsun". In the twentieth century in the Republic of China, the lengths were standardized to fit with the metric system, and in current usage in People's Republic of China and Taiwan it measures 3 ⁄3 cm (~1.312 in).
In Japan, the corresponding unit, sun (寸(すん), sun), was standardized at ⁄33 mm (~3.030 cm, ~1.193 in, or ~0.09942 ft).

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7

**Measurement System:**International System of Units

A decametre or dekametre (American spelling: dekameter, earlier decameter; symbol dam, sometimes Dm or dkm) is a very rarely used unit of length in the metric system, equal to ten metres, the SI base unit of length. It can be written in scientific notation as 1 E+1 m (exponential notation), meaning 10 × 1 m.
This measure is included mostly for completeness. One practical use of the decameter is for altitude of geopotential heights in meteorology. Meteorologists also use another seldom encountered SI prefix: hecto- in hectopascal (hPa). The volumetric form (see below) cubic decametre is convenient for describing large volumes of water such as in rivers and lakes.

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8

The cham am is a unit of length, used during 18th – 20th century in Cambodia. It is equivalent to 12 Thneap or 25 cm.

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9

**Measurement System:**Astronomical units of length

Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (≈3,947–3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 km (≈3,959 mi).
While "radius" normally is a characteristic of perfect spheres, the term as employed in this article more generally means the distance from some "center" of the Earth to a point on the surface or on an idealized surface that models the Earth. It can also mean some kind of average of such distances, or of the radius of a sphere whose curvature matches the curvature of the ellipsoidal model of the Earth at a given point.
This article deals primarily with spherical and ellipsoidal models of the Earth. See Figure of the Earth for a more complete discussion of models.
The first scientific estimation of the radius of the earth was given by Eratosthenes.
Earth radius is also used as a unit of distance, especially in astronomy and geology. It is usually denoted by .
Earth's rotation, internal density variations, and external tidal forces cause it to deviate systematically from a perfect sphere.

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10

A light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly 299,792,458 metres. It is just over 186,000 miles and almost 9.84×10 feet.
Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (just under one U.S. or imperial foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly-used light-year is also presently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not Gregorian year) of exactly 365.25 days, each of exactly 86400 SI seconds.
This definition fixes the speed of light in vacuum at exactly 299792458 m/s, and hence the light-second at exactly 299,792,458 m.
Communications signals on Earth rarely travel at precisely the speed of light in free space, but distances in fractions of a light-second are still useful for planning telecommunications networks as they indicate the minimum possible delay

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11

**Measurement System:**International System of Units

A decimetre (American spelling: decimeter, symbol dm) is a unit of length in the metric system, equal to one tenth of a metre, the SI base unit of length. In simple words there are 10 cm in a decimetre. It can be written in scientific notations as 100×10 m (engineering notation) or 1 E-1 m (exponential notation) — meaning 100 × 1 mm or 1 m / 10 respectively. The c is pronounced as /s/, unlike in decametre. The non-SI unit for volume, the litre, is defined as one cubic decimetre (other than between 1901 and 1964 where there was a slight difference between the two).

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12

**Measurement System:**Taiwanese units of measurement

The shaku (尺) is a traditional unit of measure used throughout Asia with a length approximately equal to a foot. It is variously called shaku in Japanese, chi in Mandarin, and chek in Cantonese, and is written as "呎" in Hong Kong. As with other measurements, it was originally derived from nature: the average length between nodes on bamboo. The actual length varies slightly by country.
The shaku may be divided into ten smaller units (寸), known as either cun in Mandarin, sun in Japanese, or tsun in Cantonese. Ten shaku are equal to a jō in Japanese, a zhang in Mandarin, and a jeung in Cantonese, and is traditionally written as "丈".
The English term shaku derives from the Japanese word shaku (尺). Early citations indicate that it entered the English language early in the 18th century.
In the People's Republic of China, the Chinese counterpart of the shaku, known as the chi in Mandarin (Chinese: 尺; pinyin: chǐ; Wade–Giles: chih), has been defined since 1984 as exactly 1/3 of a meter, i.e. 33⁄3 cm (approximately 1.094 ft). However, in the Hong Kong SAR the corresponding unit, pronounced chek (Chinese: 尺; Jyutping: chek) in Cantonese and spelled as "chek" in Hong Kong, is defined as

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13

**Measurement System:**Imperial unit system

A chain is a unit of length. It measures 66 feet, or 22 yards, or 100 links, or 4 rods (20.1168 m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in Britain and in some other countries influenced by British practice.
The chain was commonly used with the mile to indicate land distances and in particular in surveying land for legal and commercial purposes. In medieval times, local measures were commonly used, and many units were adopted that gave manageable units; for example the distance from London to York could be quoted in inches, but the resulting huge number would be unmemorable. The locally used units were often inconsistent from place to place.
In 1620, the clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, using what became known as Gunter's chain; this was 66 feet long and from the practice of using his chain, the word transferred to the actual measured unit. His chain had 100 links, and the link is used as a subdivision of the chain as a unit of length.
In countries

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14

The siriometer is a rarely used astronomical measure equal to one million astronomical units, i.e., one million times the average distance between the Sun and Earth. This distance is equal to about 149,597,870,000,000,000 meters (149.6 Pm) or about 15.813 light-years, which is roughly twice the distance from Earth to the star Sirius.
It was proposed in 1911 by Carl V. L. Charlier.

5.20

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15

**Measurement System:**Obsolete Tatar units of measurement

The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheroid this equalled 1855.4 metres. Any greater precision depends more on choice of standard than on more careful measurement: the length of the equator in the World Geodetic System WGS-84 is 40,075,016.6856 m which makes the geographical mile 1855.3248 m, while the International Astronomical Union standard IAU-2000 takes the equator to be 40,075,035.5351 m making the geographical mile 1855.3257 m, almost a millimetre longer.
It was closely related to the nautical mile, which was originally determined as 1 minute of arc along a great circle of the Earth, but is nowadays defined as exactly 1852 metres. The US National Institute of Standards and Technology (NIST) notes that: "The international nautical mile of 1 852 meters (6 076.115 49...feet) was adopted effective July 1, 1954, for use in the United States. The value formerly used in the United States was 6 080.20 feet = 1 nautical (geographical or sea) mile." A separate reference also identifies the geographic mile as being identical to these international nautical miles (and slightly shorter than British

7.67

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16

**Measurement System:**International System of Units

A megametre (American spelling: megameter, symbol Mm) is a unit of length in the metric system, equal to one million metres, the SI base unit of length, hence to 1,000 km or approximately 621.37 miles.
Megametres (from the Greek words megas = big and metro = count/measure) are rarely seen in practical use, e.g. "5000 km" is much more common than "5 Mm". Their symbol (Mm) can also be confused with millimetres (mm). Megametres are also occasionally found in science fiction.

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17

**Measurement System:**Imperial unit system

The nautical mile (symbol M, NM or nmi) is a unit of length that is about one minute of arc of latitude measured along any meridian, or about one minute of arc of longitude at the equator. By international agreement it is 1,852 metres (approximately 6,076 feet).
It is a non-SI unit (although accepted for use in the International System of Units by the BIPM) used especially by navigators in the shipping and aviation industries, and also in polar exploration. It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the sea mile and the related geographical mile.
The nautical mile remains in use by sea and air navigators worldwide because of its convenience when working with charts. Most nautical charts are constructed on the Mercator projection whose scale varies by approximately a factor of six from the equator to 80° north or south latitude. It is, therefore, impossible to show a single linear scale for use on charts on scales smaller than about 1/80,000. The nautical mile circumvents this problem by being equal to a minute of latitude on a chart, which allows any distance measured with a chart divider to be

7.67

3 votes

18

**Measurement System:**English unit

An ell (from Old Germanic *alinâ cognate with Latin "ulna"), is a unit of measurement, originally a cubit, i.e., approximating the length of a man's arm from the elbow ("elbow" means the bend or bow of the ell or arm) to the tip of the middle finger, or about 18 inches; in later usage, any of several longer units. In English-speaking countries, these included (until the 19th century) the Flemish ell (⁄4 of a yard), English ell (⁄4 yard) and French ell (⁄4 yard), some of which are thought to derive from a 'double ell'.
Several national forms existed, with different lengths, including the Scottish ell (≈37 inches or 94 centimetres), the Flemish ell (≈27 in or 68.6 cm), the French ell (≈54 in or 137.2 cm) the Polish ell (≈31 in or 78.7 cm) and the Danish ell (≈25 in or 63.5 cm)
Select customs were observed by English importers of Dutch textiles: although all cloths were bought by the Flemish ell, linen was sold by the English ell, but tapestry was sold by the Flemish ell.
In England, the ell was usually 45 in (1.143 m), or a yard and a quarter. It was mainly used in the tailoring business but is now obsolete. Although the exact length was never defined in English law, standards were

9.50

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19

A light-hour (also written light hour) is a unit of length. It is the distance travelled by light in vacuum in one hour, or 3,600 seconds. Based on the current definition of the metre a light-hour is equal to 1,079,252,848,800 metres (~1.08 Tm). That is about 7.214 AU.
To give an example, the semi-major axis of Pluto's orbit's is about 5.473 light-hours.

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20

Alen or aln is a traditional Scandinavian unit of distance similar to the north German elle: roughly 60 centimeters. The Danish alen, also used in Norway, was equal to 62.77 centimeters (2 Danish fod). The Swedish aln was 59.38 centimeters.
For a full list of old Danish measures, and their metric equivalents, see this Danish website

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2 votes

21

A cana was a unit of length used in the former Crown of Aragon. The exact meaning was not consistent but the use in Barcelona was a distance of 1.5708 metres.

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22

**Measurement System:**English unit

The hand is a non-SI unit of measurement of length, now used only for the measurement of the height of horses in some English-speaking countries, including Australia, Canada, the United Kingdom and the United States. With origins in ancient Egypt, it was originally based on the breadth of a human hand. It is today equal to four inches, and thus, following the adoption of the international inch in 1959, equal to exactly 10.16 centimetres. It may be abbreviated to "h" or the plural "hh". Although measurements between whole hands are usually expressed in what appears to be decimal format, the subdivision of the hand is not decimal but is in base 4, that is, subdivisions after the radix point are in quarters.Thus, a horse 62 inches high, precisely between 15 and 16 hands, is not 15.5, but rather is 15.2 hands high: likewise, 64 inches high is not 15.4, but rather is 16.0.
The hand, sometimes also called a handbreadth or handsbreadth, is an anthropic unit, originally based on the breadth of a male human hand, either with or without the thumb, or on the height of a clenched fist.
On surviving Ancient Egyptian cubit-rods, the royal cubit is divided into seven palms of four digits or

7.00

3 votes

23

**Measurement System:**International System of Units

The kilometre (American spelling: kilometer; SI symbol: km) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is the official unit used.
k (pronounced kay) is occasionally used in some English-speaking countries as an alternative for the word kilometre in everyday writing and speech. A slang term for the kilometre in the U.S. military is klick.
There are two common pronunciations for the word:
The former pronunciation follows the general pattern in English whereby SI units of measurement are pronounced with the stress on the first syllable, and the pronunciation of the actual base unit does not change irresepective of the prefix. It is generally preferred by the British Broadcasting Corporation (BBC) and the Australian Broadcasting Corporation (ABC).
Many scientists and other users, particularly in countries where the Metric System (SI) is not widely used, use the pronunciation with stress on the second

7.00

3 votes

24

**Measurement System:**US customary units

An inch (plural: inches; abbreviation or symbol: in or ″ – a double prime) is a unit of length in a number of systems of measurement, including the imperial and United States customary systems. One imperial or US customary inch is defined as ⁄12 of a foot and is therefore ⁄36 of a yard. Traditional standards for the exact length of an inch have varied, but it is now defined to be exactly 25.4 mm.
The inch is a commonly used customary unit of length in the United States, Canada, and the United Kingdom. For the United Kingdom, guidance on public sector use states that since 1 October 1995, without time limit, that the inch (along with the mile, yard and foot) is to be used as a primary unit for road signs and related measurements of distance and speed and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.
The international standard symbol for inch is in (see ISO 31-1, Annex A) but traditionally the inch is denoted by a double prime, which is often approximated by double quotes, and the foot by a prime, which is often approximated by an apostrophe. For example three feet two inches can be written as 3′ 2″.
1

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25

A light-week (also written light week) is a unit of length. It is defined as the distance light travels in an absolute vacuum in one week (seven days of 86,400 seconds each) or 181,314,478,598,400 metres (~181 Tm).
Note that this value is exact, since the metre is actually defined in terms of the speed of light. The light-week isn't very frequently used at all since there are few astronomical objects of that magnitude; the orbits of outer solar system objects are better measured in light-days or light-hours, whilst interstellar distances are on the order of light-years. The Oort cloud, for example, is thought to extend between 41 and 82 light-weeks out from the Sun.

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27

**Measurement System:**English unit

The palm may be either one of two obsolete non-SI units of measurement of length.
In English usage the palm, or small palm, also called handbreadth or handsbreadth, was originally based on the breadth of a human hand without the thumb, and has origins in ancient Egypt. It is distinct from the hand, the breadth of the hand with the thumb, and from the fist, the height of a clenched fist. It is usually taken to be equal to four digits or fingers, or to three inches, which, following the adoption of the international inch in 1959, equals exactly 7.62 centimetres. It is today used only in the field of biblical exegesis, where opinions may vary as to its precise historic length.
In other areas, such as parts of continental Europe, the palm (French: palme, Italian: palmo) related to the length of the hand, and derived from the Roman great palm, the Latin: palmus major.
On surviving Ancient Egyptian cubit-rods, the royal cubit is divided into seven palms of four digits each. Five digits are equal to a hand, with thumb; and six to a closed fist. The royal cubit measured approximately 525 mm, so the length of the ancient Egyptian palm was about 75 mm.
The ancient Roman system of linear

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28

**Measurement System:**Imperial unit system

The rod is a unit of length equal to 5½ yards, 16½ feet or ⅟320th of a statute mile. Since the adoption of the international yard on 1 July 1959, it has been equivalent to exactly 5.0292 meters. A rod is the same length as a perch or a pole. In old English, the term lug is also used.
The Ancient Roman units of measurement of length included a rod or pertica (also decempeda) of 10 pedes, which was equivalent to about 2.96 m; the related unit of square measure was the scrupulum or decempeda quadrata, equivalent to about 8.76 m².
Units comparable to the perch, pole or rod were used in many European countries, with names that include French: perche and canne, German: Ruthe, Italian: canna and pertica, Polish: pręt and Spanish: canna. They were subdivided in many different ways, and were of many different lengths. One source from 1830 lists the following:
In England, the rod is first defined in law by the Composition of Yards and Perches, one of the Statutes of uncertain date from the late 13th to early 14 centuries.
The length of the chain was standardized in 1620 by Edmund Gunter at exactly 4 rods.Fields were measured in acres, which were one chain (four rods) by one furlong (in the

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29

A light-minute (also written light minute) is a unit of length. It is defined as the distance light travels in an absolute vacuum in one minute or 17,987,547,480 metres (~18 Gm). (See 10 gigametres for similar lengths.) Note that this value is exact, since the metre is actually defined in terms of the light-second.
To give an example, the average distance from the Earth to the Sun (or 1 astronomical unit) is about 8.317 light-minutes, or about 150 million kilometers.

6.67

3 votes

30

**Measurement System:**Atomic units

The Bohr radius is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom. The precise definition of the Bohr radius is:
where:
Or, in Gaussian units the Bohr radius is simply
According to 2010 CODATA the Bohr radius has a value of 5.2917721092(17)×10 m (i.e., approximately 53 pm or 0.53 angstroms).
In the Bohr model of the structure of an atom, put forward by Niels Bohr in 1913, electrons orbit a central nucleus. The model says that the electrons orbit only at certain distances from the nucleus, depending on their energy. In the simplest atom, hydrogen, a single electron orbits the nucleus and its smallest possible orbit, with lowest energy, has an orbital radius almost equal to the Bohr radius. (It is not exactly the Bohr radius due to the reduced mass effect. They differ by about 0.1%.)
Although the Bohr model is no longer in use, the Bohr radius remains very useful in atomic physics calculations, due in part to its simple relationship with other fundamental constants. (This is why it is defined using the true electron

8.00

2 votes

31

**Measurement System:**Imperial unit system

A league is a unit of length (or, rarely, area). It was long common in Europe and Latin America, but it is no longer an official unit in any nation. The league originally referred to the distance a person or a horse could walk in an hour. Since the Middle Ages, many values have been specified in several countries.
On land, the league was most commonly defined as three miles (about 4.8 km), although the length of a "mile" could vary from place to place and at different times. At sea, a league was three nautical miles (about 5.6 km). English usage also included any of the other leagues mentioned below (for example, in discussing the Treaty of Tordesillas) .
The league was used in Ancient Rome, defined as 1.5 Roman miles (7,500 Roman feet, 2.2225 km, 1.4 mi.). The origin is the "leuga gallica" (also: leuca Gallica), the league of Gaul.
The Argentine league (legua) is 5.572 km (3.462 mi) or 6,666 varas: 1 vara is 0.83 m (33 in).
In certain districts of rural Australia, particularly far west New South Wales, one league is an approximate measurement of distance travelled in one hour by motor vehicle, usually 60 to 80 km (37 to 50 mi).
In Portugal, Brazil and other parts of the Portuguese

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32

**Measurement System:**US customary units

A mile is a unit of length, most commonly 5,280 feet (1,760 yards, or about 1,609 meters). The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile (1,852 metres, about 6,076.1 feet). There have also been many historical miles and similar units in other systems that may be translated into English as miles; they have varied in length from 1 to 15 kilometres.
The exact length of the land mile varied slightly among English-speaking countries until the international yard and pound agreement in 1959 established the yard as exactly 0.9144 metres, giving a mile of exactly 1,609.344 metres. The United States adopted this international mile for most purposes, but retained the pre-1959 mile for some land-survey data, terming it the US survey mile. In the US, statute mile formally refers to the survey mile, about 3.219 mm (⅛ inch) longer than the international mile (the international mile is exactly 0.0002% less than the US survey mile).
Use of the mile as a unit of measurement is now largely confined to the United Kingdom, the United States, and Canada.
The word mile originally derives from the Old English word mīl which in turn was

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33

**Measurement System:**International System of Units

A picometre (American spelling: picometer; symbol pm) is a unit of length in the metric system, equal to one trillionth, i.e. (/1,000,000,000,000) of a metre, which is the current SI base unit of length. It can be written in scientific notation as 1×10 m, or as 1 E−12 m in engineering notation — both meaning 1 m / 1,000,000,000,000.
It equals a millionth of a micrometre (formerly called a micron), and was formerly called micromicron, stigma, or bicron. The symbol µµ was once used for it. It is also a hundredth of an angstrom, an internationally recognised but non-SI unit of length.
The picometre's length is of an order such that its application is almost entirely confined to particle physics and quantum physics. Atoms are between 62 and 520 pm in diameter. Smaller units still may be used to describe smaller particles (some of which are the components of atoms themselves), such as hadrons and the upper limits of possible size for fermion point particles.
The Laser Interferometer Space Antenna (LISA) probe had been planned to be launched in 2025 to directly detect gravitational waves and would have been able to measure relative displacements with a resolution of 20 picometres over a

10.00

1 votes

34

**Measurement System:**International System of Units

A centimetre (American spelling: centimeter, symbol cm) is a unit of length in the metric system, equal to one hundredth of a metre, which is the SI base unit of length. Centi is the SI prefix for a factor of 10. Hence a centimetre can be written as 1×10 m, 10×10 m (engineering notation) or 1E−2 m (scientific E notation) — meaning 10 mm or 1 m/100 respectively. The centimetre is the base unit of length in the now deprecated centimetre-gram-second (CGS) system of units.
Though for many physical quantities, SI prefixes for factors of 10—like milli- and kilo-—are often preferred by technicians, the centimetre remains a practical unit of length for many everyday measurements. A centimetre is approximately the width of the fingernail of an adult person (between 3/8 in and 7/16 inch on an English inch ruler).
1 centimetre is equal to:
1 cubic centimetre is equal to 1 millilitre, under the current SI system of units.
In addition to its use in the measurement of length, the centimetre is used:
For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:
They are mostly used only with East Asian fixed-width CJK fonts, because they are equal

6.33

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35

**Measurement System:**English unit

The digit or finger is an ancient and obsolete non-SI unit of measurement of length. It was originally based on the breadth of a human finger. It was a fundamental unit of length in the Ancient Egyptian, Mesopotamian, Hebrew, Ancient Greek and Roman systems of measurement.
In astronomy a digit is one twelfth of the diameter of the sun or the moon.
The digit, also called a finger or fingerbreadth, is a unit of measurement originally based on the breadth of a human finger. In Ancient Egypt it was the basic unit of subdivision of the cubit.
On surviving Ancient Egyptian cubit-rods, the royal cubit is divided into seven palms of four digits or fingers each. The royal cubit measured approximately 525 mm, so the length of the ancient Egyptian digit was about 19 mm.
In the classical Akkadian Empire system instituted in about 2150 BC during the reign of Naram-Sin, the finger was one-thirtieth of a cubit length. The cubit was equivalent to approximately 497 mm, so the finger was equal to about 17 mm. Basic length was used in architecture and field division.
A digit (lat. digitus, "finger"), when used as a unit of length, is usually a sixteenth of a foot or 3/4" (1.905 cm for the

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**Measurement System:**International System of Units

The millimetre (American spelling: millimeter; SI unit symbol mm; informal abbreviation: mil) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length.
It is equal to 1,000 micrometres and 1,000,000 nanometres. There are 25.4 mm in one inch.
For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:
They are useful for East Asian fixed-width CJK typography, because they are equal in proportion to one Chinese character.

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**Measurement System:**French units of measurement

A toise (symbol: T) is a unit of measure for length, area and volume originating in pre-revolutionary France. In North America, it was used in colonial French establishments in early New France, French Louisiana (La Louisiane), and Quebec. The Portuguese toise (in Portuguese: toesa) was used in Portugal, Brazil and other parts of the Portuguese Empire until the adoption of the Metric system.
Historical French unit. Early Louisiana in the United States.

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**Measurement System:**US customary units

A foot (plural: feet; abbreviation or symbol: ft or ′ (the prime symbol) is a unit of length defined as being 0.3048 m exactly and used in the imperial system of units and United States customary units. It is subdivided into 12 inches.
Historically the foot was also used in a number of other systems of units including those from Ancient Greece, Ancient Rome, England, Scotland and many Continental European countries. However, the size of the measure varied from country to country and in some cases from city to city, mostly from about 250mm to 335 mm, The foot measure was also subdivided into 12 inches. Confusion between the British foot (304.8mm) and the French foot (322.5mm or 329.8mm) led to the myth of Napoleon's shortness.
The United States is the only industrialized nation that uses the international foot (a customary unit of length) and the survey foot in preference to the meter in its commercial, engineering and standards activities. The foot is still recognized as an alternative expression of length in Canada, officially defined as a unit derived from the metric metre and still commonly used in the United Kingdom, although both have partially metricated their units of

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The ångström /ˈæŋstrəm/ is a unit of length equal to 10 m (one ten-billionth of a meter) or 0.1 nm. Its symbol is the Swedish letter Å.
The ångström is often used in the natural sciences and technology to express the sizes of atoms, molecules, and microscopic biological structures, the lengths of chemical bonds, the arrangement of atoms in crystals, the wavelengths of electromagnetic radiation, and the dimensions of integrated circuit parts.
The unit was named after the Swedish physicist Anders Jonas Ångström ([ˈɔŋstrøm]; 1814–1874). The symbol is always written with a ring diacritic, as in the Swedish letter. Although the unit's name is often written in English without the diacritics, the official definitions contain diacritics.
Anders Jonas Ångström was one of the pioneers in the field of spectroscopy, and is known also for studies of astrophysics, heat transfer, terrestrial magnetism, and the aurora borealis.
In 1868, Ångström created a chart of the spectrum of solar radiation that expressed the wavelengths of electromagnetic radiation in the electromagnetic spectrum in multiples of one ten-millionth of a millimetre (or 10 mm.) Since the human eye is sensitive to wavelengths

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**Measurement System:**French units of measurement

An arpent is a unit of length and a unit of area. It is a pre-metric French unit based on the Roman actus. It is used in Quebec as well as in some areas of the United States that were part of French Louisiana.
There were various standard arpents. The most common ones were of 180 French feet, used in French North America, and 220 French feet, used in Paris.
1 arpent = 180 French feet (of approximately 32 centimetres) = about 192 English feet = about 58.47 metres
In Louisiana, parcels of land known as arpent sections or French arpent land grants also pre-date the Public Land Survey System (PLSS), but are treated as PLSS sections. An arpent is a French measurement of approximately 192 feet (59 m), and a square arpent (also referred to as an arpent) is about 0.84 acres (3,400 m).
French arpent land divisions are long narrow parcels of land usually found along the navigable streams of southern Louisiana, and also found along major waterways in other areas. This system of land subdivision was begun by French settlers in the 18th century, according to typical French practice at the time and was continued by both the Spanish and by the American government after the acquisition of the

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**Measurement System:**Imperial unit system

A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries.
Five furlongs are approximately 1 kilometre (1.0058 km is a closer approximation). Since the original definition of the metre was one-quarter of one ten-millionth of the circumference of the Earth (along the great circle coincident with the meridian of longitude passing through Paris), the circumference of the Earth is about 40,000 km or about 200,000 furlongs.
The name furlong derives from the Old English words furh (furrow) and lang (long). Dating back at least to early Anglo-Saxon times, it originally referred to the length of the furrow in one acre of a ploughed open field (a medieval communal field which was divided into strips). The system of long furrows arose because turning a team of oxen pulling a heavy plough was difficult. This offset the drainage advantages of short furrows and meant furrows were made as long as possible. An acre is an area that is one furlong long and one chain (66 feet or 22 yards) wide. For this reason,

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**Measurement System:**French units of measurement

In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The point has long been the usual unit for measuring font size and leading and other minute items on a printed page. The original printer's point, from the era of foundry metal typesetting and letter press printing, varied between 0.18 and 0.4 mm depending on various definitions of the foot. By the end of the 19th Century, it had settled to around 0.35 to 0.38 mm, depending on one’s geographical location.
In the late 1980s to the 1990s, the traditional point was supplanted by the desktop publishing point (also called the PostScript point), which was defined as 72 points to the inch (1 point = ⁄72 inches = ⁄72 mm = 0.3527 mm). In either system, there are 12 points to the pica. In metal type, the point size of the font described the size (height) of the metal body on which the typeface's characters were cast. In digital type, the body is now an imaginary design space, but is used as the basis from which the type is scaled (see em).
A measurement in picas is usually represented by placing a lower case p after the number, such as "10p" meaning "10 picas."

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**Measurement System:**English unit

A finger (sometimes fingerbreadth or finger's breadth) is any of several units of measurement that are approximately the width of an adult human finger, including:
The digit, also known as digitus or digitus transversus (Latin), dactyl (Greek) or dactylus, or finger's breadth - ⁄4 of an inch or ⁄16 of a foot.
In medicine and related disciplines (anatomy, radiology, etc.) the fingerbreadth (literally the width of a finger) is an informal but widely used unit of measure.
In the measurement of distilled spirits, two fingers of whisky refers to the amount of whiskey that would fill a glass to the level of two fingers wrapped around the glass at the bottom.
Another definition (from Noah Webster): "nearly an inch."
Finger is also the name of a longer unit of length used in cloth measurement, specifically, one eighth of a yard or 4⁄2 inches.
In English these units have mostly fallen out of use.

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The spat (symbol S), from the Latin Spatium ("Space") is an obsolete unit of distance used in astronomy. It is equal to one billion kilometres (1 Tm or 10 m). It is about 1.057×10 light-years or 3.240×10 parsecs.

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**Measurement System:**Astronomical units of length

An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length equal to exactly 149,597,870,700 metres (92,955,807.273 mi) or approximately the mean Earth–Sun distance.
The symbol ua is recommended by the International Bureau of Weights and Measures, and the international standard ISO 80000, while au is recommended by the International Astronomical Union, and is more common in Anglosphere countries. In general, the International System of Units only uses capital letters for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit. However, the use of AU to refer to the astronomical unit is widespread. The astronomical constant whose value is one astronomical unit is referred to as unit distance and is given the symbol A.
The AU was originally defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976 the International Astronomical Union (IAU) revised the definition of the AU for greater precision, defining it as that length for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical

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The smoot ( /ˈsmuːt/) is a nonstandard unit of length created as part of an MIT fraternity prank. It is named after Oliver R. Smoot, a fraternity pledge to Lambda Chi Alpha, who in October 1958 lay on the Harvard Bridge (between Boston and Cambridge, Massachusetts), and was used by his fraternity brothers to measure the length of the bridge.
One smoot is equal to Oliver Smoot's height at the time of the prank (five feet and seven inches ~1.70 m). The bridge's length was measured to be 364.4 smoots (620.1 m) plus or minus one ear, with the "plus or minus" intended to express uncertainty of measurement. Over the years the "or minus" portion has gone astray in many citations, including the markings at the site itself, but has now been enshrined in stone by Smoot's college class.
To implement his use as a measuring unit, Oliver Smoot repeatedly lay down on the bridge, let his companions mark his new position in chalk or paint, and then got up again. Eventually, he tired from all this exercise and was carried thereafter by the fraternity brothers to each new position.
Oliver Smoot graduated from MIT with the class of 1962, became a lawyer, and later became chairman of the American

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**Measurement System:**Astronomical units of length

Solar radius is a unit of distance used to express the size of stars in astronomy equal to the current radius of the Sun:
The solar radius is approximately 695,500 kilometres (432,450 miles) or about 110 times the radius of the Earth, or 10 times the average radius of Jupiter. It varies slightly from pole to equator due to its rotation, which induces an oblateness of order 10 parts per million. See 1 gigametre for similar distances.
The SOHO spacecraft was used to measure the diameter of the Sun by timing transits of Mercury across the surface during 2003 and 2006. The result was a measured radius of 696,342 ± 65 kilometres (432,687 ± 40 miles).

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**Measurement System:**English unit

The cubit is a traditional unit of length, based on the length of the forearm: from the elbow to the tip of the middle finger. Cubits of various lengths were employed in many parts of the world in Antiquity, in the Middle Ages and into Early Modern Times.
The Egyptian hieroglyph for the cubit shows the symbol of a longer than normal forearm. According to the Ancient Egyptian units of measurement, the Egyptian Royal cubit was subdivided into 7 palms of 4 fingers/digits each; surviving cubit rods are between 52.3 and 52.9 cm (20.6 to 20.8 inches) in length.
Over time, various cubits and variations on the cubit have measured:
The cubit has also been expressed as "any of various ancient units of length based on the length of the forearm from the elbow to the tip of the middle finger and usually equal to almost 18 inches (46 centimeters)."
The English word 'cubit' comes from Latin cubitum 'elbow', from the verb cubāre, -cumbĕre 'to lie down'; cf. 'recumbent'.
The earliest attested standard measure is from the Old Kingdom pyramids of Egypt and was called the royal cubit (mahe). The royal cubit was 523 to 529 mm (20.6 to 20.8 in) in length, and was subdivided into 7 palms of 4 digits

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**Measurement System:**Astronomical units of length

A convenient measure of comparing the observed/deduced radii of planetary objects to that of the planet Jupiter. It is most often used in reference with exoplanets.

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**Measurement System:**Chinese units of measurement

The li (里, lǐ) is a traditional Chinese unit of distance, which has varied considerably over time but now has a standardized length of 500 metres (1,640 feet) or half a kilometer. A modern li consists of 1,500 Chinese "feet" or chi and, in the past, was often translated as a "mile." Since the li has generally been only about a third as long as the mile, translating the character as "Chinese mile" or simply "li" is much less likely to produce confusion or error.
In practice however, as late as the 1940s, a li did not represent a fixed measure. It could be longer or shorter depending on the effort required to cover the distance.
The character 里 combines the characters for "field" (田, tián) and "earth" (土, tǔ), since it was considered to be about the length of a single village. In Chinese, li is sometimes prefaced by the character shi (市, shì) to distinguish it from the kilometer proper or gongli (公里, gōnglǐ).
There is also another li (Traditional: 釐, Simplified: 厘, lí) that indicates a unit of length 1/1000 of a chi, but it is used much less commonly. This li is used in the People's Republic of China as the equivalent of the centi- prefix in metric units, thus limi (厘米, límǐ) for

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**Measurement System:**International System of Units

The metre (meter in American English), symbol m, is the fundamental unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth′s equator to the North Pole (at sea level), its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."
The first recorded proposal for a decimal-based unit of length was the universal measure unit proposed by the English philosopher John Wilkins in 1668. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the words metro cattolico (lit. "catholic [i.e. universal] metre"), which was derived from the Greek μέτρον καθολικόν (métron katholikón), "a universal measure". This word gave rise to the French mètre which in 1797 was introduced into the English language.
In 1668, Wilkins proposed using Christopher Wren's suggestion of a pendulum with a half-period of one second to measure a standard length that Christiaan Huygens had observed to be 38 Rhineland or 39 ⁄4 English inches (997 mm)

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**Measurement System:**International System of Units

A nanometre (American spelling: nanometer; symbol nm) is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- (from the Ancient Greek νάνος, nanos, "dwarf") with the parent unit name metre (from Greek μέτρον, metrοn, "unit of measurement").
The nanometre is often used to express dimensions on the atomic scales: the diameter of a helium atom, for example, is about 0.1 nm, and that of a ribosome is about 20 nm. The nanometre is commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light, in particular, ranges from 400 to 1000 nm. In these uses, the nanometre appears to be supplanting the other common unit for atomic scale dimensions, the angstrom, which is equal to 0.1 nanometre.
This unit is often associated with the field of nanotechnology. Since late 1980s, it has also been used to describe generations of the manufacturing technology in the semiconductor industry.
The nanometre was formerly known as the millimicron, since it is 1/1000 of a micron (micrometre), and was often denoted by the symbol mµ or (more rarely) µµ.

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A light-month (also written light month) is a unit of length. It is defined as the distance light travels in an absolute vacuum in one full month (thirty days of 86,400 seconds each) or 777,062,051,136,000 metres (~777 Tm). See Orders of magnitude (length) for similar scale lengths.
Note that this value is exact, since the metre is actually defined in terms of the speed of light. Nevertheless, since the term "month" can be understood in various ways (hollow month, average julian month, etc.), use of this unit is not recommended.
The light-month is not very frequently used at all since there are few astronomical objects of that magnitude; the orbits of outer solar system objects are better measured in light-days or light-hours, whilst interstellar distances are on the order of light-years. There are, however, some exceptions. The Oort cloud, for example, is thought to extend between 10 and 20 light-months out from the Sun.

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A bamboo is an obsolete unit of length in India and Myanmar.
In India, the unit was fixed by the reforms of Akbar the Great (1556–1605) at approximately 12.8 m (42 ft). After Metrication in India in the mid-20th century, the unit became obsolete.
In Myanmar (formerly Burma) it was approximately 3.912 meters (154 in, or 12.86 ft). It was also known as the dha.
"Bamboo". Sizes, grades, units, scales, calendars, chronologies. http://www.sizes.com/units/bamboo.htm. Retrieved 2007-02-19.

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**Measurement System:**International System of Units

A micrometre (or micrometer) is 1×10 of a metre (SI Standard prefix "micro" = 10); that is, one-millionth of a metre (or one-thousandth of a millimetre, 0.001 mm, or about 0.000039 inches). Its unit symbol in the International System of Units (SI) is µm. The latter may be rendered as um if Greek fonts are not available or not admissible. "Micron" comes from Greek μικρόν (mikrón), which means "small".
The term micron and the symbol µ, representing the micrometre, were officially accepted between 1879 and 1967, but officially revoked by the ISI in 1967. Nevertheless, in practice, "micron" remains a widely used term in preference to "micrometre" in many English-speaking countries, and in American English the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. The term "micron" is still extensively used in most English-speaking countries both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing).
The micrometre is a common unit of

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**Measurement System:**Persian units of measurement

The parasang (Persian: فرسنگ/فرسخ/پرسنگ) is a historical Iranian unit of itinerant distance comparable to the European league.
In antiquity, the term was used throughout much of the Middle East, and the Old Iranian language from which it derives can no longer be determined (only two—of what must have been dozens—of Old Iranian languages are attested). There is no consensus with respect to its etymology or literal meaning. In addition to its appearance in various forms in later Iranian languages (e.g. Middle Persian farsang or Sogdian fasukh), the term also appears in Greek as Παρασάγγης parasanges, in Latin as parasanga, in Armenian as hrasakh, in Georgian as parsakhi, in Syriac as prsha, in Arabic as farsakh (فرسخ) and in Turkish fersah. The present-day New Persian word is also farsakh, and should not be confused with the present-day farsang (فرسنگ), which is now a metric unit of 6 km. or 4 miles
The parasang may have originally been some fraction of the distance an infantryman could march in some predefined period of time. Herodotus (v.53) speaks of [an army] traveling the equivalent of five parasangs per day.
The earliest surviving mention of the parasang comes from the mid-5th

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The beard-second is a unit of length created as teaching exercise inspired by the light year, but for nuclear physics instead of astronomy. The beard-second is defined as the length an average beard grows in a second, or about 5 nanometers.
The beard-second is less popular than the Ångström (10 m). In physics it is also common to use the SI units, usually with the nano prefix; 1 nm = 10 m.

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A light day (also written light-day) is a unit of length. It is defined as the distance light travels in an absolute vacuum in one day (of 86,400 seconds) or 25,902,068,371,200 metres (~26 Tm).
Note that this value is exact, since the metre is actually defined in terms of the speed of light. The light day isn't very frequently used at all since there are few astronomical objects or distances of that magnitude; the Oort cloud, for example, is thought to extend between 290 and 580 light-days out from the Sun.
Every known object in the solar system is less than a light-day from the Sun. A light-day is about 173 astronomical units. Sedna is currently 90 AU (0.52 light-days) from the Sun, though it goes out as far as 975 AU (5.64 light-days). While Eris, the outermost known dwarf planet, is 97 AU (0.56 light-days) distant and currently the most distant known object in orbit around the sun. The existence of the Oort cloud has been deduced from comets; no object in the Oort cloud has yet been seen.

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**Measurement System:**Astronomical units of length

A light-year, also light year or lightyear (symbol: ly), is a unit of length equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.
The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.
1 light-year = 9460730472580800 metres (exactly)
The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86400 SI seconds, totalling 31557600 seconds) and a defined speed of light of 299792458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb's J1900.0 mean tropical year of

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**Measurement System:**Imperial unit system

A fathom (abbreviation: ftm) = 1.8288 meters, is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.
There are two yards (6 feet) in an imperial or U.S. fathom. Originally based on the distance between the man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around 5–5+⁄2 feet (1.5–1.7 m).
The name derives from the Old English word fæðm, corresponding to the old Frisian (Northern Dutch) word "fadem" meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme. A cable length, based on the length of a ship's cable, has been variously reckoned as equal to 100 or 120 fathoms. At one time, a quarter meant a fourth of a fathom.
Abbreviations: f, fath, fm, fth, fthm.
One fathom is equal to:
In the international yard and pound agreement of 1959 the United States, Australia, Canada, New Zealand, South Africa, and the United Kingdom defined the length of the international yard to be exactly 0.9144

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**Measurement System:**Imperial unit system

The link (usually abbreviated as "l.", "li." or "lnk."), also called a Gunter’s link, is a unit of length in the imperial system. The unit was based on Gunter's measurement where a metal chain consisting of 100 links was used in surveying real property. In the English-speaking world prior to the 20th century, links were commonly used for this function but are rarely used now.
A link is exactly ⁄50 of a survey foot. Twenty-five links make a rod (16.5 feet). One hundred links make a chain. One thousand links make a furlong. Eight thousand links make a statute mile.

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**Measurement System:**English unit

A nail, as a unit of cloth measurement, is generally a sixteenth of a yard or 2⁄4 inches (5.715 cm). The nail was apparently named after the practice of hammering brass nails into the counter at shops where cloth was sold. On the other hand, R D Connor, in The weights and measures of England (p 84) states that the nail was the 16th part of a Roman foot, i.e., digitus or finger, although he provides no reference to support this. Zupko's A dictionary of weights and measures for the British Isles (p 256) states that the nail was originally the distance from the thumbnail to the joint at the base of the thumb, or alternately, from the end of the middle finger to the second joint.
An archaic usage of the term nail is as a sixteenth of a (long) hundredweight for mass, or 1 clove of 7 pound avoirdupois (3.175 kg).
Explanation: Katherine and Petruchio are getting married. At the tailor shop, they examine the wedding dress, which is nearly finished. Petruchio is concerned that it has too many frills, wonders what it will cost, and suspects that he has been cheated. Katherine says she likes it, and complains that Petruchio is making a fool of her. The taylor repeats Katherine's words: Sir,

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**Measurement System:**Astronomical units of length

The parsec (symbol: pc) is a unit of length used in astronomy. It is about 3.26 light-years, which is about 30.9 trillion (3.09×10) kilometres or about 19.2 trillion (1.92×10) miles.
The name parsec is "an abbreviated form of 'a distance corresponding to a parallax of one second'." It was coined in 1913 at the suggestion of British astronomer Herbert Hall Turner. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond.
1 parsec = 30856775814671900 metres (approximate)
The parsec is equal to the length of the adjacent side of an imaginary right triangle in space. The two dimensions on which this triangle is based are the angle (which is defined as 1 arcsecond), and the opposite side (which is defined as 1 astronomical unit, which is the distance from the Earth to the Sun). Using these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.
One of the oldest methods for astronomers to calculate the distance to a star was to record the difference in angle between two measurements of the position of the star in the sky. The first measurement was taken from the Earth on one

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**Measurement System:**English unit

A span is the distance measured by a human hand, from the tip of the thumb to the tip of the little finger. In ancient times, a span was considered to be half a cubit. Sometimes the distinction is made between the great span (thumb to little finger) and little span (index finger to little finger)
Ancient Greek texts show that the span was used as a fixed measure in ancient Greece since at least archaic period. The word spithame (Greek: "σπιθαμή"), "span", is attested in the work of Herodotus in the 5th century BC, however span was used in Greece long before, since the word trispithamos (Greek: "τρισπίθαμος"), "three spans long", occurs as early as the 8th century BC in Hesiod.
See also: English unit
In Slavic languages, the analogue of the span is various words derived from Proto-Slavic *pędь (Polish, piędź; Russian, пядь, Slovenian, ped, etc.). In various Slavic languages it is the distance from the tip of the thumb to the tip of the little finger or index finger. For example, Slovenian velika ped = great span (23 cm), mala ped = little span (9.5 cm); Russian piad = 4 vershok = 17.8 cm. See also: Obsolete Russian weights and measures.
In Swahili, the equivalent of the great span

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**Measurement System:**Imperial unit system

A thousandth of an inch is a unit of length. Equal to 0.001 inch, it is normally referred to as either a thou ( /ˈθaʊ/), or particularly in the United States, a mil.
Joseph Whitworth is generally credited with introducing thousandths of an inch as an engineering unit.
The plural of thou is also thou (one hundredth of an inch is written 10 thou), while the plural of mil is mils.
The thou, or mil, is most commonly used in engineering. For example in specifying:
There are also compound units such as "mils per year" used to express corrosion rates.
A related measurement for area known as the circular mil, is based on a circle having a diameter of one mil.
In machining, where the thou is treated as a basic unit, 0.0001 inches can be referred to as "one tenth", meaning "one tenth of a thou". On CNC machine tools, all linear motion is based on the minimum increment of the control system, which in the past was commonly one "tenth" (0.0001 inch). Machining "to within a few tenths" is usually considered very accurate. However, in recent decades production tolerances for many parts have been trending downward, and production-run tolerance ranges of 2 tenths are no longer rare, and tolerances

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**Measurement System:**US customary units

A yard (abbreviation: yd) is a unit of length in several different systems including United States customary units, Imperial units and the former English units. It is equal to 3 feet or 36 inches. Under an agreement in 1959 between Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States, the yard (known as the "international yard" in the United States) was legally defined to be exactly 0.9144 metres. Prior to that date, the legal definition of the yard when expressed in terms of metric units varied slightly from country to country.
For purposes of measuring cloth, the early yard was divided by the binary method into two, four, eight and sixteen parts. The two most common divisions were the fourth and sixteenth parts. The quarter of a yard was known as the "quarter" without further qualification, while the sixteenth of a yard was called a nail. The eighth of a yard was sometimes called a finger, but was more commonly referred to simply as an eighth of a yard, while the half-yard was called "half a yard".
Other units related to the yard, but not specific to cloth measurement: two yards are a fathom, a quarter of a yard (when not referring to cloth) is a

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