Landrace

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Landrace

 

Aerial roots of a maize landrace grown in nitrogen-depleted soils in the Sierra Mixe. Known for extensive aerial roots with a bacterial gel supplying 29%–82% of the plant's nitrogen supply.

A landrace is a domesticated, locally adapted, often traditional variety of a species of animal or plant that has developed over time, through adaptation to its natural and cultural environment of agriculture and pastoralism, and due to isolation from other populations of the species. Landraces are distinct from cultivars and from standard breeds.

A significant proportion of farmers around the world grow landrace crops, and most plant landraces are associated with traditional agricultural systems. Landraces of many crops have probably been grown for millennia. Increasing reliance upon modern plant cultivars that are bred to be uniform has led to a reduction in biodiversity, because most of the genetic diversity of domesticated plant species lies in landraces and other traditionally used varieties. Some farmers using scientifically improved varieties also continue to raise landraces for agronomic reasons that include better adaptation to the local environment, lower fertilizer requirements, lower cost, and better disease resistance. Cultural and market preferences for landraces include culinary uses and product attributes such as texture, color, or ease of use.

Domestic shorthair cat, a landrace of Felis catus

Plant landraces have been the subject of more academic research, and the majority of academic literature about landraces is focused on botany in agriculture, not animal husbandry. Animal landraces are distinct from ancestral wild species of modern animal stock, and are also distinct from separate species or subspecies derived from the same ancestor as modern domestic stock. Not all landraces derive from wild or ancient animal stock; in some cases, notably dogs and horses, domestic animals have escaped in sufficient numbers in an area to breed feral populations that form new landraces through evolutionary pressure.

Characteristics

There are differences between authoritative sources on the specific criteria which describe landraces, although there is broad consensus about the existence and utility of the classification. Individual criteria may be weighted differently depending on a given source's focus (e.g., governmental regulation, biological sciences, agribusiness, anthropology and culture, environmental conservation, pet -keeping and -breeding, etc.). Additionally, not all cultivars agreed to be landraces exhibit every characteristic of a landrace. General features that characterize a landrace may include:

A basket of landrace snap melons Cucumis melo subspecies agrestis, cultivar group Momordica from Pemba town, northern Mozambique. The landrace incorporates different colours and patterns of the fruit surface and is the only melon cultivar group in northern Mozambique.

• It is morphologically distinctive and identifiable (i.e., has particular and recognizable characteristics or properties), yet remains "dynamic".
• It is genetically adapted to, and has a reputation for being able to withstand, the conditions of the local environment, including climate, disease and pests, even cultural practices.
• It is not the product of formal (governmental, organizational, or private) breeding programs, and may lack systematic selection, development and improvement by breeders.
• It is maintained and fostered less deliberately than a standardized breed, with its genetic isolation principally a matter of geography acting upon whatever animals that happened to be brought by humans to a given area.
• It has a historical origin in a specific geographic area, will usually have its own local name(s), and will often be classified according to intended purpose.
• Where yield (e.g. of a grain or fruit crop) can be measured, a landrace will show high stability of yield, even under adverse conditions, but a moderate yield level, even under carefully managed conditions.
• At the level of genetic testing, its heredity will show a degree of integrity, but still some genetic heterogeneity (i.e. genetic diversity).

Terminology

See also: Breed, Cultivar, and Ecotype

Landrace literally means 'country-breed' (German: Landrasse) and close cognates of it are found in various Germanic languages. The first known reference to the role of landraces as genetic resources was made in 1890 at an agriculture and forestry congress in Vienna, Austria. The term was first defined by Kurt von Rümker in 1908, and more clearly described in 1909 by U. J. Mansholt, who wrote that landraces have more stable characteristics and better resistance to adverse conditions, but have lower production capacity than cultivars, and are apt to change genetically when moved to another environment. H. Kiessling added in 1912 that a landrace is a mixture of phenotypic forms despite relative outward uniformity, and a great adaptability to its natural and human environment.

The word landrace entered non-academic English in the early 1930s, by way of the Danish Landrace pig, a particular breed of lop-eared swine. Many other languages do not use separate terms, like landrace and breed, but instead rely on extended description to convey such distinctions. Spanish is one such language.

Geneticist D. Phillip Sponenberg described animal breeds within these classes: the landrace, the standardized breed, modern "type" breeds, industrial strains, and feral populations. He describes landraces as an early stage of breed development, created by a combination of founder effect, isolation, and environmental pressures. Human selection for production goals is also typical of landraces.

As discussed in more detail in breed, that term itself has several definitions from various scientific and animal husbandry perspectives. Some of those senses of breed relate to the concept of landraces. A Food and Agriculture Organization of the United Nations (FAO) guideline defines landrace and landrace breed as "a breed that has largely developed through adaptation to the natural environment and traditional production system in which it has been raised." This is in contrast to its definition of standardized breed: "a breed of livestock that was developed according to a strict programme of genetic isolation and formal artificial selection to achieve a particular phenotype."

In various domestic species (including pigs, goats, sheep and geese) some standardized breeds include "Landrace" in their names, but do not meet widely used definitions of landraces. For example, the British Landrace pig is a standardized breed, derived from earlier breeds with "Landrace" names.

Farmers' variety, usually applied to local cultivars, or seen as intermediate between a landrace and a cultivar, may also include landraces when referring to plant varieties not subjected to formal breeding programs.

Autochthonous and allochthonous landraces

A landrace native to, or produced for a long time within the agricultural system in which it is found is referred to as an autochthonous landrace, while a more recently introduced one is termed an allochthonous landrace.

Within academic agronomy, the term autochthonous landrace is sometimes used with a more technical, productivity-related definition, synthesized by A. C. Zeven from previous definitions beginning with Mansholt's: "an autochthonous landrace is a variety with a high capacity to tolerate biotic and abiotic stress, resulting in a high yield stability and an intermediate yield level under a low input agricultural system."

The terms autochthonous and allochthonous are most often applied to plants, with animals more often being referred to as indigenous or native. Examples of references in sources to long-term local landraces of livestock include constructions such as "indigenous landraces of sheep", and "Leicester Longwool sheep were bred to the native landraces of the region". Some usage of autochthonous does occur in reference to livestock, e.g. "autochthonous races of cattle such as the Asturian mountain cattle – Ratina and Casina – and Tudanca cattle."

Biodiversity and conservation

A morphologically diverse group of fruit from the Zapallo Plomo landrace of Cucurbita maxima squash

A significant proportion of farmers around the world grow landrace crops. However, as industrialized agriculture spreads, cultivars, which are selectively bred for high yield, rapid growth, disease and drought resistance, and other commercial production values, are supplanting landraces, putting more and more of them at risk of extinction.

In 1927 at the International Agricultural Congress, organized by the predecessor of the FAO, an extensive discussion was held on the need to conserve landraces. A recommendation that members organize nation-by-nation landrace conservation did not succeed in leading to widespread conservation efforts.

Landraces are often free from many intellectual property and other regulatory encumbrances. However, in some jurisdictions, a focus on their production may result in missing out on some benefits afforded to producers of genetically selected and homogenous organisms, including breeders' rights legislation, easier availability of loans and other business services, even the right to share seed or stock with others, depending on how favorable the laws in the area are to high-yield agribusiness interests.

As Regine Andersen of the Fridtjof Nansen Institute (Norway) and the Farmers' Rights Project puts it, "Agricultural biodiversity is being eroded. This trend is putting at risk the ability of future generations to feed themselves. In order to reverse the trend, new policies must be implemented worldwide. The irony of the matter is that the poorest farmers are the stewards of genetic diversity." Protecting farmer interests and protecting biodiversity is at the heart of the International Treaty on Plant Genetic Resources for Food and Agriculture (the "Plant Treaty" for short), under the Food and Agriculture Organization of the United Nations (FAO), though its concerns are not exclusively limited to landraces.

Landraces played a basic role in the development of the standardized breeds but are today threatened by the market success of the standardized breeds. In developing countries, landraces still play an important role, especially in traditional production systems. Specimens within an animal landrace tend to be genetically similar, though more diverse than members of a standardized or formal breed.

Carosello and Barattiere, Italian landraces of Cucumis melo whose fruits are eaten unripe

In situ and ex situ landrace conservation

Two approaches have been used to conserve plant landraces:

• in situ where the landrace is grown and conserved by farmers on farms.
• ex situ where the landrace is conserved in an artificial environment such as a gene-bank, using controls such as laminated packets kept frozen at −18 °C (0 °F).

As the amount of agricultural land dedicated to growing landrace crops declines, such as in the example of wheat landraces in the Fertile Crescent, landraces can become extinct in cultivation. Therefore ex situ landrace conservation practices are considered a way to avoid losing the genetic diversity completely. Research published in 2020 suggested that existing ways of cataloging diversity within ex situ genebanks fall short of cataloging the appropriate information for landrace crops.

An in situ conservation effort to save the Berrettina di Lungavilla squash landrace made use of participatory plant breeding practices in order to incorporate the local community into the work.

Preserving cereal landraces

Preservation efforts for cereal strains are ongoing including in situ and in online-searchable germplasm collections (seed banks), coordinated by Biodiversity International and the National Institute of Agricultural Botany (NIAB, UK). However, more may need to be done, because plant genetic variety, the source of crop health and seed quality, depends on a diversity of landraces and other traditionally used varieties. Efforts (as of 2008) were mostly focused on Iberia, the Balkans, and European Russia, and dominated by species from mountainous areas. Despite their incompleteness, these efforts have been described as "crucial in preventing the extinction of many of these local ecotypes".

An agricultural study published in 2008 showed that landrace cereal crops began to decline in Europe in the 19th century such that cereal landraces "have largely fallen out of use" in Europe. Landrace cultivation in central and northwest Europe was almost eradicated by the early 20th century, due to economic pressure to grow improved, modern cultivars. While many in the region are already extinct, some have survived by being passed from generation to generation, and have also been revived by enthusiasts outside Europe to preserve European agriculture and food culture elsewhere. These survivals are usually for specific uses, such as thatch, and traditional European cuisine and craft beer brewing.

Plants

Plant landrace development

See also: Plant breeding § Evolutionary plant breeding

The label landrace includes regional cultigens that are genetically heterogeneous, but with enough characteristics in common to permit their recognition as a group. These characteristics are used by farmers to manage diversity and purity within landraces.

In some cultures, development of new landraces is typically limited to members of specific social groups, such as women or shaman. Maintaining existing landraces, like developing new landraces, requires that farmers be able to identify crop-specific characteristics and that those characteristics are passed on to following generations.

Over time, the process of identifying the distinguishing characteristic or characteristics of a new landrace is reinforced by cultivation processes; for example, descendants of a plant that is notably drought tolerant may become iteratively more so through selective breeding as farmers regard it as better for dry areas and prioritize planting it in those locations. This is one way in which farming systems can develop a portfolio of landraces over time that have specific ecological niches and uses.

Conversely, modern cultivars can also be developed into a landrace over time when farmers save seed and practice selective breeding.

Although landraces are often discussed once they have become endemic to a particular geographical region, landraces have always been moved over long and short distances. Some landraces can adapt to various environments, while others only thrive within specific conditions. Self-fertilizing and vegetatively populated species adapt by changing the frequencies of phenotypes. Outbreeding crops absorb new genotypes through intentional and unintentional hybridization, or through mutation.

Cultivars developed from landraces

Members of a landrace variety, selected for uniformity with regards to a unique feature over a period of time, can be developed into a farmers' variety or cultivar. Traits from landraces are valuable for incorporation into elite lines. Crop disease resistance genes from landraces can provide eternally-needed resistances to more widely-used, modern varieties.

Parsec

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Parsec

 

Parsec
A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale)
General information
Unit system	astronomical units
Unit of	length/distance
Symbol	pc
Conversions
1 pc in ...	... is equal to ...
metric (SI) units	3.0857×1016 m    ~31 petametres
imperial & US units	1.9174×1013 mi
astronomical units	2.06265×105 au    3.26156 ly

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (au), i.e. 30.9 trillion kilometres (19.2 trillion miles). The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 au subtends an angle of one arcsecond (1/3600 of a degree). The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand.

The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to make calculations of astronomical distances from only raw observational data easy for astronomers. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.

In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648000/π au, or approximately 30.856775814913673×10¹⁵ metres (based on the IAU 2012 definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many astronomical references.

History and derivation

See also: Stellar parallax

The parsec is defined as being equal to the length of the adjacent leg (opposite leg being 1 AU) of an extremely elongated imaginary right triangle in space. The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit (the average Earth-Sun distance), and the subtended angle of the vertex opposite that leg, measuring one arcsecond. Applying the rules of trigonometry to these two values, the unit length of the other leg of the triangle (the parsec) can be derived.

One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is taken approximately half a year later, when the Earth is on the opposite side of the Sun. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the distant vertex. Then the distance to the star could be calculated using trigonometry. The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate the 3.5-parsec distance of 61 Cygni.

Stellar parallax motion from annual parallax

The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the semimajor axis of the Earth's orbit. The star, the Sun and the Earth form the corners of an imaginary right triangle in space: the right angle is the corner at the Sun, and the corner at the star is the parallax angle. The length of the opposite side to the parallax angle is the distance from the Earth to the Sun (defined as one astronomical unit, au), and the length of the adjacent side gives the distance from the sun to the star. Therefore, given a measurement of the parallax angle, along with the rules of trigonometry, the distance from the Sun to the star can be found. A parsec is defined as the length of the side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond.

The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the reciprocal of the parallax angle in arcseconds (i.e. if the parallax angle is 1 arcsecond, the object is 1 pc from the Sun; if the parallax angle is 0.5 arcseconds, the object is 2 pc away; etc.). No trigonometric functions are required in this relationship because the very small angles involved mean that the approximate solution of the skinny triangle can be applied.

Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He proposed the name astron, but mentioned that Carl Charlier had suggested siriometer and Herbert Hall Turner had proposed parsec. It was Turner's proposal that stuck.

Calculating the value of a parsec

By the 2015 definition, 1 au of arc length subtends an angle of 1″ at the center of the circle of radius 1 pc. That is, 1 pc = 1 au/tan(1″) ≈ 206,264.8 au by definition. Converting from degree/minute/second units to radians,

${\displaystyle \frac{1\text{ pc}}{1\text{ au}}=\frac{180\times 60\times 60}{\pi }}$, and

${\displaystyle 1\text{ au}=149597870700\text{ m}}$ (exact by the 2012 definition of the au)

Therefore,

${\displaystyle \pi \mathrm{p}\mathrm{c}=180\times 60\times 60\mathrm{a}\mathrm{u}=180\times 60\times 60\times 149597870700\mathrm{m}=96939420213600000\mathrm{m}}$ (exact by the 2015 definition)

Therefore,

$${\displaystyle 1\mathrm{p}\mathrm{c}=\frac{96939420213600000}{\pi }\mathrm{m}=30856775814913673\mathrm{m}}$$

(to the nearest metre)

Approximately,

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. Thus the distance ES is one astronomical unit (au). The angle SDE is one arcsecond (1/3600 of a degree) so by definition D is a point in space at a distance of one parsec from the Sun. Through trigonometry, the distance SD is calculated as follows:

$${\displaystyle \begin{array}{rl}\mathrm{S}\mathrm{D}& =\frac{\mathrm{E}\mathrm{S}}{\tan 1^{″}}\\ & =\frac{\mathrm{E}\mathrm{S}}{\tan \left(\frac{1}{60\times 60}\times \frac{\pi }{180}\right)}\\ & \approx \frac{1\mathrm{a}\mathrm{u}}{\frac{1}{60\times 60}\times \frac{\pi }{180}}=\frac{648000}{\pi }\mathrm{a}\mathrm{u}\approx 206264.81\mathrm{a}\mathrm{u}.\end{array}}$$

Because the astronomical unit is defined to be 149597870700 m, the following can be calculated:

Therefore, 1 parsec	≈ 206264.806247096 astronomical units
	≈ 3.085677581×1016 metres
	≈ 30.856775815 trillion kilometres
	≈ 19.173511577 trillion miles

Therefore, if 1 ly ≈ 9.46×10¹⁵ m,

Then 1 pc ≈ 3.261563777 ly

A corollary states that a parsec is also the distance from which a disc one astronomical unit in diameter must be viewed for it to have an angular diameter of one arcsecond (by placing the observer at D and a diameter of the disc on ES).

Mathematically, to calculate distance, given obtained angular measurements from instruments in arcseconds, the formula would be:

$${\displaystyle {\text{Distance}}_{\text{star}}=\frac{{\text{Distance}}_{\text{earth-sun}}}{\tan \frac{\theta }{3600}}}$$

where θ is the measured angle in arcseconds, Distance_earth-sun is a constant (1 au or 1.5813×10⁻⁵ ly). The calculated stellar distance will be in the same measurement unit as used in Distance_earth-sun (e.g. if Distance_earth-sun = 1 au, unit for Distance_star is in astronomical units; if Distance_earth-sun = 1.5813×10⁻⁵ ly, unit for Distance_star is in light-years).

The length of the parsec used in IAU 2015 Resolution B2 (exactly 648000/π astronomical units) corresponds exactly to that derived using the small-angle calculation. This differs from the classic inverse-tangent definition by about 200 km, i.e. only after the 11th significant figure. As the astronomical unit was defined by the IAU (2012) as an exact length in metres, so now the parsec corresponds to an exact length in metres. To the nearest meter, the small-angle parsec corresponds to 30856775814913673 m.

Usage and measurement

The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01″, and thus to stars no more than 100 pc distant. This is because the Earth's atmosphere limits the sharpness of a star's image. Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100000 stars with an astrometric precision of about 0.97 mas, and obtained accurate measurements for stellar distances of stars up to 1000 pc away.

ESA's Gaia satellite, which launched on 19 December 2013, is intended to measure one billion stellar distances to within 20 microarcsecond, producing errors of 10% in measurements as far as the Galactic Centre, about 8000 pc away in the constellation of Sagittarius.

Distances in parsecs

Distances less than a parsec

Distances expressed in fractions of a parsec usually involve objects within a single star system. So, for example:

• One astronomical unit (au), the distance from the Sun to the Earth, is just under 5×10⁻⁶ pc.
• The most distant space probe, Voyager 1, was 0.000703 pc from Earth as of January 2019. Voyager 1 took 41 years to cover that distance.
• The Oort cloud is estimated to be approximately 0.6 pc in diameter

As observed by the Hubble Space Telescope, the astrophysical jet erupting from the active galactic nucleus of M87 subtends 20″ and is thought to be 1.5 kiloparsecs (4,892 ly) long (the jet is somewhat foreshortened from Earth's perspective).

Parsecs and kiloparsecs

Distances expressed in parsecs (pc) include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of 1,000 parsecs (3,262 ly) is denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to express distances between parts of a galaxy or within groups of galaxies. So, for example (NB one parsec is approximately equal to 3.26 light-years):

• Proxima Centauri, the nearest known star to earth other than the sun, is about 1.3 parsecs (4.24 ly) away by direct parallax measurement.
• The distance to the open cluster Pleiades is 130±10 pc (420±30 ly) from us per Hipparcos parallax measurement.
• The centre of the Milky Way is more than 8 kiloparsecs (26,000 ly) from the Earth and the Milky Way is roughly 34 kiloparsecs (110,000 ly) across.
• The Andromeda Galaxy (M31) is about 780 kpc (2.5 million ly) away from the Earth.

Megaparsecs and gigaparsecs

Astronomers typically express the distances between neighbouring galaxies and galaxy clusters in megaparsecs (Mpc). A megaparsec is one million parsecs, or about 3,260,000 light years. Sometimes, galactic distances are given in units of Mpc/h (as in "50/h Mpc", also written "50 Mpc h⁻¹"). h is a constant (the "dimensionless Hubble constant") in the range 0.5 < h < 0.75 reflecting the uncertainty in the value of the Hubble constant H for the rate of expansion of the universe: h = H/100 (km/s)/Mpc. The Hubble constant becomes relevant when converting an observed redshift z into a distance d using the formula d ≈ c/H × z.

One gigaparsec (Gpc) is one billion parsecs — one of the largest units of length commonly used. One gigaparsec is about 3.26 billion ly, or roughly 1/14 of the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Astronomers typically use gigaparsecs to express the sizes of large-scale structures such as the size of, and distance to, the CfA2 Great Wall; the distances between galaxy clusters; and the distance to quasars.

For example:

• The Andromeda Galaxy is about 0.78 Mpc (2.5 million ly) from the Earth.
• The nearest large galaxy cluster, the Virgo Cluster, is about 16.5 Mpc (54 million ly) from the Earth.
• The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc (650 million ly) from the Earth.
• The galaxy filament Hercules–Corona Borealis Great Wall, currently the largest known structure in the universe, is about 3 Gpc (9.8 billion ly) across.
• The particle horizon (the boundary of the observable universe) has a radius of about 14 Gpc (46 billion ly).

Volume units

To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs (kpc³) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds, and interstellar gas is determined in a similar fashion. To determine the number of galaxies in superclusters, volumes in cubic megaparsecs (Mpc³) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge Boötes void is measured in cubic megaparsecs.

In physical cosmology, volumes of cubic gigaparsecs (Gpc³) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars. The Sun is currently the only star in its cubic parsec, (pc³) but in globular clusters the stellar density could be from 100–1000 pc⁻³.

The observational volume of gravitational wave interferometers (e.g., LIGO, Virgo) is stated in terms of cubic megaparsecs (Mpc³) and is essentially the value of the effective distance cubed.

In popular culture

The parsec was seemingly used incorrectly as a measurement of time by Han Solo in the first Star Wars film, when he claimed his ship, the Millennium Falcon "made the Kessel Run in less than 12 parsecs". The claim was repeated in The Force Awakens, but this was changed in Solo: A Star Wars Story, by stating the Millennium Falcon traveled a shorter distance (as opposed to a quicker time) due to a more dangerous route through the Kessel Run, enabled by its speed and maneuverability. It is also used ambiguously as a spatial unit in The Mandalorian as opposed to a unit of distance.

In the book A Wrinkle in Time, "Megaparsec" is Mr. Murry's nickname for his daughter Meg.

Light-year

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Light-year

 

Light-year
Map showing the stars that lie within 12.5 light-years of the Sun
General information
Unit system	astronomy units
Unit of	length
Symbol	ly
Conversions
1 ly in ...	... is equal to ...
metric (SI) units	9.4607×1015 m    9.46075 Pm
imperial and US units	5.8786×1012 mi
astronomical units	63241 au    0.3066 pc

A light-year, alternatively spelled light year, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×10¹² km), or 5.88 trillion miles (5.88×10¹² mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the word "year", the term is sometimes misinterpreted as a unit of time.

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit (au) subtends an angle of one second of arc.

Definitions

As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light (299792458 m/s). Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this, the following conversions can be derived. The IAU-recognized abbreviation for light-year is "ly", although other standards like ISO 80000:2006 (now superseded) have used "l.y." and localized abbreviations are frequent, such as "al" in French (from année-lumière), Spanish (from año luz), Italian (from anno luce), "Lj" in German (from Lichtjahr), etc.

1 light-year	= 9460730472580800 metres (exactly)
	≈ 9.461 petametres
	≈ 9.461 trillion kilometres (5.879 trillion miles)
	≈ 63241.077 astronomical units
	≈ 0.306601 parsecs

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 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530×10¹⁵ m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×10¹⁵ m found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405×10¹⁵ m, is the product of the J1900.0 mean tropical year and the defined speed of light.

Abbreviations used for light-years and multiples of light-years are

• "ly" for one light-year
• "kly" for a kilolight-year (1,000 light-years)
• "Mly" for a megalight-year (1,000,000 light-years)
• "Gly" for a gigalight-year (1,000,000,000 light-years)

History

The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a 160-millimetre (6.2 in) heliometre designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9×10¹³ km; 6.1×10¹³ mi). Bessel added that light takes 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

The speed of light was not yet precisely known in 1838; the estimate of its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.

The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.

Although modern astronomers often prefer to use the parsec, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.

Usage of term

Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.

List of orders of magnitude for length

Scale (ly)	Value	Item
10−9	4.04×10−8 ly	Reflected sunlight from the Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly 350000 to 400000 kilometres).
10−6	1.58×10−5 ly	One astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.
	1.27×10−4 ly	The Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.
	5.04×10−4 ly	New Horizons encounters Pluto at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−3	2.04×10−3 ly	The most distant space probe, Voyager 1, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014. It will take about 17500 years to reach one light-year at its current speed of about 17 km/s (38000 mph, 61 200 km/h) relative to the Sun. On September 12, 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on August 25, 2012, becoming the first manmade object to leave the Solar System.
	2.28×10−3 ly	Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.
100	1.6×100 ly	The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 au, with its outer edge at 100000 au.
	2.0×100 ly	Maximum extent of the Sun's gravitational dominance (Hill sphere/Roche sphere, 125000 au). Beyond this is the deep ex-solar gravitational interstellar medium.
	4.24×100 ly	The nearest known star (other than the Sun), Proxima Centauri, is about 4.24 light-years away.
	8.6×100 ly	Sirius, the brightest star of the night sky. Twice as massive and 25 times more luminous than the Sun, it outshines more luminous stars due to its relative proximity.
	1.19×101 ly	HD 10700 e, an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of star Tau Ceti.
	2.05×101 ly	Gliese 581, a red-dwarf star with several detectable exoplanets.
	3.1×102 ly	Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9 bright giant 10700 times more luminous than the Sun.
103	3×103 ly	A0620-00, the second-nearest known black hole, is about 3000 light-years away.
	2.6×104 ly	The centre of the Milky Way is about 26000 light-years away.
	1×105 ly	The Milky Way is about 100000 light-years across.
	1.65×105 ly	R136a1, in the Large Magellanic Cloud, the most luminous star known at 8.7 million times the luminosity of the Sun, has an apparent magnitude 12.77, just brighter than 3C 273.
106	2.5×106 ly	The Andromeda Galaxy is approximately 2.5 million light-years away.
	3×106 ly	The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.
	5.9×107 ly	The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.
	1.5×108 – 2.5×108 ly	The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).
109	1.2×109 ly	The Sloan Great Wall (not to be confused with Great Wall and Her–CrB GW) has been measured to be approximately one billion light-years distant.
	2.4×109 ly	3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1. 3C 273 is about 2.4 billion light-years away.
	4.57×1010 ly	The comoving distance from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving radius of the observable universe. This is larger than the age of the universe dictated by the cosmic background radiation; see here for why this is possible.

Related units

Distances between objects within a star system tend to be small fractions of a light-year, and are usually expressed in astronomical units. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly 299792458 metres or 1⁄31557600 of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.