A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree.

The parsec (symbol pc) is a unit of length, equal to just over 30 trillion kilometres. Astronomers use a number of different length units for different objects This list compares various sizes of positive Numbers including counts of things Dimensionless quantity and probabilities. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The parsec is used in astronomy, and its length is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Circle-trig6svg|300px|thumb|right|All of the Trigonometric functions of an angle θ can be constructed geometrically in terms of a unit circle centered at O. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth

The name parsec stands for "parallax of one second of arc", and one parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond when the viewing position changes by 1 AU. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The actual length of a parsec is approximately 30. 86 petametres, 3. 262 light-years or 1. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by 918×10¹³ miles.

History

The first direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used the width of the Earth's orbit as a baseline to calculate the distance of 61 Cygni using parallax and trigonometry. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions Not be confused with 16 Cygni, a more distant system containing two G-type stars harboring the Gas giant planet 16 Cygni Bb. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between Circle-trig6svg|300px|thumb|right|All of the Trigonometric functions of an angle θ can be constructed geometrically in terms of a unit circle centered at O. ^[1] The parallax of a star is half of the angular distance a star appears to move relative to the celestial sphere as Earth orbits around the Sun; or, reciprocally, it is the angle subtended, from that star's perspective, by the semi-major axis of the Earth's orbit. In Mathematics (in particular Geometry and Trigonometry) and all Natural sciences (including astronomy geophysics etc the angular distance In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star The Sun (Sol is the Star at the center of the Solar System. In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called In Geometry, an arc subtended by an Angle is a Curve whose endpoints are on the angle's two rays The use of the parsec as a unit of distance follows naturally from this method, since distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds). In Mathematics, a multiplicative inverse for a number x, denoted by 1&frasl x or x &minus1 is a number which That is, it is the distance at which the semi-major axis of the Earth's orbit would subtend an angle of one second of arc. (See diagram above. )

Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance: he proposes the name astron, but mentions that Carl Charlier had suggested siriometer, and Herbert Hall Turner had suggested parsec (par-allax sec-ond). Astronomer Royal is a senior post in the Royal Household of the Sovereign of the United Kingdom. Sir Frank Watson Dyson FRS ( January 8 1868 &ndash May 25 1939) was an English Astronomer who is remembered Carl Vilhelm Ludwig Charlier ( April 1 1862 &ndash November 5 1934) was a Swedish Astronomer. Herbert Hall Turner ( August 13 1861 &ndash August 20 1930) was a British Astronomer and Seismologist. ^[2]

Usage and Measurement

The parallax method is the fundamental calibration step for distance determination in astrophysics, and the obvious unit for such measurements, the parsec, has become the most commonly used unit of distance in scholarly astronomical publications. The cosmic distance ladder (also known as the Extragalactic Distance Scale) is the succession of methods by which astronomers determine the Distances to celestial Articles aimed at a wider audience, such as in newspapers and popular science magazines, often use a more intuitive unit, the light-year. For the 1935-1949 film series see Popular Science (film. Popular Science is an American monthly Magazine founded in 1872 A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by

Other than the Sun, which has a parallax of 90 degrees, there is no known star whose parallax is more than one arcsecond (that is, there is no known star whose distance from Earth is less than one parsec). The Sun (Sol is the Star at the center of the Solar System. The next closest star is Proxima Centauri with a parallax of 0. Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 77233 arcseconds; it is thus 1. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. 295 pc (4. 225 ly) away from the Earth.

Refraction caused by the atmosphere, also known as astronomical seeing, limits ground-based telescopes to parallax angle measurement accuracies of less than approximately 0. Refraction is the change in direction of a Wave due to a change in its Speed. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by Turbulence in the Earth's atmosphere A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. 01 arcsec, so reliable measurements, those with errors of 10% or less, can only be achieved at stellar distances of no more than about 100 pc, or 326 ly. 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 100,000 stars with an astrometric precision of about 0. Hipparcos (an Acronym for Hi gh P recision Par allax Co llecting S atellite) was an Astrometry mission The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member Astrometry is the branch of Astronomy that relates to precise measurements and explanations of the positions and movements of Stars and other celestial bodies 97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. ^[3] NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Full-sky Astrometric Mapping Explorer (or FAME) is an astrometric Satellite designed to determine with unprecedented accuracy the positions distances However, the mission's funding was withdrawn by NASA in January 2002. ^[4] ESA's GAIA satellite, due to be launched in December 2011, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Center, about 8,000 pc away in the constellation of Sagittarius. Gaia is a European Space Agency (ESA Astrometry space mission and a successor to the ESA Hipparcos mission The Galactic Center is the rotational center of the Milky Way Galaxy. In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture ^[5]

Distances in parsecs

Distances less than a parsec

Distances measured 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 4. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 85×10⁻⁶ pc.
• the most distant space probe, Voyager 1, was 5. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, 1×10⁻⁴ pc away from Earth in March 2008. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common It took Voyager 30 years to cover that distance.
• the Oort cloud is postulated to be approximately 0. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, 6 pc in diameter.

Parsecs and kiloparsecs

Distances measured in parsecs include distances between nearby stars, such as those in the same spiral arm or globular cluster. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. A distance of one thousand parsecs (approximately 3,262 ly) is commonly denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to measure distances between parts of a galaxy, or within groups of galaxies. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation So, for example:

• one parsec is approximately 3. 262 light-years.
• the nearest known star to the Earth, other than the Sun, is Proxima Centauri, 1. Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 29 parsecs away.
• the center of the Milky Way is about 8 kpc from the Earth, and the Milky Way is about 30 kpc across. The Galactic Center is the rotational center of the Milky Way Galaxy. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply
• the Andromeda Galaxy (M31), the most distant object visible to the naked eye, is a little under 800 kpc away from the Earth. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses

Megaparsecs and gigaparsecs

A distance of one million parsecs (approximately 3,262,000 ly or 2×10¹⁹ miles) is commonly denoted by the megaparsec (Mpc). The word million In standard English, the -lli- in million is pronounced with an l-sound followed by a Astronomers typically measure the distances between neighboring galaxies and galaxy clusters in megaparsecs. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation

Galactic distances are sometimes given in units of Mpc/h (as in "50/h Mpc"). h is a parameter in the range [0. 5,0. 75] reflecting the uncertainty in the value of the Hubble constant for the rate of expansion of the universe (H = 100h km/s/Mpc). Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance The Hubble constant becomes relevant when converting an observed redshift z into a distance using the formula d ≈ (c / H) × z (where c is the velocity of light). In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by ^[6]

One gigaparsec (Gpc) is one billion parsecs — one of the largest distance measures commonly used. One gigaparsec is about 3. 262 billion light-years, or roughly one fourteenth of the distance to the horizon of the observable universe (dictated by the cosmic background radiation). The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that Astronomers typically use gigaparsecs to measure large-scale structures such as the size of, and distance to, the Great Wall; the distances between clusters of galaxies; and the distance to quasars. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light The Great Wall (also called Coma Wall) sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus.

For example:

• the Andromeda Galaxy is 0. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda 77 Mpc away from the Earth.
• the nearest large galaxy cluster, the Virgo Cluster, is about 18 Mpc away from the Earth. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18
• the galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc away from the Earth. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply
• the particle horizon (the observable part of the universe) has a radius of about 14 Gpc (46. In Physical cosmology, particle horizon is the maximum distance from which particles could have traveled to the observer in the Age of the universe The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy 5 billion light-years). See observable universe. In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that

Volume units

In order to determine the number of stars in the Milky Way Galaxy volumes in cubic kiloparsecs^[7] (kpc³) are selected in various directions. All the stars in these volumes are counted and the total number of stars is statistically determined. The number of globular clusters, dust clouds and interstellar gas is determined in a similar fashion.

In order to determine the number of galaxies in superclusters volumes in cubic megaparsecs^[7] (Mpc³) are selected. Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the Cosmos. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge void in Bootes^[8] is measured in cubic megaparsecs.

In Cosmology volumes of cubic gigaparsecs^[7] (Gpc³) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars. Cosmology (from Greek grc κοσμολογία - grc κόσμος kosmos, "universe" and grc -λογία -logia) is study

The Sun is alone in its cubic parsec^[7] (pc³) but in globular clusters the stellar density per cubic parsec could be from 100 to 1,000.

Calculating the value of a parsec

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 D is a point in space at a distance of one parsec from the Sun. By definition, the angle D is one arcsecond, and the distance ES is one astronomical unit (AU). A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the By trigonometry, the distance SD is

One AU = 149,597,870,700 m, so 1 parsec ≈ 3. Circle-trig6svg|300px|thumb|right|All of the Trigonometric functions of an angle θ can be constructed geometrically in terms of a unit circle centered at O. 085 678×10¹⁶ metres ≈ 3. 261 564 light-years. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by

References

Notes

See also

• Conversion of units
• Small-angle approximation
• Attoparsec: a unit humorously used by some hackers. Conversion of units refers to conversion factors between different Units of measurement for the same Quantity. Small-angle approximation is a useful simplification of the laws of Trigonometry which is only approximately true for finite angles but correct in the limit as the This article describes unusual units of measurement that are sometimes used by Anglophone Scientists, especially Physicists and Mathematicians, and other See also Hacker In home computing a hacker is a person who heavily modifies the software or hardware of their computer system It is approximately 3. 1 centimetres — just a little over an inch.
• Light-year
• Cosmic Distance Ladder