The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond,[1] which corresponds to 648000/π astronomical units. One parsec is equal to about 3.26 light-years or 31 trillion kilometres (31×1012 km) or 19 trillion miles (19×1012 mi).[a] The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun.[2] Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun.

The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner.[3] Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and 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 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 astronomical units, or approximately 3.08567758149137×1016 metres (based on the IAU 2012 exact SI definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references.[4]

Stellarparallax parsec1
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 systemastronomical units
Unit oflength/distance
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

History and derivation

The parsec is defined as being equal to the length of the longer leg 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 arc second. 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.[5] 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.[6]

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.[3] It was Turner's proposal that stuck.

Calculating the value of a parsec

Diagram of parsec.
Diagram of parsec.

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:

Because the astronomical unit is defined to be 149597870700 m,[7] the following can be calculated:

Therefore, 1 parsec 206264.806247096 astronomical units
3.085677581×1016 metres
19.173511577 trillion miles
3.261563777 light-years

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:

where θ is the measured angle in arcseconds, Distanceearth-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 Distanceearth-sun (e.g. if Distanceearth-sun = 1 AU, unit for Distancestar is in astronomical units; if Distanceearth-sun = 1.5813×10−5 ly, unit for Distancestar is in light years).

The length of the parsec used in IAU 2015 Resolution B2[8] (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 SI length in metres, so now the parsec corresponds to an exact SI length in metres. To the nearest meter, the small-angle parsec corresponds to 30,856,775,814,913,673 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 arcseconds, and thus to stars no more than 100 pc distant.[9] This is because the Earth's atmosphere limits the sharpness of a star's image.[10] 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 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1000 pc away.[11]

ESA's Gaia satellite, which launched on 19 December 2013, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Centre, about 8000 pc away in the constellation of Sagittarius.[12]

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 parsecs.
  • The most distant space probe, Voyager 1, was 0.000703 parsecs 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 parsecs in diameter
M87 jet
The jet erupting from the active galactic nucleus of M87 is thought to be 1.5 kiloparsecs (4890 ly) long. (image from Hubble Space Telescope)

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 1000 parsecs (3262 light-years) is commonly 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:

  • One parsec is approximately 3.26 light-years.
  • Proxima Centauri, the nearest known star to earth other than the sun, is about 1.30 parsecs (4.24 light-years) 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 (26000 ly) from the Earth, and the Milky Way is roughly 34 kpc (110000 ly) across.
  • The Andromeda Galaxy (M31) is about 780 kpc (2.5 million light-years) away from the Earth.

Megaparsecs and gigaparsecs

A distance of one million parsecs is commonly denoted by the megaparsec (Mpc). Astronomers typically express the distances between neighbouring galaxies and galaxy clusters in megaparsecs.

Galactic distances are sometimes given in units of Mpc/h (as in "50/h Mpc", also written "50 Mpc h−1"). h is a parameter 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 dc/H × z.[13]

One gigaparsec (Gpc) is one billion parsecs — one of the largest units of length commonly used. One gigaparsec is about 3.26 billion light-years, 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:

Volume units

To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs[b] (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[b] (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.[16]

In physical cosmology, volumes of cubic gigaparsecs[b] (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 the only star in its cubic parsec,[b] (pc3) but in globular clusters the stellar density could be from 100 to 1000 per cubic parsec.

In popular culture

The parsec is apparently used incorrectly as a measurement of time not distance in Han Solo's claim in A New Hope, the first Star Wars film, that he "made the Kessel Run in less than 12 parsecs". This was retconned in Solo: A Star Wars Story.[17]

See also


  1. ^ One trillion here is taken to be 1012 (one million million, or billion in long scale).
  2. ^ a b c d
    1 pc3 2.938×1049 m3
    1 kpc32.938×1058 m3
    1 Mpc32.938×1067 m3
    1 Gpc32.938×1076 m3
    1 Tpc³2.938×1085 m3


  1. ^ "Cosmic Distance Scales - The Milky Way". Retrieved 24 September 2014.
  2. ^ Benedict, G. F.; et al. "Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri" (PDF). Proceedings of the HST Calibration Workshop. pp. 380–384. Retrieved 11 July 2007.
  3. ^ a b Dyson, F. W. (March 1913). "Stars, Distribution and drift of, The distribution in space of the stars in Carrington's Circumpolar Catalogue". Monthly Notices of the Royal Astronomical Society. 73 (5): 334–342. Bibcode:1913MNRAS..73..334D. doi:10.1093/mnras/73.5.334. There is a need for a name for this unit of distance. Mr. Charlier has suggested Siriometer ... Professor Turner suggests parsec, which may be taken as an abbreviated form of 'a distance corresponding to a parallax of one second'.
  4. ^
    • Cox, Arthur N., ed. (2000). Allen's Astrophysical Quantities (4th ed.). New York: AIP Press / Springer. ISBN 978-0387987460.
    • Binney, James; Tremaine, Scott (2008). Galactic Dynamics (2nd ed.). Princeton, NJ: Princeton University Press. ISBN 978-0-691-13026-2.
  5. ^ High Energy Astrophysics Science Archive Research Center (HEASARC). "Deriving the Parallax Formula". NASA's Imagine the Universe!. Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center. Retrieved 26 November 2011.
  6. ^ Bessel, F. W. (1838). "Bestimmung der Entfernung des 61sten Sterns des Schwans" [Determination of the distance of the 61st star of Cygnus]. Astronomische Nachrichten. 16 (5): 65–96. Bibcode:1838AN.....16...65B. doi:10.1002/asna.18390160502. Archived from the original on 2007-06-24.
  7. ^ International Astronomical Union, ed. (31 August 2012), "RESOLUTION B2 on the re-definition of the astronomical unit of length" (PDF), RESOLUTION B2, Beijing: International Astronomical Union, The XXVIII General Assembly of International Astronomical Union recommends [adopted] that the astronomical unit be redefined to be a conventional unit of length equal to exactly 149597870700 m, in agreement with the value adopted in IAU 2009 Resolution B2
  8. ^ "Four Resolutions to be Presented for Voting at the IAU XXIX GA".
  9. ^ Pogge, Richard. "Astronomy 162". Ohio State University.
  10. ^ "Parallax Measurements".
  11. ^
  12. ^ "GAIA". European Space Agency.
  13. ^ "Galaxy structures: the large scale structure of the nearby universe". Archived from the original on 5 March 2007. Retrieved 22 May 2007.
  14. ^ Mei, S.; Blakeslee, J. P.; Côté, P.; et al. (2007). "The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-dimensional Structure of the Virgo Cluster". The Astrophysical Journal. 655 (1): 144–162. arXiv:astro-ph/0702510. Bibcode:2007ApJ...655..144M. doi:10.1086/509598.
  15. ^ Lineweaver, Charles H.; Davis, Tamara M. (2005-03-01). "Misconceptions about the Big Bang". Scientific American. 292 (3): 36–45. Bibcode:2005SciAm.292c..36L. doi:10.1038/scientificamerican0305-36. Archived from the original on 2011-08-10. Retrieved 2016-02-04.
  16. ^ Kirshner, R. P.; Oemler, A., Jr.; Schechter, P. L.; Shectman, S. A. (1981). "A million cubic megaparsec void in Bootes". The Astrophysical Journal. 248: L57. Bibcode:1981ApJ...248L..57K. doi:10.1086/183623. ISSN 0004-637X.
  17. ^ "'Solo' Corrected One of the Most Infamous 'Star Wars' Plot Holes". 2018-05-30.

External links

Astronomical unit

The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres, or about 150 million kilometres (93 million miles). The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

Binary black hole

A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.

For many years, proving the existence of BBHs was made difficult because of the nature of black holes themselves, and the limited means of detection available. However, in the event that a pair of black holes were to merge, an immense amount of energy should be given off as gravitational waves, with distinctive waveforms that can be calculated using general relativity. Therefore, during the late 20th and early 21st century, BBHs became of great interest scientifically as a potential source of such waves, and a means by which gravitational waves could be proven to exist. BBH mergers would be one of the strongest known sources of gravitational waves in the Universe, and thus offer a good chance of directly detecting such waves. As the orbiting black holes give off these waves, the orbit decays, and the orbital period decreases. This stage is called binary black hole inspiral. The black holes will merge once they are close enough. Once merged, the single hole settles down to a stable form, via a stage called ringdown, where any distortion in the shape is dissipated as more gravitational waves. In the final fraction of a second the black holes can reach extremely high velocity, and the gravitational wave amplitude reaches its peak.

The existence of stellar-mass binary black holes (and gravitational waves themselves) were finally confirmed when LIGO detected GW150914 (detected September 2015, announced February 2016), a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 solar masses each, occurring about 1.3 billion light years away. In its final moments of spiraling inward and merging, GW150914 released around 3 solar masses as gravitational energy, peaking at a rate of 3.6×1049 watts — more than the combined power of all light radiated by all the stars in the observable universe put together - during its last brief moments. Supermassive binary black hole candidates have been found but as yet, not categorically proven.

Brian Dunning (author)

Brian Andrew Dunning (born 1965) is an American writer and producer who focuses on science and skepticism. He has hosted a weekly podcast, Skeptoid, since 2006, and he is an author of a series of books on the subject of scientific skepticism, some of which are based on the podcast. Skeptoid has been the recipient of several podcast awards such as the Parsec Award. Dunning has also created the spin-off video series, inFact, and The Feeding Tube both available on YouTube.

Dunning has produced two educational films on the subject of critical thinking, Here be Dragons in 2008, and Principles of Curiosity in 2017.Dunning co-founded Buylink, a business-to-business service provider, in 1996, and served at the company until 2002. He later became eBay's second biggest affiliate marketer; he has since been convicted of wire fraud through a cookie stuffing scheme. In August 2014, he was sentenced to 15 months in prison, to be followed by three years of supervised release for the company obtaining between $200,000 and $400,000 through wire fraud.

Buzz Lightyear of Star Command

Buzz Lightyear of Star Command is an American animated science fiction/adventure/comedy series produced by Walt Disney Television Animation. The series originally aired on UPN and ABC from October 2000 to January 2001 as part of Disney's One Saturday Morning and Disney's One Too programming blocks. It follows the adventures of space ranger Buzz Lightyear, who first appeared in the film Toy Story as an action figure and one of the film's protagonists. It is set shortly after the events of the film and set within the timelines between Toy Story 2 and Toy Story 3.

Galactic Center

The Galactic Center, or Galactic Centre, is the rotational center of the Milky Way. It is 8,122 ± 31 parsecs (26,490 ± 100 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. It coincides with the compact radio source Sagittarius A*.

There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf-Rayet stars from a star formation event around one million years ago, and one supermassive black hole of 4.100 ± 0.034 million solar masses at the Galactic Center, which powers the Sagittarius A* radio source.


Global Mobile Information System Simulator (GloMoSim) is a network protocol simulation software that simulates wireless and wired network systems. GloMoSim is designed using the parallel discrete event simulation capability provided by Parsec, a parallel programming language. GloMoSim currently supports protocols for a purely wireless network.

It uses the Parsec compiler to compile the simulation protocols.

Horror podcast

Horror is a genre of podcasts covering fiction, non-fiction, and reviews of the horror genre generally.

Horror podcasts are typically created and run by volunteers in their free time. As some podcasts such as Archive 81 and The Deep Vault have grown they have been able to attract advertisers.Notable podcasts include Alice Isn't Dead, Archive 81, The Black Tapes, The Deep Vault, Knifepoint Horror, Last Podcast on the Left, Lore, The NoSleep Podcast, Pseudopod, Rabbits, Small Town Horror, Tales to Terrify, Tanis, Uncanny County, Welcome to Night Vale, We're Alive, and The White Vault. The world's longest running, active horror podcast is which has been going since August, 2006 as a semi-regular weekly series with celebrity interviews. Horror podcasts have featured in the Parsec Awards, and in 2013 The NoSleep Podcast won the award for "Best New Speculative Fiction Podcaster/Team" while in 2014 a story from Pseudopod won the award for "Best Speculative Fiction Story: Small Cast (Short Form)" and We're Alive won "Best Speculative Fiction Audio Drama (Long Form)".


The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year 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 nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).


MonsterTalk is an audio podcast presented by the Skeptics Society's Skeptic magazine. The show critically examines the science behind cryptozoological creatures, such as Bigfoot, the Loch Ness Monster, and werewolves. It is hosted by Blake Smith and Karen Stollznow, and produced by Blake Smith. In 2012, MonsterTalk was awarded the Parsec Award for the "Best Fact Behind the Fiction Podcast".

NGC 1647

NGC 1647 is an open cluster in the constellation Taurus. It contains nearly 90 stars and it lies at a distance of 550 parsec. It is visible even with binoculars close to Aldebaran. It was discovered by William Herschel in 1784. It is located behind Taurus dark nebula complex, approximately 160 parsec away. The brightest main sequence stars are of spectral type B7. Its age is estimated to be 150 million years.

Pamela L. Gay

Pamela L. Gay (born December 12, 1973) is an American astronomer, educator, podcaster, and writer, best known for her work in astronomical podcasting and citizen science astronomy projects. She is a Senior Education and Communication Specialist and Senior Scientist for the Planetary Science Institute. Her research interests include analysis of astronomy data, as well as examination of the impact of citizen science initiatives. Gay has also appeared as herself in various television documentary series.

Gay takes part in science popularization efforts and educational outreach as director of CosmoQuest, a citizen science project aimed at engaging the public in astronomy research, speaking on science and scientific skepticism topics internationally, and through educational podcasting.

Parsec (video game)

Parsec is a computer game for the Texas Instruments TI-99/4A. Perhaps the best-remembered of all TI-99/4A games, it is a side-scrolling shooter, programmed in 1982 by Jim Dramis (who also programmed the popular TI-99/4A games Car Wars and Munch Man) and Paul Urbanus.

Parsec Awards

The Parsec Awards are a set of annual awards created to recognize excellence in science fiction podcasts and podcast novels. The awards were created by Mur Lafferty, Tracy Hickman and Michael R. Mennenga and awarded by FarPoint Media. They were first presented in 2006 at DragonCon and have since become "one of the most recognizable honors in science and fiction podcasting".Nominations are accepted from the listening public annually in each of the categories. The list is vetted for eligibility by the steering committee, before producers are invited to submit samples of work for consideration by a panel of judges. The panel reduces the list of nominees to five finalists in each category. The finalists' work is submitted for judging and the winner is selected by that panel of authors, podcasters, and others knowledgeable in the field of speculative fiction, podcasting, and/or publishing. Past finalist judges have included Catherine Asaro, Charles de Lint, Cory Doctorow, and Evo Terra.

Princeton Application Repository for Shared-Memory Computers

Princeton Application Repository for Shared-Memory Computers (PARSEC) is a benchmark suite composed of multithreaded emerging workloads that is used to evaluate and develop next-generation chip-multiprocessors. It was collaboratively created by Intel and Princeton University to drive research efforts on future computer systems. Since its inception the benchmark suite has become a community project that is continued to be improved by a broad range of research institutions. PARSEC is freely available and is used for both academic and non-academic research.

Stellar parallax

Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at exactly opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).

Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. It was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.

Once a star's parallax is known, its distance from Earth can be computed trigonometrically. But the more distant an object is, the smaller its parallax. Even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs (or roughly 300 light years) too approximate to be useful when obtained by this technique. This limits the applicability of parallax as a measurement of distance to objects that are relatively close on a galactic scale. Other techniques, such as spectral red-shift, are required to measure the distance of more remote objects.

Stellar parallax measures are given in the tiny units of arcseconds, or even in thousandths of arcseconds (milliarcseconds). The distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is 1 AU long. Because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes (in arcseconds) to distance (in parsecs). The approximate distance is simply the reciprocal of the parallax: For example, Proxima Centauri (the nearest star to Earth other than the Sun), whose parallax is 0.7687, is 1 / 0.7687 parsecs = 1.3009 parsecs (4.243 ly) distant.

Thousand Parsec

Thousand Parsec (TP) is a free and open source project with the goal of creating a framework for turn-based space empire building games.

Thousand Parsec is a framework for creating a specific group of games, which are often called 4X games, from the main phases of gameplay that arise: eXplore, eXpand, eXploit and eXterminate. Some examples of games from which Thousand Parsec draws ideas are Reach for the Stars, Stars!, VGA Planets, Master of Orion and Galactic Civilizations.

Unlike commercial alternatives, it is designed for long games supporting universes as large as the player's computer can handle. It allows a high degree of player customization, and features a flexible technology system, where new technologies may be introduced mid-game.

Uncanny Magazine

Uncanny Magazine is an American science fiction and fantasy online magazine, edited and published by Lynne M. Thomas and Michael Damian Thomas. Issues appear bimonthly, starting November 2014 after receiving funding through Kickstarter. Uncanny Magazine has maintained a regular bimonthly schedule since, publishing original works by authors such as Neil Gaiman, Elizabeth Bear, Paul Cornell, Catherynne M. Valente, Charlie Jane Anders, Seanan McGuire, Javier Grillo-Marxuach, Alex Bledsoe, Kameron Hurley and Ken Liu.In 2017, Uncanny won the 2016 Hugo Award for Best Semiprozine, and one of its published stories, "Folding Beijing" by Hao Jingfang translated by Ken Liu, won the Hugo Award for Best Novelette.

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