Light_year

Light-year

Light-year

Distance that light travels in one year

This article is about the unit of length. For other uses, see Light year (disambiguation).

A light-year, alternatively spelled light year (ly), is a unit of length used to express astronomical distances and is equal to exactly 9,460,730,472,580.8 km, which is approximately 5.88 trillion 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).[2] Because it includes the word "year", the term is sometimes misinterpreted as a unit of time.[3]

Quick Facts General information, Unit system ...

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.[3] The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years).[2]

Definitions

As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year[note 1] (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).[note 2] Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.[5] From this, the following conversions can be derived. The IAU-recognized abbreviation for light-year is "ly",[2] although other standards like ISO 80000:2006 (now superseded) have used "l.y."[6][7] and localized abbreviations are frequent, such as "al" in French, Spanish, and italian (from année-lumière, año luz and anno luce, respectively), "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 parsec

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.[8] 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[9][10][11] was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,[12] which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).[13]

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources[14][15] 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,[16] 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[2]
  • "kly" for a kilolight-year (1,000 light-years)[17]
  • "Mly" for a megalight-year (1,000,000 light-years)[18]
  • "Gly" for a gigalight-year (1,000,000,000 light-years)[19]

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.[20] 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.[21] 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.[22] In 1868 an English journal labelled the light-year as a unit used by the Germans.[23] Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.[24]

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.

More information Scale (ly), Value ...

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.[34][35] The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.[36]

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.[37]

See also

Notes

  1. [N 1]
    One Julian year is exactly 365.25 days (or 31557600 s based on a day of exactly 86400 SI seconds)[4]
  2. [N 2]
    The speed of light is precisely 299792458 m/s by definition of the metre.

References

  1. [1]
    "The Universe within 12.5 Light Years – The Nearest stars". www.atlasoftheuniverse.com. Retrieved 2 April 2022.
  2. [2]
    International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013
  3. [3]
    Bruce McClure (31 July 2018). "How far is a light-year?". EarthSky. Retrieved 15 October 2019.
  4. [4]
    IAU Recommendations concerning Units, archived from the original on 16 February 2007
  5. [5]
    "Selected Astronomical Constants Archived 2014-07-26 at the Wayback Machine" in Astronomical Almanac, p. 6.
  6. [6]
    ISO 80000-3:2006 Quantities and Units – Space and Time
  7. [7]
    IEEE/ASTM SI 10-2010, American National Standard for Metric Practice
  8. [8]
    P. Kenneth Seidelmann, ed. (1992), Explanatory Supplement to the Astronomical Almanac, Mill Valley, California: University Science Books, p. 656, ISBN 978-0-935702-68-2
  9. [9]
    Basic Constants, Sierra College
  10. [10]
    Marc Sauvage, Table of astronomical constants, archived from the original on 11 December 2008
  11. [11]
    Robert A. Braeunig, Basic Constants
  12. [12]
    C. W. Allen (1973), Astrophysical Quantities (third ed.), London: Athlone, p. 16, ISBN 978-0-485-11150-7
  13. [13]
    Arthur N. Cox, ed. (2000), Allen's Astrophysical Quantities (fourth ed.), New York: Springer-Valeg, p. 12, ISBN 978-0-387-98746-0
  14. [14]
    Nick Strobel, Astronomical Constants
  15. [15]
    KEKB, Astronomical Constants, archived from the original on 9 September 2007, retrieved 5 November 2008
  16. [16]
    Thomas Szirtes (1997), Applied dimensional analysis and modeling, New York: McGraw-Hill, p. 60, ISBN 978-0-07-062811-3
  17. [17]
    Comins, Neil F. (2013), Discovering the Essential Universe (fifth ed.), W. H. Freeman, p. 365, ISBN 978-1-4292-5519-6
  18. [18]
    Hassani, Sadri (2010), From Atoms to Galaxies, CRC Press, p. 445, ISBN 978-1-4398-0850-4
  19. [19]
    Deza, Michel Marie; Deza, Elena (2016), Encyclopedia of Distances (fourth ed.), Springer, p. 620, ISBN 978-3-662-52843-3
  20. [20]
    Bessel, Friedrich (1839). "On the parallax of the star 61 Cygni". London and Edinburgh Philosophical Magazine and Journal of Science. 14: 68–72. Bessel's statement that light employs 10.3 years to traverse the distance.
  21. [21]
    Ule, Otto (1851). "Was wir in den Sternen lesen". Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben. 1: 721–738.
  22. [22]
    Diesterweg, Adolph Wilhelm (1855). Populäre Himmelskunde u. astronomische Geographie. p. 250.
  23. [23]
    The Student and Intellectual Observer of Science, Literature and Art. Vol. 1. London: Groombridge and Sons. 1868. p. 240.
  24. [24]
    "Stellar movements and the structure of the universe". Retrieved 1 November 2014.
  25. [25]
    "Chapter 1, Table 1-1", IERS Conventions (2003)
  26. [26]
    WHERE ARE THE VOYAGERS?, retrieved 14 October 2014
  27. [27]
    NASA Spacecraft Embarks on Historic Journey Into Interstellar Space, retrieved 14 October 2014
  28. [28]
    NASA, Cosmic Distance Scales – The Nearest Star
  29. [29]
    "Proxima Centauri (Gliese 551)", Encyclopedia of Astrobiology, Astronomy, and Spaceflight
  30. [30]
    "Tau Ceti's planets nearest around single, Sun-like star". BBC News. 19 December 2012. Retrieved 1 November 2014.
  31. [31]
    Tuomi, Mikko; Jones, Hugh R. A.; Jenkins, James S.; Tinney, Chris G.; Butler, R. Paul; Vogt, Steve S.; Barnes, John R.; Wittenmyer, Robert A.; O'Toole, Simon; Horner, Jonathan; Bailey, Jeremy; Carter, Brad D.; Wright, Duncan J.; Salter, Graeme S.; Pinfield, David (March 2013). "Signals embedded in the radial velocity noise: periodic variations in the τ Ceti velocities" (PDF). Astronomy & Astrophysics. 551: A79. arXiv:1212.4277. Bibcode:2013A&A...551A..79T. doi:10.1051/0004-6361/201220509. S2CID 2390534.
  32. [32]
    Eisenhauer, F.; Schdel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T. (2003), "A Geometric Determination of the Distance to the Galactic Center", The Astrophysical Journal, 597 (2): L121, arXiv:astro-ph/0306220, Bibcode:2003ApJ...597L.121E, doi:10.1086/380188, S2CID 16425333
  33. [33]
    McNamara, D. H.; Madsen, J. B.; Barnes, J.; Ericksen, B. F. (2000), "The Distance to the Galactic Center", Publications of the Astronomical Society of the Pacific, 112 (768): 202, Bibcode:2000PASP..112..202M, doi:10.1086/316512
  34. [34]
    Fujisawa, K.; Inoue, M.; Kobayashi, H.; Murata, Y.; Wajima, K.; Kameno, S.; Edwards, P. G.; Hirabayashi, H.; Morimoto, M. (2000), "Large Angle Bending of the Light-Month Jet in Centaurus A", Publications of the Astronomical Society of Japan, 52 (6): 1021–26, Bibcode:2000PASJ...52.1021F, doi:10.1093/pasj/52.6.1021, archived from the original on 2 September 2009
  35. [35]
    Junor, W.; Biretta, J. A. (1994), "The Inner Light-Month of the M87 Jet", in Zensus, J. Anton; Kellermann; Kenneth I. (eds.), Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11–12, 1994, Green Bank, WV: National Radio Astronomy Observatory (NRAO), p. 97, Bibcode:1994cers.conf...97J
  36. [36]
    Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.
  37. [37]
    David Mermin (2009). It's About Time: Understanding Einstein's Relativity. Princeton, New Jersey: Princeton University Press. p. 22. ISBN 978-0-691-14127-5.
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