More Info on LightyearSimilar Undetermined MusicSearch Artistopia
Biography
Otheruses2|Light yearwiktionary|light yearA light-year , also light year or lightyear (symbol: ly) is a Units of measurement|unit of length , equal to just under 10 1000000000000 (number)|trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that Speed of light|light travels in a vacuum in one Julian year (astronomy)|Julian year .citation| url = http://www.iau.org/public_press/themes/measuring/ | title = The IAU and astronomical units | publisher = International Astronomical Union | accessdate=2008-07-05
The light-year is often used to measure distances to stars and other distances on a Galaxy|galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.
Numerical value
Unit of length|m=9460730472580800 |accuracy=5
1 light-year = gaps|9|460|730|472|580|800 metre s (exactly) :::& nbsp;& nbsp;˜ gaps|5|878|625 milllion mile s :::& nbsp;& nbsp;˜ gaps|63|241 astronomical unit s :::& nbsp;& nbsp;˜ gaps|0.306|601 parsec s
The figures above are based on a Julian year (astronomy)|Julian year (not Gregorian calendar|Gregorian year ) of exactly 365.25 days (each of exactly gaps|86|400 International System of Units|SI seconds, totalling gaps|31|557|600 seconds) http://www.iau.org/Units.234.0.html IAU Recommendations concerning Units and a defined speed of light of gaps|299|792|458& nbsp;m/s, both included in the Astronomical constant|IAU (1976) System of Astronomical Constants , used since 1984. http://asa.usno.navy.mil/SecK/2009/Astronomical_Constants_2009.pdf Astronomical Constants page K6 of the Astronomical Almanac .
Other values
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.P. Kenneth Seidelmann, ed., http://books.google.com/books? ct=result& id=uJ4JhGJANb4C& pg=PP1& lpg=PP1#PPA656,M1 Explanatory Supplement to the Astronomical Almanac (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7 The product of Simon Newcomb 's J1900.0 mean tropical year of gaps|31|556|925.9747 ephemeris second s and a speed of light of gaps|299|792.5& nbsp;km/s produced a light-year of val|9.460530|e=15|u=m (rounded to the seven significant digits in the speed of light) found in several modern sourcesSierra College, http://astronomy.sierracollege.edu/Resources/Reference/basic%20Contants%20app2.htm Basic ConstantsMarc Sauvage, http://marc.sauvage.free.fr/astro_book/Cts_pages/astro.html Table of astronomical constantsRobert A. Braeunig, http://www.braeunig.us/space/constant.htm Basic Constants was probably derived from an old source such as C. W. Allen 's 1973 Astrophysical Quantities reference work,C. W. Allen, Astrophysical Quantities (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0 which was updated in 2000.Arthur N. Cox, ed., '' http://books.google.com/books? id=w8PK2XFLLH8C& pg=PP1& lpg=PP1#PPA12,M1 Allen's Astrophysical Quantities (fourth edition, New York: Springer-Valeg, 2000) 12. ISBN 0-387-98746-0
Other high-precision values are not derived from a coherent IAU system. A value of val|9.460536207|e=15|u=m found in some modern sourcesNick Strobel, http://www.astronomynotes.com/tables/tablesa.htm Astronomical Constants KEK B http://www-acc.kek.jp/kekb/Introduction/misc/astronomical_constant.html Astronomical Constants is the product of a mean Gregorian year of 365.2425 days (gaps|31|556|952& nbsp;s) and the defined speed of light (gaps|299|792|458& nbsp;m/s). Another value, val|9.460528405|e=15|u=m,Thomas Szirtes, http://books.google.com/books? id=8Fk -TUdCEC& pg=PA60 Applied dimensional analysis and modeling (New York: McGraw-Hill, 1997) 60. http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-7/Sun-Moon-and-Earth.html Sun, Moon, and Earth: Light-year is the product of the J1900.0 mean tropical year and the defined speed of light.
History
The first successful measurement of the distance to a star other than our Sun was made by Friedrich Bessel in 1838. The star was 61 Cygni , and he used a superlative convert|6.2|in|adj=on heliometer designed by Joseph von Fraunhofer . The largest unit for measuring distances across space at that time was the astronomical unit (AU), equal to the radius of the Earth's orbit nowrap|(val|1.50|e=8|u=km;nowrap|val|9.30|e=7|u=mi). The use of this unit in trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, gave the distance to the star as convert|660|e3AU|km mi. Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible.
James Bradley had stated in 1729 that light travelled gaps|10|210 times faster than the Earth in its orbit. In 1769, a transit of Venus revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as val|3.01|e=8|u=m/s, very close to the modern value.
Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec , it is popularly used to gauge the expanses of interstellar and intergalactic space.
Distances in light-years
Distances measured in fractions of a light-year (or in light-months) usually involve objects within a star system . Distances measured in light-years include distances between nearby star s, such as those in the same spiral arm or globular cluster .
One kilolight-year, abbreviated "kly", is one thousand light-years (about 307 parsecs). Kilolight-years are typically used to measure distances between parts of a galaxy .
One megalight-year, abbreviated "Mly", is one million light-years (about 307 kiloparsecs). Megalight-years are typically used to measure distances between neighbouring galaxies and Galaxy groups and clusters|galaxy clusters .
One gigalight-year, abbreviation "Gly", is one billion light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasar s and the Great Wall (astronomy)|Sloan Great Wall .
Scale (ly)
Value
Item
10-9
E>-9 ly
gaps>376
10-6
E>-6 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.http:/ / www.iers.org/ MainDisp.csl? pid=46-25776 IERS Conventions (2003) , Chapter 1, Table 1-1.
E>-6 ly
Titan and transmits images from its surface gaps
10-3
E>-3 ly
As of>2011
100
E>0 ly
gaps>50
E>0 ly
gaps>125
E>0 ly
The nearest known star (other than the Sun), Proxima Centauri , is about 4.22 light-years away. NASA : http:/ / heasarc.gsfc.nasa.gov/ docs/ cosmic/ nearest_star_info.html Cosmic Distance Scales - The Nearest Starhttp:/ / www.daviddarling.info/ encyclopedia/ P/ ProximaCen.html Proxima Centauri (Gliese 551), Encyclopedia of Astrobiology, Astronomy, and Spaceflight
E>0 ly
luminous than the Sun, it outshines more luminous stars due to its relative proximity.
E>0 ly
Gliese 581g , the first discovered extrasolar candidate for habitable planet. Three or four times as massive as the Earth, it is in the middle of the habitable zone of star Gliese 581 .
E>0 ly
gaps>15
103
E>3 ly
centre of our galaxy , the Milky Way , is about 26 kilolight-years away.F. Eisenhauer, et al. , "http:/ / www.journals.uchicago.edu/ ApJ/ journal/ issues/ ApJL/ v597n2/ 17431/ 17431.web.pdf A Geometric Determination of the Distance to the Galactic Center" (pdf, 93KB), Astrophysical Journal 597 (2003) L121-L124McNamara, D. H., et al. , "http:/ / www.journals.uchicago.edu/ PASP/ journal/ issues/ v112n768/ 200037/ 200037.web.pdf The Distance to the Galactic Center" (pdf, 298KB), The Publications of the Astronomical Society of the Pacific , 112 (2000), pp. 202–216.
E>3 ly
Milky Way is about gaps>100
E>3 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
E>6 ly
The Andromeda Galaxy is approximately 2.5 megalight-years away.
E>6 ly
M33 ), at about 3 megalight-years away, is the most distant object visible to the naked eye.
E>6 ly
The nearest large galaxy cluster , the Virgo Cluster , is about 59 megalight-years away.
E>6 – 250E
The Great Attractor lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
109
E>9 ly
Great Wall ) has been measured to be approximately one gigalight-year distant.
E>9 ly
3C 273 , optically the brightest quasar , of apparent magnitude 12.9, just dimmer than R136a1 .
E>9 ly
size of the universe: misconceptions for why this is possible.
Related units
Other units of length can similarly be formed by multiplying units of time by the speed of light. For example, the light-second , useful in astronomy, telecommunications and relativistic physics, is exactly gaps|299|792|458 metres or frac|gaps|31|557|600 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.citation | author = Fujisawa, K.; Inoue, M.; Kobayashi, H.; Murata, Y.; Wajima, K.; Kameno, S.; Edwards, P. G.; Hirabayashi, H.; Morimoto, M. | year = 2000 | title = Large Angle Bending of the Light-Month Jet in Centaurus A | url = http://sciencelinks.jp/j-east/article/200123/000020012301A0179284.php | journal = Publ. Astron. Soc. Jpn. | volume = 52 | issue = 6 | pages = 1021–26|bibcode = 2000PASJ...52.1021F citation | author = Junor, W.; Biretta, J. A. | year = 1994 | contribution = The Inner Light-Month of the M87 Jet | bibcode = 1994cers.conf...97J | title = Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11–12, 1994 | editor = Zensus, J. Anton; Kellermann; Kenneth I. | location = Green Bank, WV | publisher = National Radio Astronomy Observatory (NRAO)| page = 97 The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. http://www.haydenplanetarium.org/universe/duguide/app_light_travel_time_dista.php Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.
