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Light-year

A '''light-year''' or '''light year''' (symbol: '''ly''') is a Units of measurement|unit of length, equal to just under ten orders_of_magnitude_%28numbers%29#1012|trillion kilometres. As defined by the International Astronomical Union (which is the body which has the jurisdictional authority to promulgate the definition), a light-year is the distance that light travels in a vacuum in one Julian year (astronomy)|Julian year. The light-year is often used to measure distances to stars. In astronomy, the preferred unit of measurement for such distances is the parsec, which is defined as the distance at which an object will appear to move one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer. This is equal to approximately 3.26 light-years. The parsec is preferred because it can be more easily derived from, and compared with, observational data. However, outside scientific circles, the term light-year is more widely used.

Numerical value

A light-year is equal to:
- exactly 9,460,730,472,580.8 kilometre|km (about 10 Petametre|Pm)
- about 5,878,630,000,000 mile|international miles
- about 63,241.1 astronomical units
- about 0.306601 parsecs The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)IAU Recommendations concerning Units and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.Astronomical Constants page K6 of the Astronomical Almanac. The DE405 value of the astronomical unit, 149,597,870,691 m,USNO Circular 179 page 32. is used for the light-year in astronomical units and parsecs.

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., ''Explanatory Supplement to the Astronomical Almanac'' (Miil Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7 The product of Simon Newcomb's J1900.0 mean tropical year of 31,556,925.9747 ephemeris seconds and a speed of light of 299,792.5 km/s produced a light-year of 9.460530 metres (rounded to the seven significant digits in the speed of light) found in several modern sourcesSierra College, Basic ConstantsMarc Sauvage, Table of astronomical constantsRobert A. Braeunig, Basic Constants was probably derived from an old source such as a reputable 1973 referenceC. W. Allen, ''Astrophysical Quantities'' (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0 which was not updated until 2000.Arthur N. Cox, ed., ''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 9.460536207 metres found in some modern sourcesNick Strobel, Astronomical ConstantsKEKB Astronomical Constants is the product of a mean Gregorian year of 365.2425 days (31,556,952 s) and the defined speed of light (299,792,458 m/s). The Live Search value, 9.460528405 metres,Microsoft's Live Search convert light year to meters is the product of the J1900.0 mean tropical year and the defined speed of light.

Distances in light-years

Distances measured in ''fractions of a light-year'' usually involve objects within a star system. Distances measured in ''light-years'' include distances between nearby stars, such as those in the same spiral arm or globular cluster. One ''kilolight-year'', abbreviated "kly", is one thousand light-years, or 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, or about 306,600 parsecs. Megalight-years are typically used to measure distances between neighboring galaxy|galaxies and galaxy clusters. One ''gigalight-year'', abbreviation "Gly", is one 1000000000 (number)|billion light-years — one of the largest distance measures used. One gigalight-year is about 306.6 million parsecs. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Great Wall (astronomy)|Great Wall.

References

Related Images

- Outer shell is one light year from Sun and left yellow line is Comet 1910 A1's orbit. Inner shell is one light month.

Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL

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