AskDefine | Define lightyear

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Extensive Definition

A light-year or light year (symbol: ly) is a unit of length, equal to just under ten 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.

Numerical value

A light-year is equal to:
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) as defined by the IAU.
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.

Definitions on the web

Internet search engines base their light-years on the 1900.0 mean tropical year, specified in the definition of the ephemeris second as 31,556,925.9747 s (12 significant digits), but round the number of metres to fewer significant digits. Microsoft's Live Search rounds to ten digits, Google and AOL round to eight digits, rounds to seven digits, while Yahoo rounds to only five digits. All of these are smaller than the IAU definition.

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 galaxies and galaxy clusters.
One gigalight-year, abbreviation "Gly", is one 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.


See also

lightyear in Old English (ca. 450-1100): Lēohtgēar
lightyear in Arabic: سنة ضوئية
lightyear in Aragonese: Año luz
lightyear in Asturian: Añu lluz
lightyear in Bengali: আলোক বর্ষ
lightyear in Min Nan: Kng-nî
lightyear in Belarusian (Tarashkevitsa): Сьветлавы год
lightyear in Bosnian: Svjetlosna godina
lightyear in Breton: Bloavezh-gouloù
lightyear in Bulgarian: Светлинна година
lightyear in Catalan: Any-llum
lightyear in Czech: Světelný rok
lightyear in Welsh: Blwyddyn golau
lightyear in Danish: Lysår
lightyear in German: Lichtjahr
lightyear in Estonian: Valgusaasta
lightyear in Modern Greek (1453-): Έτος φωτός
lightyear in Spanish: Año luz
lightyear in Esperanto: Lumjaro
lightyear in Basque: Argi-urte
lightyear in Persian: سال نوری
lightyear in French: Année-lumière
lightyear in Irish: Solasbhliain
lightyear in Gan Chinese: 光年
lightyear in Galician: Ano luz
lightyear in Gujarati: પ્રકાશવર્ષ
lightyear in Korean: 광년
lightyear in Croatian: Svjetlosna godina
lightyear in Ido: Lumyaro
lightyear in Indonesian: Tahun cahaya
lightyear in Icelandic: Ljósár
lightyear in Italian: Anno luce
lightyear in Hebrew: שנת אור
lightyear in Kannada: ಜ್ಯೋತಿರ್ವರ್ಷ
lightyear in Georgian: სინათლის წელიწადი
lightyear in Swahili (macrolanguage): Mwaka wa nuru
lightyear in Kurdish: Saletîrêj
lightyear in Lao: ປີແສງ
lightyear in Latin: Spatium lucis annuae
lightyear in Latvian: Gaismas gads
lightyear in Luxembourgish: Liichtjoer
lightyear in Lithuanian: Šviesmetis
lightyear in Limburgan: Leechjaor
lightyear in Hungarian: Fényév
lightyear in Macedonian: Светлосна година
lightyear in Malayalam: പ്രകാശവര്‍ഷം
lightyear in Maltese: Sena dawl
lightyear in Malay (macrolanguage): Tahun cahaya
lightyear in Mongolian: Гэрлийн жил
lightyear in Dutch: Lichtjaar
lightyear in Japanese: 光年
lightyear in Norwegian: Lysår
lightyear in Norwegian Nynorsk: Lysår
lightyear in Oromo: Wagga Iffa
lightyear in Uzbek: Yorugʻlik yili
lightyear in Low German: Lichtjohr
lightyear in Polish: Rok świetlny
lightyear in Portuguese: Ano-luz
lightyear in Romanian: An-lumină
lightyear in Quechua: Achkiy wata
lightyear in Russian: Световой год
lightyear in Albanian: Dritëviti
lightyear in Sicilian: Annu luci
lightyear in Simple English: Light year
lightyear in Slovak: Svetelný rok
lightyear in Slovenian: Svetlobno leto
lightyear in Serbian: Светлосна година
lightyear in Serbo-Croatian: Svjetlosna godina
lightyear in Finnish: Valovuosi
lightyear in Swedish: Ljusår
lightyear in Tamil: ஒளியாண்டு
lightyear in Telugu: కాంతి సంవత్సరం
lightyear in Thai: ปีแสง
lightyear in Vietnamese: Năm ánh sáng
lightyear in Turkish: Işık yılı
lightyear in Ukrainian: Світловий рік
lightyear in Urdu: نوری سال
lightyear in Volapük: Litayel
lightyear in Yiddish: ליכט יאר
lightyear in Contenese: 光年
lightyear in Chinese: 光年
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