# Light-year

Light-year
Map showing the stars that lie within 12.5 light-years of the Sun[1]
General information
Unit systemastronomy units
Unit oflength
Symbolly[2]
Conversions
1 ly[2] in ...... is equal to ...
metric (SI) units   9.4607×1015 m
9.4607 Pm
imperial & US units   5.8786×1012 mi
astronomical units   63241 au
0.3066 pc

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).[note 1] 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).[2] 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).[2]

## Definitions

As defined by the IAU, the light-year is the product of the Julian year[note 2] (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s).[note 3] Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.[4] From this, the following conversions can be derived. The IAU recognized abbreviation for light-year is ly,[2] although other standards like ISO 80000 use "l.y."[5][6] and localized symbols are frequent, such as "al" in French (from année-lumière) and Spanish (from año luz), "Lj" in German (from Lichtjahr), etc.

 1 light-year = 9460730472580800 metres (exactly) ≈ 9.461 petametres ≈ 9.461 trillion kilometres ≈ 5.878625 trillion miles ≈ 63241.077 astronomical units ≈ 0.306601 parsecs

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.[7] 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[8][9][10] was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,[11] which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).[12]

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources[13][14] 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,[15][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]
Other Languages
Afrikaans: Ligjaar
Ænglisc: Lēohtgēar
العربية: سنة ضوئية
aragonés: Anyo luz
অসমীয়া: আলোক-বৰ্ষ
asturianu: Añu lluz
azərbaycanca: İşıq ili
تۆرکجه: ایشیق ایلی
বাংলা: আলোক বর্ষ
Bân-lâm-gú: Kong-liân
беларуская: Светлавы год
беларуская (тарашкевіца)‎: Сьветлавы год
भोजपुरी: प्रकाश-बरिस
brezhoneg: Bloavezh-gouloù
català: Any llum
čeština: Světelný rok
Chi-Chewa: Chaka Chowala
dansk: Lysår
Deutsch: Lichtjahr
Ελληνικά: Έτος φωτός
emiliàn e rumagnòl: An luś
español: Año luz
Esperanto: Lumjaro
estremeñu: Añu lus
euskara: Argi-urte
فارسی: سال نوری
Fiji Hindi: Prakash saal
français: Année-lumière
Frysk: Ljochtjier
Gaeilge: Solasbhliain
galego: Ano luz

ગુજરાતી: પ્રકાશવર્ષ
한국어: 광년
Ido: Lumyaro
Bahasa Indonesia: Tahun cahaya
interlingua: Anno luminal
íslenska: Ljósár
italiano: Anno luce
עברית: שנת אור
Basa Jawa: Taun cahya
қазақша: Жарық жылы
Kiswahili: Mwakanuru
Kreyòl ayisyen: Ane-limyè
Кыргызча: Жарык жылы
ລາວ: ປີແສງ
Lëtzebuergesch: Liichtjoer
lietuvių: Šviesmetis
Ligure: Annô lüxe
Limburgs: Leechjaor
Lingua Franca Nova: Anio de lus
lumbaart: An lüce
magyar: Fényév
македонски: Светлосна година
Malagasy: Taon-kazavana
മലയാളം: പ്രകാശവർഷം
Malti: Sena dawl
Bahasa Melayu: Tahun cahaya
монгол: Гэрлийн жил
မြန်မာဘာသာ: အလင်းနှစ်
Nederlands: Lichtjaar

norsk: Lysår
norsk nynorsk: Lysår
ଓଡ଼ିଆ: ଆଲୋକବର୍ଷ
Oromoo: Wagga Iffa
oʻzbekcha/ўзбекча: Yorugʻlik yili
پنجابی: چانن ورہ
ភាសាខ្មែរ: ឆ្នាំពន្លឺ
Piemontèis: Ann lus
Plattdüütsch: Lichtjohr
português: Ano-luz
română: An-lumină
Runa Simi: Achkiy wata
русиньскый: Світловый рік
русский: Световой год
саха тыла: Сырдык сыла
संस्कृतम्: प्रकाशवर्षः
Scots: Licht-year
Seeltersk: Luchtjier
shqip: Dritëviti
sicilianu: Annu luci
Simple English: Light year
سنڌي: نوري سال
slovenčina: Svetelný rok
slovenščina: Svetlobno leto
Soomaaliga: Sano Ilays
српски / srpski: Светлосна година
srpskohrvatski / српскохрватски: Svjetlosna godina
suomi: Valovuosi
svenska: Ljusår
Tagalog: Sinag-taon
татарча/tatarça: Яктылык елы
ไทย: ปีแสง
Türkçe: Işık yılı
Türkmençe: Ýagtylyk ýyly
удмурт: Югыт ар
українська: Світловий рік
اردو: نوری سال
vèneto: Ano luxe
Tiếng Việt: Năm ánh sáng
Volapük: Litayel

Winaray: Tuig-lamrag

ייִדיש: ליכט יאר

Zazaki: Serra roşti