Thelight-year is most often used when expressing distances to stars and other distances on agalactic scale, especially innon-specialist contexts andpopular science publications.[4] The unit most commonly used in professionalastronomy is theparsec (symbol: pc, about 3.26 light-years).[2]
The abbreviation used by the IAU for light-year is "ly",[2] International standards likeISO 80000:2006 (now superseded) have used "l.y."[7][8] and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (fromannée-lumière,año luz andanno luce, respectively), "Lj" in German (fromLichtjahr), etc.
Before 1984, thetropical 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.[9] The product ofSimon Newcomb'sJ1900.0 mean tropical year of31556925.9747ephemeris seconds and a speed of light of299792.5 km/s produced a light-year of9.460530×1015 m (rounded to the sevensignificant digits in the speed of light) found in several modern sources[10][11][12] was probably derived from an old source such asC. W. Allen's 1973Astrophysical Quantities reference work,[13] which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).[14]
Other high-precision values are not derived from acoherent IAU system. A value of9.460536207×1015 m found in some modern sources[15][16] is the product of a mean Gregorian year (365.2425 days or31556952 s) and the defined speed of light (299792458 m/s). Another value,9.460528405×1015 m,[17] 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:
The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, byFriedrich Bessel in 1838. The star was61 Cygni, and he used a 160-millimetre (6.2 in)heliometre designed byJoseph von Fraunhofer. The largest unit for expressing distances across space at that time was theastronomical 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'sparallax of 0.314 arcseconds, showed the distance to the star to be660000 astronomical units (9.9×1013 km; 6.1×1013 mi). Bessel added that light takes 10.3 years to traverse this distance.[23] 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 theaether or space was still enigmatic.
The light-year unit appeared in 1851 in a German popular astronomical article byOtto Ule.[24] 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.[25] In 1868 an English journal labelled the light-year as a unit used by the Germans.[26]Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.[27]
Although modern astronomers often prefer to use theparsec, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.
Distances expressed in light-years include those betweenstars in the same general area, such as those belonging to the samespiral arm orglobular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies andgalaxy clusters by millions of light-years. Distances to objects such asquasars and theSloan Great Wall run up into the billions of light-years.
Reflected sunlight from theMoon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly350000 to400000 kilometres).
10−6
1.58×10−5 ly
Oneastronomical unit (the distance from theSun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.[28]
1.27×10−4 ly
TheHuygens probe lands onTitan offSaturn and transmits images from its surface, 1.2 billion kilometres from Earth.
5.04×10−4 ly
New Horizons encountersPluto at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−3
2.04×10−3 ly
The most distantspace probe,Voyager 1, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014[update].[29] It will take about17500 years to reach one light-year at its current speed of about 17 km/s (38000 mph, 61 200 km/h) relative to the Sun. On 12 September 2013, NASA scientists announced thatVoyager 1 had entered theinterstellar medium of space on 25 August 2012, becoming the first manmade object to leave theSolar System.[30]
2.28×10−3 ly
Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.
100
1.6×100 ly
TheOort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at50000 au ≈ 0.8 ly, with its outer edge at100000 au ≈ 1.6 ly.
Sirius, the brightest star of the night sky. Twice as massive and 25 times moreluminous than the Sun, it outshines more luminous stars due to its relative proximity.
1.19×101 ly
Tau Ceti e, an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of starTau Ceti.[33][34]
2.05×101 ly
Gliese 581, a red-dwarf star with several detectable exoplanets.
3.1×102 ly
Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9bright giant10700 times more luminous than the Sun.
103
3×103 ly
A0620-00, the second-nearest knownblack hole, is about3000 light-years away.
Distances between objects within astar system tend to be small fractions of a light-year, and are usually expressed inastronomical units. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, thelight-second, useful in astronomy, telecommunications and relativistic physics, is exactly299792458 metres or1/31557600 of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used inpopular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[37][38] TheHayden Planetarium specifies the light month more precisely as 30 days of light travel time.[39]
Light travels approximately one foot in ananosecond; the term "light-foot" is sometimes used as an informal measure of time.[40]
^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.
^Ule, Otto (1851)."Was wir in den Sternen lesen".Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben.1:721–738.
^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
