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A light-year, alternatively spelled light year, is a large
unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units ...
used to express astronomical distances and is equivalent to about 9.46  trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 (one million million, or billion in long scale). As defined by the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
(IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the time-measurement word "
year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hou ...
", the term ''light-year'' is sometimes misinterpreted as a
unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI) and by extension most of the Western world, is the second, defined as ab ...
. The ''light-year'' is most often used when expressing distances to stars and other distances on a
galactic Galactic is an American jam band from New Orleans, Louisiana. Origins and background Formed in 1994 as an octet (under the name Galactic Prophylactic) and including singer Chris Lane and guitarist Rob Gowen, the group was soon pared down to ...
scale, especially in non-specialist contexts and
popular science ''Popular Science'' (also known as ''PopSci'') is an American digital magazine carrying popular science content, which refers to articles for the general reader on science and technology subjects. ''Popular Science'' has won over 58 awards, inclu ...
publications. The unit most commonly used in professional
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galax ...
is the
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...
(symbol: pc, about 3.26 light-years) which derives from
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...
; it is the distance at which one
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
subtends an angle of one
second of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
.


Definitions

As defined by the
IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
, the light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day
Tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...
that both approximate) and the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit for ...
().The speed of light is precisely by definition of the metre. Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this, the following conversions can be derived. The IAU-recognized abbreviation for light-year is "ly", although other standards like
ISO 80000 ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ). It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrote ...
uses "l.y." and localized abbreviations are frequent, such as "al" in French (from ''année-lumière''), Spanish (from ''año luz''), Italian (from ''anno luce''), "Lj" in German (from ''Lichtjahr''), etc. : Before 1984, the
tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...
(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. The product of
Simon Newcomb Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian– American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in ...
's
J1900.0 In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to per ...
mean tropical year of ephemeris seconds and a speed of light of produced a light-year of (rounded to the seven
significant digits Significant figures (also known as the significant digits, ''precision'' or ''resolution'') of a number in positional notation are digits in the number that are reliable and necessary to indicate the quantity of something. If a number expres ...
in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 ''Astrophysical Quantities'' reference work, which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits). Other high-precision values are not derived from a
coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deriv ...
IAU system. A value of found in some modern sources is the product of a mean Gregorian year (365.2425 days or ) and the defined speed of light (). Another value, , 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 *"kly" for a kilolight-year (1,000 light-years) *"Mly" for a megalight-year (1,000,000 light-years) *"Gly" for a gigalight-year (1,000,000,000 light-years)


History

The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by
Friedrich Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method ...
in 1838. The star was 61 Cygni, and he used a heliometre designed by
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffract ...
. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at . In those terms, trigonometric calculations based on 61 Cygni's
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
of 0.314 arcseconds, showed the distance to the star to be . Bessel added that light takes 10.3 years to traverse this distance. 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; 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 the aether or space was still enigmatic. The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. 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. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations. Although modern astronomers often prefer to use the
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...
, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.


Usage of term

Distances expressed in light-years include those between
star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...
s in the same general area, such as those belonging to the same
spiral arm Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''globular cluster A globular cluster is a spheroidal conglomeration of stars. wikt:globular, Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many mil ...
. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and
galaxy clusters A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...
by millions of light-years. Distances to objects such as
quasar A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass rangi ...
s and the
Sloan Great Wall The Sloan Great Wall (SGW) is a cosmic structure formed by a giant wall of galaxies (a galaxy filament). Its discovery was announced from Princeton University on October 20, 2003, by J. Richard Gott III, Mario Jurić, and their colleagues, b ...
run up into the billions of light-years.


Calculation

The way to calculate the light-year distance: 299 792 458 \times 60 \times 60 \times 24 \times 365.25 where: * 299 792 458 is the number of meters light travels in one second * 60 is the number of seconds in a minute * The other 60 is the number of minutes in an hour * 24 is the number of hours in a day * 365.25 is the number of days in a Julian year


Related units

Distances between objects within a
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking ...
tend to be small fractions of a light-year, and are usually expressed in
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
s. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the
light-second The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly . Just as the second forms the basis for ...
, useful in astronomy, telecommunications and relativistic physics, is exactly metres or of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in
popular science ''Popular Science'' (also known as ''PopSci'') is an American digital magazine carrying popular science content, which refers to articles for the general reader on science and technology subjects. ''Popular Science'' has won over 58 awards, inclu ...
publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. Light travels approximately one
foot The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
in a
nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...
; the term "light-foot" is sometimes used as an informal measure of time.


See also

* 1 petametre (examples of distances on the order of one light-year) * Distance measures (cosmology) * Einstein protocol * Hubble length *
Orders of magnitude (length) The following are examples of orders of magnitude for different lengths. __TOC__ Overview Detailed list To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10^ ...


Notes


References


External links

* {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science Light Units of length Units of measurement in astronomy Concepts in astronomy 1838 in science