Light-year
Encyclopedia
A light-year, also light year or lightyear (symbol: ly) is a unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

of length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

, equal to just under 10 trillion
Trillion
-Numbers:Either of the two numbers :* 1,000,000,000,000 for all short scale countries...

kilometre
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...

s (10 metres, 10 petametres or about 6 trillion miles). As defined by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

(IAU), a light-year is the distance that light travels
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

in one Julian year
Julian year (astronomy)
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86 400 SI seconds each, totaling 31 557 600 seconds. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is...

.

The light-year is often used to measure distances to stars and other distances on a galactic
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

scale, especially in non-specialist and popular science
Popular science
Popular science, sometimes called literature of science, is interpretation of science intended for a general audience. While science journalism focuses on recent scientific developments, popular science is broad-ranging, often written by scientists as well as journalists, and is presented in many...

publications. The preferred unit in astrometry
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...

is the parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of parallax
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. The term is derived from the Greek παράλλαξις , meaning "alteration"...

when the observer moves one astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.

## Numerical value

One light-year is equal to:
• exactly 9,460,730,472,580.8 km
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...

• about 5,878,625,373,183.608 mile
Mile
A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...

s (about 6 trillion miles)
• about 63,241.1 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s
• about 0.306601 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s
• exactly 31,557,600 light-second
Light-second
A 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 299,792,458 metres...

s

The figures above are based on a Julian year
Julian year (astronomy)
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86 400 SI seconds each, totaling 31 557 600 seconds. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is...

(not Gregorian year
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

) of exactly 365.25 days (each of exactly 86,400 SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

seconds, totalling 31,557,600 seconds) and a defined speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants
Astronomical constant
An astronomical constant is a physical constant used in astronomy. A formal set of constants, along with recommended values, was defined by the International Astronomical Union in 1976, and a new set of recommended values was produced in 1994...

, used since 1984.

### Other values

Before 1984, the tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

(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
Simon Newcomb was a Canadian-American astronomer and mathematician. Though he had little conventional schooling, he made important contributions to timekeeping as well as writing on economics and statistics and authoring a science fiction novel.-Early life:Simon Newcomb was born in the town of...

'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 (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as a reputable 1973 reference which was not updated until 2000.

Other high-precision values are not derived from a coherent IAU system. A value of found in some modern sources 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). Another value, , is the product of the J1900.0 mean tropical year and the defined speed of light.

## History

The first successful measurement of the distance to a star other than our Sun was made by Friedrich Bessel
Friedrich Bessel
-References:* John Frederick William Herschel, A brief notice of the life, researches, and discoveries of Friedrich Wilhelm Bessel, London: Barclay, 1847 -External links:...

in 1838. The star was 61 Cygni
61 Cygni
61 Cygni,Not to be confused with 16 Cygni, a more distant system containing two G-type stars harboring the gas giant planet 16 Cygni Bb. sometimes called Bessel's Star or Piazzi's Flying Star, is a binary star system in the constellation Cygnus...

, and he used a superlative 6.2 inches (15.7 cm) heliometer
Heliometer
Heliometer is an instrument originally designed for measuring the variation of the sun's diameter at different seasons of the year, but applied now to the modern form of the instrument which is capable of much wider use....

designed by Joseph von Fraunhofer
Joseph von Fraunhofer
Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...

. The largest unit for measuring distances across space at that time was the Astronomical Unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

(AU), equal to the radius of the Earth's orbit The use of this unit in trigonometric calculations based on 61 Cygni's parallax
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. The term is derived from the Greek παράλλαξις , meaning "alteration"...

of 0.314 arcseconds, gave the distance to the star as 660000 AU. Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible.

James Bradley FRS was an English astronomer and served as Astronomer Royal from 1742, succeeding Edmund Halley. He is best known for two fundamental discoveries in astronomy, the aberration of light , and the nutation of the Earth's axis...

had stated in 1729 that light travelled 10,210 times faster than the Earth in its orbit. In 1769, a transit of Venus
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...

revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as , very close to the modern value.

Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

, it is popularly used to gauge the expanses of interstellar and intergalactic space.

## Distances in light-years

Distances measured in fractions of a light-year (or in light-months) usually involve objects within a star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...

. Distances measured in light-years include distances between nearby star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, such as those in the same spiral arm or globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

.

One kilolight-year, abbreviated "kly", is one thousand light-years (about 307 parsecs). Kilolight-years are typically used to measure distances between parts of a galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

.

One megalight-year, abbreviated "Mly", is one million light-years (about 307 kiloparsecs). Megalight-years are typically used to measure distances between neighbouring galaxies and galaxy clusters
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...

.

One gigalight-year, abbreviation "Gly", is one billion (109) light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s and the Great Wall
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...

.
List of orders of magnitude for length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

Scale (ly) Value Item
10−9 40.4 ly Reflected sunlight from the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

's surface takes 1.2–1.3 seconds to travel the distance to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface. (The surface of the Moon is roughly 376,300 kilometres from the surface of the Earth, on average. 376,300 km ÷ 300,000 km/s (roughly the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

) ≈ 1.25 seconds)
10−6 15.8 ly One astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

(the distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.
127 ly The Huygens probe
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....

lands on Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

and transmits images from its surface 1200 million kilometres to the Earth.
10−3 3.2 ly The most distant space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

, Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

, was about 16 light-hours away from the Earth . It will take about 17,500 years to reach one light-year at its current speed of about 17 km/s (38,000 mph) relative to the Sun.
100 1.6 ly The Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

is approximately two light-years in diameter. Its inner boundary is speculated to be at 50,000 AU, with its outer edge at 100,000 AU.
2.0 ly Maximum extent of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's gravitational dominance (hill sphere
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...

/roche sphere, 125,000 AU). Beyond this is the true interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

.
4.22 ly The nearest known star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

(other than the Sun), Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...

, is about 4.22 light-years away.
8.60 ly Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

, the brightest star of the night sky. Twice as massive and 25 times more luminous than the Sun, it outshines more luminous stars due to its relative proximity.
20.5 ly Gliese 581g, the first discovered extrasolar candidate for habitable planet. Three or four times as massive as the Earth, it is in the middle of the habitable zone of star Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

.
310 ly Canopus
Canopus
Canopus |Alpha]] Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.Canopus is a supergiant of spectral...

, second in brightness in the terrestrial sky only to Sirius, a type F supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

15,000 times more luminous than the Sun.
103 26 ly The centre
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

of our galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, is about 26 kilolight-years away.
100 ly The Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

is about 100,000 light-years across.
165 ly R136a1
R136a1
R136a1 is a blue hypergiant star and the most massive star known. It is an estimated 265 solar masses. The star is also the most luminous at 8,700,000 times the luminosity of the Sun....

, in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

, the most luminous star known at 8,700,000 times the luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

of the Sun, has an apparent magnitude 12.77, just brighter than 3C 273
106 2.5 ly The Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

is approximately 2.5 megalight-years away.
3 ly The Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...

(M33
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...

), at about 3 megalight-years away, is the most distant object visible to the naked eye.
59 ly The nearest large galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

, the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

, is about 59 megalight-years away.
150 – 250 ly The Great Attractor
Great Attractor
The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated...

lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
109 1.2 ly The Sloan Great Wall
Sloan Great Wall
The Sloan Great Wall is a cosmic structure formed by a giant wall of galaxies , and to the present day it is the largest known structure in the universe. Its discovery was announced on October 20, 2003 by J. Richard Gott III of Princeton University and Mario Jurić and their colleagues, based on...

(not to be confused with the Great Wall
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...

) has been measured to be approximately one gigalight-year distant.
2.4 ly 3C 273, optically the brightest quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

, of apparent magnitude 12.9, just dimmer than R136a1
R136a1
R136a1 is a blue hypergiant star and the most massive star known. It is an estimated 265 solar masses. The star is also the most luminous at 8,700,000 times the luminosity of the Sun....

.
46.5 ly The comoving distance
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...

from the Earth to the edge of the visible universe is about 46.5 gigalight-years in any direction; this is the comoving radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

of the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

. This is larger than the age of the universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

dictated by the cosmic background radiation; see size of the universe: misconceptions for why this is possible.

## Light-month

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium
Hayden Planetarium
The Hayden Planetarium is a public planetarium, part of the Rose Center for Earth and Space of the American Museum of Natural History in New York City, currently directed by astrophysicist Dr. Neil deGrasse Tyson....

specifies the light month more precisely as 30 days of light travel time.

• Einstein protocol
Einstein protocol
Einstein protocol is a standard used for precisely measuring the distance between two objects in space....

• Light-second
Light-second
A 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 299,792,458 metres...

• 1 petametre (examples of distances on the order of one light-year)
• Orders of magnitude (length)
• Hubble length