# light year

1 light-year =
SI units
01015 m01012 km
Astronomical units
0103 AUpc
US customary / Imperial units
01015 ft01012 mi
A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance that light travels in a vacuum in one year. While there is no authoritative decision on which year is used, the International Astronomical Union (IAU) recommends the Julian year.

## Numerical value

A light-year is equal to:
The exact length of the light-year depends on the length of the reference year used in the calculation, and there is no wide consensus on the reference to be used. The figures above are based on a reference year of exactly 365.25 days (each of exactly 86,400 SI seconds). This is the value recommended by the IAU. However, other reference years are often used (e.g. Yahoo's and Google's calculators use a smaller value than the IAU), thus the light-year is not an appropriate unit to use when extremely high precision is required.

The IAU Style Manual[1] recommends the use of Julian calendar years (not Gregorian) of 365.25 days, or exactly 31,557,600 seconds. This gives the light-year an exact value of 9,460,730,472,580,800 metres.

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 generate one arcsecond of parallax when the observing object moved 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.

### Other light years

The exact length of a light year depends on the exact length used for one “Earth year”. The IAU uses a Julian year of 365.25 days, while other sources may use a Gregorian year of 365.2425 days, or another year altogether.
Source year (days) light year (metres) light year (miles)
IAU: Julian year365.259,460,730,472,580,8005,878,625,373,184
Gregorian year365.24259,460,536,207,068,0205,878,504,662,190
1900 mean tropical year365.2421999.460 528 4 ×10155.878 499 81 ×1012

Internet search engines use various definitions. Google uses a light-year based on the 1900 mean tropical year. Yahoo's light-year definition works out to a year length of ~365.2411 days.

## 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, or roughly one-thirteenth the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Gigalight-years are typically used to measure distances to supergalactic structures, such as clusters of quasars or the Great Wall.

List of orders of magnitude for length
Factor (ly) Value Item
10-9
40.410-9 lyReflected sunlight from the Moon's surface takes 1.2-1.3 seconds to travel the distance to the Earth's surface. (The moon is roughly 384400 kilometers from Earth, on average. 384400 km ÷ 300000 km/s (roughly the speed of light) ≈ 1.28 seconds)
10-6
15.810-6 lyOne astronomical unit (the distance from the Sun to the Earth). It takes approximately 8.31 minutes for light to travel this distance. [2]
10-3
3.210-3 lyThe most distant space probe, Voyager 1, was about 14 light-hours away from Earth in the week ending March 9, 2007. It took that space probe 30 years to cover that distance. [3]
100
2100 lyThe Oort cloud is approximately 2 light-years in diameter.
4.21100 lyThe nearest known star (other than the Sun), Proxima Centauri is about 4.22 light-years away. [4] [5]
10³
26103 lyThe center of our galaxy, the Milky Way, is about 8 kiloparsecs away. [6] [7]
100103 lyThe Milky Way is about 100,000 light-years across.
106
2.5106 lyThe Andromeda Galaxy is approximately 2.5 megalight-years away.
3.14106 lyThe Triangulum Galaxy (M33), at 3.14 megalight-years away, is the most distant object visible to the naked eye.
59106 lyThe nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
150106 - 250106 lyThe Great Attractor lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
109
1.2109 lyThe Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
46.5109 lyThe comoving distance from the Earth to the edge of the visible universe is about 46.5 gigalight-years in any direction; this is the comoving radius of the Observable universe.

## References

1. ^ IAU Recommendations concerning Units
2. ^ University of Western Ontario The Sun-Earth Connection
3. ^ NASA pressrelease (05-131) 2005-05-24: Voyager Mission Operations Status Report Week Ending March 9, 2007
4. ^ NASA: Cosmic Distance Scales - The Nearest Star
5. ^ Proxima Centauri (Gliese 551), Encyclopedia of Astrobiology, Astronomy, and Spaceflight
6. ^ F. Eisenhauer, et al., "A Geometric Determination of the Distance to the Galactic Center" (pdf, 93KB), Astrophysical Journal 597 (2003) L121-L124
7. ^ McNamara, D. H., et al., "The Distance to the Galactic Center" (pdf, 298KB), The Publications of the Astronomical Society of the Pacific, 112 (2000), pp. 202–216.

Light year may refer to:
• Light-year, a unit of length, the distance light travels in a vacuum in one year.
• Light Years (film), a 1988 animated science fiction film.
• Light Years (Glen Campbell album), a 1988 Glen Campbell album.

International System of Units (abbreviated SI from the French Le Système international d'unités) is the modern form of the metric system.
1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
1 kilometre =
SI units
0 m 0106 mm
US customary / Imperial units
0 ft 0 mi
A kilometre (American spelling: kilometer, symbol km
1 astronomical unit =
SI units
0109 m 0106 km
Astronomical units
010-6 pc 010−6 ly
US customary / Imperial units
0109 ft 0106 mi
The
parsec (symbol pc) is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars.
U.S. customary units, also known in the United States as English units[1] (but see English unit) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units
Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced.
1 foot =
SI units
0 m 0 mm
US customary / Imperial units
0 yd 0 in
A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes,
1 mile =
SI units
0 m 0 km
US customary / Imperial units
0 ft 0 yd

units of measurement have played a crucial role in human endeavour from early ages up to this day. Disparate systems of measurement used to be very common. Now there is a global standard, the International System (SI) of units, the modern form of the metric system.
Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. This may be distinguished from height, which is vertical extent, and width or breadth
Light is electromagnetic radiation of a wavelength that is visible to the eye (visible light). In a scientific context, the word "light" is sometimes used to refer to the entire electromagnetic spectrum.
A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than standard atmospheric pressure. The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum.
A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.
International Astronomical Union (IAU) unites national astronomical societies from around the world. It also acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.
Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of 86,400 SI seconds each, totalling 31,557,600 seconds. That is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the
1 kilometre =
SI units
0 m 0106 mm
US customary / Imperial units
0 ft 0 mi
A kilometre (American spelling: kilometer, symbol km
1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
1 astronomical unit =
SI units
0109 m 0106 km
Astronomical units
010-6 pc 010−6 ly
US customary / Imperial units
0109 ft 0106 mi
The
parsec (symbol pc) is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars.
Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
• One of two Italian words:
• (accented) for "yes"
• si

International Astronomical Union (IAU) unites national astronomical societies from around the world. It also acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.
1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation).
parsec (symbol pc) is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars.
A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60) of one degree. [1] Since one degree is defined as one three hundred sixtieth (1/360) of a circle, 1 MOA is 1/21600 of the amount of arc in a closed circle, or
Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an
1 astronomical unit =
SI units
0109 m 0106 km
Astronomical units
010-6 pc 010−6 ly
US customary / Imperial units
0109 ft 0106 mi
The