light-year

Dictionary: light-year

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also light year (līt'yîr')
n.

1. The distance that light travels in a vacuum in one year, approximately 9.46 trillion (9.46 × 1012) kilometers or 5.88 trillion (5.88 × 1012) miles.
2. Informal. A long way. Often used in the plural: a small town light-years away from the action of the capital.


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Britannica Concise Encyclopedia:
light-year
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Distance traveled by light moving in a vacuum in one year, at its accepted speed of 186,282 mi/second (299,792 km/second). It equals about 5.9 trillion mi (9.5 trillion km), 63,240 astronomical units, or 0.307 parsec.

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Sci-Tech Encyclopedia:
Light-year
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A unit of measurement of astronomical distance. A light-year is the distance light travels in 1 sidereal year. One light-year is equivalent to 9.461 × 1012 km, or 5.879 × 1012 mi. Distances to some of the nearer celestial objects, measured in units of light time, are shown in the table.
Distances from the Earth to some celestial objects

Object


Distance from Earth (in light time)

Moon (mean)


1.3 s

Sun (mean)


8.3 min

Mars (closest)


3.1 min

Jupiter (closest)


33 min

Pluto (closest)


5.3 h

Nearest star (Proxima Centauri)


4.3 years

Andromeda Nebula (M31)


2,300,000 years

This unit, while useful for its graphic presentation of the enormous scale of stellar distances, is seldom used technically except in cosmology. See also Astronomical unit; Parallax (astronomy); Parsec.

Computer Desktop Encyclopedia:
light year
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The distance light travels in one year, which is approximately 5.9 quadrillion miles. Abbreviated "ly," light years are used to measure distances in the universe. For example, the galaxy is presumed to be about 100 ly across. Proxima Centauri, at about 4.22 ly from the sun, is the closest star to the sun.

It's About Distance, Not Time

Because the word "year" is used, light years are often erroneously used to measure time. The phrase "I spent light years doing that" might sound triumphant, but it is like saying "I spent a million miles doing that."

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Measures and Units:
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length. Symbol ly. The distance travelled by light in one mean year, = 9.460 536~ × 1012 km (63 239.8~ AU, 5.878 505~ × 1012 mi) ≈ 10-10 hubble.

Columbia Encyclopedia:
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light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 1012 km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant. See also parsec.


Science Q&A:
What is a light year?
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A light year is a measure of distance, not time. It is the distance that light, which travels in a vacuum at the rate of 186,282 miles (299,792 kilometers) per second, can travel in a year (365.25 days). This is equal to 5.87 trillion miles (9.46 trillion kilometers).

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Unit Conversions:
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To convert from light-years to:

astronomical unit, multiply by 63239.7.
miles, multiply by 5.9E+12.
kilometers, multiply by 9.46091E+12.

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Result:


Cosmic Lexicon:
Light year
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Distance that light can travel in 1 year. Light travels at a speed of 300,000 km / sec, so this distance is equal to 9.46 x 1012 kilometers.

Science Dictionary:
light year
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The distance traveled by light in a year (over five trillion miles); a unit for measuring distances outside the solar system. The star nearest to our sun, Alpha Centauri, is more than four light years away.

Wikipedia:
Light-year
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For other uses, see Light year (disambiguation).
Outer shell is one light year from Sun and left yellow line is Comet 1910 A1's orbit. Inner shell is one light month.
Look up light year in Wiktionary, the free dictionary.

A light-year, also light year or lightyear, (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (1016 metres or 10 petametres). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.[1]

The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec, 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 when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.[1]

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[2][3]
Contents

* 1 Numerical value
o 1.1 Other values
* 2 Distances in light-years
* 3 See also
* 4 References

Numerical value
1 light-year = SI units
9.461×10^12 km 9.461×10^15 m
Astronomical units
63.24×10^3 AU 0.3066 pc
US customary / Imperial units
5.879×10^12 mi 31.04×10^15 ft

A light-year is equal to:

* exactly 9,460,730,472,580.8 km (about 9.5 Pm)
* about 5,878,630,000,000 miles (about 6 trillion miles)
* about 63,241.1 astronomical units
* about 0.306601 parsecs

The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[4] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.[5] The DE405 value of the astronomical unit, 149,597,870,691 m,[6] is used for the light-year in astronomical units and parsecs.

The light-month does not have a precise definition or value, as there is no unique definition of a month in astronomical or SI units: however it may be understood to be approximately one-twelfth of a light-year, in line with the various mundane definitions of the month (hollow month, average Julian month, etc.).
Other values

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 31,556,925.9747 ephemeris seconds and a speed of light of 299,792.5 km/s produced a light-year of 9.460530 × 1015 metres (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 a reputable 1973 reference[11] which was not updated until 2000.[12]

Other high precision values are not derived from a coherent IAU system. A value of 9.460536207 × 1015 metres found in some modern sources[13][14] 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, 9.460528405 × 1015 metres,[15][16] is the product of the J1900.0 mean tropical year and the defined speed of light.
Distances in light-years

Distances measured in fractions of a light-year (or in light-months) 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. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Great Wall.
List of orders of magnitude for length