Light-year

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For other uses, see Light year (disambiguation).

**1 light-year** =
SI units
9.461×10¹² km	9.461×10¹⁵ m
Astronomical units
63.24×10³ AU	0.3066 pc
US customary / Imperial units
5.879×10¹² mi	31.04×10¹⁵ ft

A light-year or light year (symbol: ly) is a unit of length, equal to just under ten trillion kilometres. As defined by the International Astronomical Union (which is the body which has the jurisdictional authority to promulgate the definition), 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. In astronomy, the preferred unit of measurement for such distances is the parsec, which 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. 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.

1 Numerical value
2 Other values
3 Distances in light-years
4 References
5 See also
6 External links

[edit] Numerical value

A light-year is equal to:

exactly 9,460,730,472,580.8 km (about 10 Pm)
about 5,878,625,373,183.61 international miles
about 63,241.0770881 astronomical units
about 0.306601393806 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)^[2] 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.^[3] The DE405 value of the astronomical unit, 149,597,870,691 m,^[4] is used for the light-year in astronomical units and parsecs.

[edit] 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.^[5] 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^[6]^[7]^[8] was probably derived from an old source such as a reputable 1973 reference^[9] which was not updated until 2000.^[10]

Other high precision values are not derived from a coherent IAU system. A value of 9.460536207×10¹⁵ metres found in some modern sources^[11]^[12] 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). The Live Search value, 9.460528405×10¹⁵ metres,^[13] is the product of the J1900.0 mean tropical year and the defined speed of light.

[edit] 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. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Great Wall.

**List of orders of magnitude for length**
Factor (ly)	Value	Item
10^-9	40.4×10^-9 ly	Reflected sunlight from the Moon's surface takes 1.2-1.3 seconds to travel the distance to the Earth's surface. (The surface of the moon is roughly 376300 kilometers from the surface of the Earth, on average. 376300 km ÷ 300000 km/s (roughly the speed of light) ≈ 1.25 seconds)
10^-6	15.8×10^-6 ly	One astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.^[14]
10^-3	3.2×10^-3 ly	The 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.^[15]
10⁰	1.6×10⁰ ly	The Oort cloud 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
	2.0×10⁰ ly	Maximum extent of the Sun's gravitational pull (hill sphere/roche sphere, 125,000 AU). Beyond this is true interstellar space
	4.22×10⁰ ly	The nearest known star (other than the Sun), Proxima Centauri, is about 4.22 light-years away.^[16]^[17]
10³	26×10³ ly	The center of our galaxy, the Milky Way, is about 8 kiloparsecs away.^[18]^[19]
10³	100×10³ ly	The Milky Way is about 100,000 light-years across.
10⁶	2.5×10⁶ ly	The Andromeda Galaxy is approximately 2.5 megalight-years away.
	3.14×10⁶ ly	The Triangulum Galaxy (M33), at 3.14 megalight-years away, is the most distant object visible to the naked eye.
	59×10⁶ ly	The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
	150×10⁶ - 250×10⁶ ly	The Great Attractor lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
10⁹	1.2×10⁹ ly	The Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
10⁹	46.5×10⁹ ly	The 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. This is larger than the age of the universe dictated by the cosmic background radiation; see size of the universe: misconceptions for why this is possible.