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:

• 9,460,730,472,580.8 km (about 9.461 Pm)
• 5,878,625,373,183.61 statute miles
• about 63,240 astronomical units
• about 0.3066 parsecs

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 year	365.25	9,460,730,472,580,800	5,878,625,373,184
Gregorian year	365.2425	9,460,536,207,068,020	5,878,504,662,190
1900 mean tropical year	365.242199	9.460 528 4 ×1015	5.878 499 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 ly	Reflected 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 ly	One 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 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. [3]
100
	2100 ly	The Oort cloud is approximately 2 light-years in diameter.
	4.21100 ly	The nearest known star (other than the Sun), Proxima Centauri is about 4.22 light-years away. [4] [5]
10³
	26103 ly	The center of our galaxy, the Milky Way, is about 8 kiloparsecs away. [6] [7]
	100103 ly	The Milky Way is about 100,000 light-years across.
106
	2.5106 ly	The Andromeda Galaxy is approximately 2.5 megalight-years away.
	3.14106 ly	The Triangulum Galaxy (M33), at 3.14 megalight-years away, is the most distant object visible to the naked eye.
	59106 ly	The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
	150106 - 250106 ly	The Great Attractor lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
109
	1.2109 ly	The Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
	46.5109 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.

References

See also

• Astronomical unit
• Parsec
• Light-second
• Light-minute
• Light-hour
• Light-day
• Light-week
• Light-month
• 1 E15 m (examples of distances on the order of one light-year)
• Orders of magnitude (length)