Light-year

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.

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 astronomical units
• about 0.3066 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) as defined by the IAU.

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.

Definitions on the web

Internet search engines base their light-years on the 1900.0 mean tropical year, specified in the definition of the ephemeris second as 31,556,925.9747 s (12 significant digits), but round the number of metres to fewer significant digits. Microsoft's Live Search rounds to ten digits, Google and AOL round to eight digits, Ask.com rounds to seven digits, while Yahoo rounds to only five digits. All of these are smaller than the IAU definition.

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 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 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.
10-3	3.2 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.
100	1.6 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 ly	Maximum extent of the Sun's gravitational pull (hill sphere/roche sphere, 125,000 AU). Beyond this is true interstellar space
	4.22 ly	The nearest known star (other than the Sun), Proxima Centauri, is about 4.22 light-years away.
103	26 ly	The center of our galaxy, the Milky Way, is about 8 kiloparsecs away.
	100 ly	The Milky Way is about 100,000 light-years across.
106	2.5 ly	The Andromeda Galaxy is approximately 2.5 megalight-years away.
	3.14 ly	The Triangulum Galaxy (M33), at 3.14 megalight-years away, is the most distant object visible to the naked eye.
	59 ly	The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
	150 - 250 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 ly	The Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
	46.5 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.

References

See also

• 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)