A light-year or light year (symbol: ly) is a unit of length, equal to just under ten trillion kilometres. A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit 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 list compares various sizes of positive Numbers including counts of things Dimensionless quantity and probabilities. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand 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. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. In Astronomy, a Julian year (symbol a) is a unit of measurement of Time defined ^[1]

Contents 1 Numerical value 2 Definitions on the web 3 Distances in light-years 4 References 5 See also

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. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International 61 international miles
• about 63,241 astronomical units
• about 0. A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the 3066 parsecs

The figures above are based on a Julian year (not Gregorian year) of exactly 365. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds) as defined by the IAU^[2].

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. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between 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. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units 9747 s^[3] (12 significant digits), but round the number of metres to fewer significant digits. The significant figures (also called significant digits and abbreviated sig figs) of a number are those digits that carry meaning contributing to its accuracy 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. A star system or stellar system is a small number of Stars which orbit each other bound by gravitational attraction. Distances measured in light-years include distances between nearby stars, such as those in the same spiral arm or globular cluster. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite.

One kilolight-year, abbreviated "kly", is one thousand light-years, or about 307 parsecs. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Kilolight-years are typically used to measure distances between parts of a galaxy. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter

One megalight-year, abbreviated "Mly", is one million light-years, or about 306,600 parsecs. The word million In standard English, the -lli- in million is pronounced with an l-sound followed by a Megalight-years are typically used to measure distances between neighboring galaxies and galaxy clusters. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation

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. A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. The Great Wall (also called Coma Wall) sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the

List of orders of magnitude for length

Factor (ly)	Value	Item
10-9	40. 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 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. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 (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). The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 It takes approximately 499 seconds (8. 32 minutes) for light to travel this distance. [4]
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. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Events 590 - Bahram Chobin is crowned as king Barham VI of Persia. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. It took that space probe 30 years to cover that distance. [5]
100	2×100 ly	The Oort cloud is approximately two light-years in diameter. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, Its inner boundary is speculated to be at 50,000 AU, with its outer edge at 100,000 AU
	1. 8×100 ly	Maximum extent of the Sun's gravitational pull (hill sphere/roche sphere, 125,000 AU). The Sun (Sol is the Star at the center of the Solar System. A Hill sphere is roughly the volume around an Astronomical body (such as a Planet) where it dominates in attraction of Satellites to that body rather A Hill sphere is roughly the volume around an Astronomical body (such as a Planet) where it dominates in attraction of Satellites to that body rather Beyond this is true interstellar space
	4. Interstellar Space was the one of the final studio albums recorded by the saxophonist John Coltrane before his death in 1967 originally-released posthumously 21×100 ly	The nearest known star (other than the Sun), Proxima Centauri, is about 4. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 22 light-years away. [6] [7]
103	26×103 ly	The center of our galaxy, the Milky Way, is about 8 kiloparsecs away. The Galactic Center is the rotational center of the Milky Way Galaxy. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply [8] [9]
	100×103 ly	The Milky Way is about 100,000 light-years across. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply
106	2. 5×106 ly	The Andromeda Galaxy is approximately 2. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda 5 megalight-years away.
	3. 14×106 ly	The Triangulum Galaxy (M33), at 3. The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses 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. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18
	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). The Great Attractor is a Gravity anomaly in Intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised
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. The Sloan Great Wall is a giant wall of Galaxies which may be the largest known structure in the Universe. The Great Wall (also called Coma Wall) sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the
	46. 5×109 ly	The comoving distance from the Earth to the edge of the visible universe is about 46. In standard cosmology, ' comoving' distance and ' proper distance' are two closely related distance measures used by cosmologists to define distances between 5 gigalight-years in any direction; this is the comoving radius of the observable universe. Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that This is larger than the age of the universe dictated by the cosmic background radiation. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day

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)

A light-second is a Unit of length. It is defined as the Distance Light travels in an absolute Vacuum in one Second or 299792458 A light-minute (also written light minute) is a Unit of length. A light-hour (also written light hour) is a unit of Length. It is the distance travelled by Light in Vacuum in one Hour A light day (also written light-day) is a Unit of length. It is defined as the Distance Light travels in an absolute Vacuum in one A light-week (also written light week) is a Unit of length. It is defined as the Distance Light travels in an absolute Vacuum in one A light-month (also written light month) is a Unit of length. To help compare different Distances this page lists lengths starting at 1015 m (1 Pm or 1000000 million km or 6700 Astronomical units To help compare different Orders of magnitude, the following list describes various Lengths between 1

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