# Light-year is the measuement of distance not time.

A light-year or lightyear (symbol: ly) is a unit of measurement of Length, specifically the distance light travels in vacuum in one year. While there is no authoritative decision on which year is used, the IAU recommends the Julian year.

Numerical value of light-year. A light-year is equal to:

The exact length of the light-year depends on the length of the reference year used in the calculation.
• 9,460,730,472,580.8 km (about 9.461 Pm).

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 calculators use a smaller value than the IAU), such that the light-year is not an appropriate unit to use when extremely high precision is required.

The IAU Style Manual 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 (again about 9.461 Pm).

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.

Distances in light-years.

1 light-year =
International units
9.461×1015 m 9.461×1012 km
9.461×1018 mm 94.607×1024 Å
63.241×103 AU ly
US customary / Imperial units
372.47×1015 in 31.039×1015 ft
10.346×1015 yd 5.879×1012 mi

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.ValueItem
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 moon is roughly 384400 kilometers from Earth, on average. 384400km ÷ 300000km/second (roughly the speed of light) 1.28 seconds)

10-6
15.8×10-6 ly One astronomical unit (the distance from the Sun to the Earth). It takes approximately 8.31 minutes for light to travel this distance.
10-3
3.2×10-3 ly The most distant Space probe, Voyager 1, was about 28 light-hours (round-trip light time) away from Earth in the week ending March 9 2007. It took Voyager 30 years to cover that distance.
100
2×100 ly The Oort cloud is approximately 2 light-years in diameter.
4.21×100 ly The nearest known star (other than the Sun), Proxima Centauri is about 4.22 light-years away.
103
26×103 ly The center of our Galaxy, the Milky Way, is about 8 kiloparsecs away.
100×103 ly The Galaxy 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.
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.

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