Light-year

A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one Julian year.

Numerical value

A light-year is equal to:

• 9,460,730,472,580.8 km (about 9.461 Pm)
• about 5,879,000,000,000 statute miles
• about 63,240 astronomical units
• about 0.3066 parsecs

The actual, 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 24 hours). This is the value recommended by the IAU. However, other reference years are often used (e.g. Yahoo's calculator uses 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 guide [1] recommends the use of calendar years, specifically Julian (and not Gregorian) calendar years 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

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 306,601 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 306.601 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.

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)

References

1. ^ University of Western Ontario The Sun-Earth Connection
2. ^ NASA pressrelease (05-131) 2005-05-24: Voyager Spacecraft Enters Solar System's Final Frontier
3. ^ NASA: Cosmic Distance Scales - The Nearest Star
4. ^ Encyclopedia of Astrobiology, Astronomy, and Spaceflight: Proxima Centauri (Gliese 551)
5. ^ F. Eisenhauer, R. Schoedel, R. Genzel, T. Ott, M. Tecza, R. Abuter, A. Eckart, T. Alexander: A Geometric Determination of the Distance to the Galactic Center, Astrophys.J. 597 (2003) L121-L124
6. ^ McNamara, D. H.; Madsen, J. B.; Barnes, J.; Ericksen, B. F.: The Distance to the Galactic Center, The Publications of the Astronomical Society of the Pacific, Volume 112, Issue 768, pp. 202-216.

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