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A light-year or '''lightyear''' (symbol: '''ly''') is a Unit Of Measurement of Length , specifically the distance 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:
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 IAU Recommendations concerning Units 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. 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 Star s, such as those in the same Spiral Arm or Globular Cluster . One ''kilolight-year'', abbreviated "kly", is One Thousand light-years, or about 307 Parsec s. 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 Parsec s. Megalight-years are typically used to measure distances between neighboring Galaxies and Galaxy Cluster s. 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 Parsec s, 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 Quasar s or the Great Wall . REFERENCES SEE ALSO
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