Thursday, September 6, 2007

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 IAU recommends the julian year.

Light-year Other 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, 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.
(The moon is roughly 384400 kilometers from Earth, on average. 384400km ÷ 300000km/second (roughly the speed of light) ≈ 1.28 seconds)

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