Light Year - MSN Encarta

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eSky: Light Year
A unit of interstellar distance, defined as the distance light travels in a period of one year. The speed of light is constant, at about 300,000 km per second: a light year is ...
Light-year - Wikipedia, the free encyclopedia
A light-year or light year (symbol: ly) is a unit of length, equal to just under ten trillion kilometres. As defined by the International Astronomical Union (which is the body ...
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Light Year, unit of length used in astronomy to measure vast distances. It is equal to the distance that light travels in a mean solar or astronomical year. At the rate of approximately 300,000 km/s (186,000 mi/s), a light year is approximately equal to 9,461,000,000,000 km (5,880,000,000,000 mi). See Parsec.