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by JP1967 on January 7th, 2004

JP1967

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How far is a light-year? In miles and kilometers.

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  • by Relsqui on March 17th, 2006

    Relsqui

    Great Answer

    Hand-picked by Answerbag's staff. (What's this?)

    A light year is a unit of distance (not time!) equal to the speed of light in a vacuum times the duration of one Julian year--in other words, how far light could travel in a vacuum in a year, assuming no influence by gravitational or magnetic fields. It's used, mostly by laypeople, to discuss the differences in stars. (Professionals prefer the parsec.)

    So how far is that? Exactly 9,460,528,410,545,436.2688 meters, and when I say "exactly" I mean it--the meter is defined relative to the speed of light in a vacuum (specifically, as the distance it can travel in 1/299,792,458 of a second), and as the number of seconds in a year is a fixed constant, you can't get more precise than that. You can express the same value slightly less precisely, though, as 5,878,625,373,184 miles.

    For what it's worth, a light year is also about 63,241.077 astronomical units, which are the defined as the average distance between the earth and the sun. That's even less precise than the above, though, seeing as the currently accepted length of one AU comes with a 30-meter margin of error.

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    • The answer above uses an exact definition of meter (which is now defined in terms of the speed of light) which is good and correct. However, the answer also depends on the definition of "year" used. The above answer uses a mean solar year which is APPROXIMATELY 365.2422 days. To make the answer "exact", scientists and astronomers define the light-year in terms of the Julian year which is EXACTLY 365.25 days of EXACTLY 86,400 seconds per day. So if you use these definitions, this gives an EXACT answer of 9,460,730,472,580.8 meters.
      Reference: http://en.wikipedia.org/wiki/Light-year

      EagleEye88

      by EagleEye88 on April 10th, 2011

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