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How light years are measured. Once i remember the distance between earth and moon are measured by light which travels and comeback and by the delay the distance is calculated.. But how light years are calculated

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If the question is supposed to be How are distances of the order of light-years measured?, see also physics.stackexchange.com/q/14016/2451 –  Qmechanic Dec 7 '11 at 8:42
    
Hi Jeyanth, and welcome to Physics Stack Exchange! It's really not clear what you're asking. Could you edit your question to clarify that? –  David Z Dec 7 '11 at 10:35
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We don't directly measure distances to anything light years away by timing light.

"Light year" is just a unit of distance like a mile or kilometer. We know how fast light goes in 1 second, we know how many seconds in a year and so we know how many meters (or miles) are in a light year.

To learn how we actually measure distances to stars see How do you measure distance to stars within the galaxy?

edit A light year is now defined as 9,460,730,472,580,800 m = 299 792 458 m / s * 365.25 * 24 * 60 * 60s (http://www.iau.org/public/measuring/)

Since the speed of light is constant and we are able to measure time very accurately we now actually define the meter in terms of the distance light travels in a fraction of a second - so as measurements of the speed of light improve it's actually the length of a meter that changes.

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The specific conversion factor is 1 light year = 9,460,730,500,000,000 meters. –  AdamRedwine Dec 7 '11 at 17:20
    
I thought it was 9,460,730,472,580,800 m (299,792,458 × 365.25 × 86400). –  Edgar Bonet Dec 8 '11 at 15:43
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