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How are light years are measured? I remember the distance between earth and moon are measured by the delay in light which travels and comes back. But how are light years calculated?

Brandon Enright
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Jey
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1 Answers1

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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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Martin Beckett
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