# How light years is measured? [closed]

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?

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## closed as unclear what you're asking by Rob Jeffries, Kyle Kanos, Brandon Enright, Danu, John RennieDec 23 '14 at 7:29

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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

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
I object to quoting the light year to this many (13) significant digits. Although the IAU "defined" a year as 365.25 days (which is of course not the mean length of a year over a longer period - because of the "no leap year in 100's except 400's" rule) and the speed of light is also set by definition (9 digits), you do everyone a disfavor by assuming this means you can just multiply and consider every digit significant. Makes my skin crawl. – Floris Dec 23 '14 at 3:11
@Floris if you are in charge of fixing the defn of 'year' and the speed of light is fixed then you can have pretty much all the decimal places you want ;-) – Martin Beckett Dec 24 '14 at 4:25
@MartinBeckett I know that - I am saying that "all the decimal places I want" is fewer than the number presented. I don't want that many. I can think of no use for them. Can you? There are just so many reasons why a length measurement that spans light years can never be accurate to that many digits. – Floris Dec 24 '14 at 4:30