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
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.