A light year is a unit of distance in astronomy. It is defined as the distance light will travel in one year (Earth years).
We can determine the actual metric distance by knowing two things: the speed of light in meters per second (c=299,792,458 m/s) and how many seconds are in one year (31,557,600 seconds). For simplicity and within an order of magnitude, we can set c to 3x10^8 m/s and the number of seconds in a year to 3.16x10^7 seconds.
Multiple speed and time to get distance: 3x10^8 * 3.16x10^7 = 9.48x10^15 m, 9.48 pentameters, 9.5 trillion kilometers (long scale), or 9.5 quadrillion kilometers (short scale, which is what the US uses).
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