The light year is easily derived from the speed of light which is about 300,000,000 meters per second = 3×108 m/s.
Since there are 31,536,000 seconds in one year, a light year can then be calculated to be about 3×108 m/s × 31536000 s = 9.46×1012 km, or 5.9×1012 miles (5900 billion miles).
(To put this in perspective: a person would have to live 31 years, 251 days, 7 hours, 46 minutes, and 39 seconds to reach the age of 1 billion seconds-old.)
The distance in light years is equal to the time a light wave needs to cover that distance. Many objects in the universe are more than 10,000 light years away, giving rise to the starlight problem in Creation science. A similar problem occurs (the Horizon problem) in the big bang model of the Universe. In the Big bang theory, this problem is resolved by the fact the universe is expanding.
- Light-year from britannica.com
- Wile, Dr. Jay L. Exploring Creation With Physical Science. Apologia Educational Ministries, Inc. 1999, 2000