# Light year

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A light year is the distance light travels in one year in a vacuum. It is a measure of distance used in popular science for distances to great too express in kilometers or miles.

Professional astronomers prefer to use the parsec (approximately 3.26 light years) instead of the light year.

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.