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 = <B><math>3x10^8</math></B>m/s.
A light year can then be calculated to be about <math>3.00x10^8</math>m/s times 3600s times 24 times 365 = <B><math>9.46x10^12</math></B> kilometers, or <B><math>5.9x10^12</math></B> miles (5900 billion miles).
The distance in light years is equal to the time a light wave needs to cover that distance<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref> . Many objects in the universe are more than 10,000 light years away, giving rise to the [[starlight problem]].