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The distance light travels in one year ''in vacuo''.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref>
The distance light travels in one year in a vacuum.
It is a measure of distance used in popular science to for distances to great to 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 300,000,000 meters per second or <B><math>3x10^8</math></B>m/s.
A light year can then be calculated to be <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.
For instance, the [[Andromeda galaxy]] is 2.5 million light years from earth, meaning that light we see from this galaxy has been traveling for over two and a half million years.<ref>[http://www.solstation.com/x-objects/andromeda.htm]</ref>
<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref>
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