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A light year, abbreviation ly, is the distance light travels in one year: roughly 10 million million kilometers (or about 5.85 trillion miles). More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely far away from any gravitational or magnetic fields, in one Julian year (365.25 days of 86400 seconds each). Since the speed of light in vacuum is 299,792,458 m/s, one light year is approximately equal to 9.46 × 10^{15} m = 9.46 petameter.
The light year is used to measure large distances, like the distance from the solar system to a nearby star. A light year is not a unit of time. In astronomy, the parsec is nowadays the preferred unit for large distances; one parsec equals 3.26 lightyears.
A light year is also equal to 63,240 astronomical units. For a list of lengths on the order of one light year, see the article 1 E15 m.
Miscellaneous facts:
Caveat: since our galaxy is 100,000 light years across, a hypothetical spaceship travelling close to the speed of light would need somewhat longer than 100,000 years to cross it. However, this is only true for an observer at rest with respect to the galaxy; the space ship's crew will experience the trip across the galaxy in a matter of minutes. This is because of the time dilation[?] of moving clocks explained by special relativity.
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