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A light-year, abbreviation ly, is the distance light travels in one year: roughly 9.46 trillion kilometers (or about 5.88 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 × 1015 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 light-years.

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:

• It takes 8 minutes for light to travel from the Sun to the Earth (thus we are about 8 light minutes in distance away from the Sun).
• The most distant space probe, Voyager 1, was 12.5 light hours away in January 2004.
• The nearest known star, Proxima Centauri is 4.22 light years away
• Our galaxy, the Milky Way, is 100,000 light years in diameter.
• The observable universe has a radius of about 15,000,000,000 light years.

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.