- Astronomical unit of distance equal to the space covered by light in a year in a vacuum. Equal to 9.3 billion km or 0.3 parsec. Light years are frowned upon by serious astronomers, who like to work in parsecs, even though parallaxes cannot be detected for any but the nearest astronomical objects while the time taken for light to reach the observer from distant parts of the universe is of direct interest in astronomy and cosmology.
- The distance light travels through a vacuum in one year. It is unit of distance utilized in astronomy, and is equal to about 9.4605 × 1015 meters, or about 0.306591 par-sec. Its abbreviation is ly.
- abbreviation ly
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