Glossary · Thiên văn học
Light-year
A unit of distance, not of time: one light-year is the distance light travels in a vacuum in one year. It is commonly used to express distances to stars and galaxies.
According to the International Astronomical Union (IAU), a light-year is the distance light travels in a vacuum in one Julian year of 365.25 days. The IAU gives the value as 9,460,730,472,580.8 km, or about 9.46 trillion km.
Because light takes time to reach us, looking at a more distant object means seeing it as it was further in the past.
The IAU notes that the light-year is used mostly in popular publications. In technical work, such as studies of galactic structure, astronomers generally use the parsec, which is a larger unit.