Encyclopedia · Glossary
Light-Year
The distance light travels in one year — a measure of distance.
Definition
A light-year is the distance that light, traveling at its fixed speed in a vacuum, covers in one year — about 9.46 trillion kilometers (about 5.88 trillion miles). Despite the word 'year' in its name, it measures distance, not time.
In context
Astronomical distances are so vast that ordinary units become unwieldy, so astronomers measure the gaps between stars and galaxies in light-years. The nearest star system beyond the Sun, Alpha Centauri, lies about four light-years away.
Related ideas
Because light takes time to reach us, looking far across space is also looking back in time: we see distant objects as they were when their light set out. For distances within the Solar System, astronomers more often use the astronomical unit.
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Astronomical Unit
The average Earth–Sun distance, about 149.6 million km.
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Star
A self-gravitating sphere of plasma powered by fusion.
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Galaxy
A gravitationally bound system of stars, gas, dust, and dark matter.
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Planet
A round body that orbits the Sun and has cleared its orbit.