Light-year
A unit of distance equal to the distance light travels through a vacuum in one Julian year, about 9.46 trillion kilometers.
Speed of light: The speed at which electromagnetic radiation propagates through a vacuum, exactly 299,792,458 meters per second. This fixed speed sets the distance represented by one light-year.
Proxima Centauri: The nearest known star to the Sun, about 4.24 light-years away. Its distance gives a nearby, concrete example of the unit’s scale.
Light-second: The distance light travels through a vacuum in one second, exactly 299,792,458 meters. It uses the same distance-by-light-travel definition on a much smaller scale.
Finite speed of light: The physical fact that light travels at a finite speed rather than arriving instantaneously. Because light takes time to cross a light-year, distant observations show objects as they were in the past.
Julian year: A time interval of exactly 365.25 days, each day defined as 86,400 seconds. The light-year convention uses this standardized year to fix its length.
Milky Way: The barred spiral galaxy containing the Solar System and billions of other stars. Its roughly 100,000-light-year diameter shows the unit’s use on galactic scales.
Light-minute: The distance light travels through a vacuum in one minute. It demonstrates that “light” units measure distance, not duration.
Cosmic distance ladder: A sequence of astronomical methods used to measure distances from nearby stars to remote galaxies. Light-years express results from distance measurements across the ladder’s many scales.
Vacuum: A region containing no matter, or matter at a pressure far below ordinary atmospheric pressure. The unit’s definition specifies light traveling through a vacuum.
Andromeda Galaxy: A large spiral galaxy about 2.5 million light-years from the Milky Way. Its distance illustrates how light-years describe distances between galaxies.