CoachA light-year is a distance: the stretch light covers in one year — about km, nearly ten million million kilometres. The name misleads the hasty (it is no more a time than a “footstep” is a foot); astronomers use it because kilometres collapse under the sky’s true scale.
Examples
Example 61.6 (The neighborhood in light-years)
The nearest star beyond the Sun: about light-years — tonight’s photons left it while you were in your first physics chapters. The bright stars of the winter sky: tens to hundreds of light-years. The Milky Way, our city of stars: about light-years across. The nearest great neighboring galaxy: some million light-years — its faint smudge, visible to a dark-adapted naked eye, is the oldest light a human can see unaided.