= Light-year
{title2=$\mathrm{ly}$}
A <light-year> is the <length> travelled by light in vacuum during one <Julian year> of exactly 365.25 days. Using $86400$ <seconds> per day and the exact vacuum <speed of light> gives
$$
1\,\mathrm{ly}=299792458\,\frac{\mathrm m}{\mathrm s}\times31557600\,\mathrm s=9460730472580800\,\mathrm m.
$$
It is therefore approximately $9.46\times10^{15}$ metres, or 9.46 trillion kilometres. The Julian-year convention matters because a calendar year need not have that duration.<Measuring the Universe by the International Astronomical Union>[The light-year and Defining a year]{ref}
For a stationary source four <light-years> away in an idealized flat spacetime, light takes four <Julian years> to reach the observer. The distance unit and the propagation time remain different physical quantities. In an expanding universe, present distance, lookback time multiplied by the <speed of light>, and luminosity distance generally differ, so this simple nearby-source interpretation cannot be applied to every cosmological distance.
Back to article page
