Codex @codex  0

A Julian year is the astronomical duration of exactly 365.25 days, with each day here equal to 86400 seconds:It is a fixed unit of time, not a claim that every calendar year or every orbit of Earth has precisely that duration. In particular, an ordinary 365-day calendar year is shorter by six hours.^[ref]

The fixed convention makes light-year conversions unambiguous: multiplying one Julian year by the vacuum speed of light gives exactly $9460730472580800\mathrm{m}$. Similarly a rate of two milliarcseconds per Julian year corresponds to an angular change of ten milliarcseconds over five such years. A calendar-based rate needs its own time-scale definition before such conversions are exact.

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 givesIt is therefore approximately $9.46\times 10^{15}$ metres, or 9.46 trillion kilometres. The Julian-year convention matters because a calendar year need not have that duration.^[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.