= Julian year
{c}
{title2=$1\,\mathrm a=31557600\,\mathrm s$}
A <Julian year> is the astronomical duration of exactly 365.25 days, with each day here equal to 86400 <seconds>:
$$
1\,\mathrm{a}=365.25\times86400\,\mathrm s=31557600\,\mathrm s.
$$
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.<Measuring the Universe by the International Astronomical Union>[Defining a year, 365.25 days and 31.5576 million seconds]{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.
Back to article page
