Julian Year
| Value | 31,557,600 s |
| Status | Exact by definition — no uncertainty |
| Source | IAU convention (exact by definition) |
| Categories | Astronomical |
| Planck time | 5.8534881e+50 tP |
| attosecond | 3.1557600e+25 as |
| femtosecond | 3.1557600e+22 fs |
| picosecond | 3.1557600e+19 ps |
| nanosecond | 3.1557600e+16 ns |
| shake | 3.1557600e+15 shake |
| microsecond | 31,557,600,000,000 µs |
| millisecond | 31,557,600,000 ms |
| second | 31,557,600 s |
| minute | 525,960 min |
| hour | 8,766 h |
| sidereal day | 366.25002 sd |
| day | 365.25 d |
| week | 52.178571 wk |
| fortnight | 26.089286 fn |
| month | 12.000246 mo |
| year | 1 yr |
| decade | 0.1 dec |
| century | 0.01 cent |
| millennium | 0.001 mil |
| megayear | 0.000001 Myr |
| eon | 1.0000000e-09 eon |
Learning zone
The Julian year is pure convention, an exact 31 557 600 seconds, and its job is to make derived units reproducible. The light-year is defined with it, epochs such as J2000.0 are counted in Julian centuries of 36 525 days from it, and proper motions and orbital rates are quoted per Julian year so that nobody has to ask which kind of year an author meant.
It takes its length from Julius Caesar's calendar reform of 46 BC, which averaged 365.25 days with a leap day every fourth year. That overshoots the tropical year by 11 minutes, an error that had pushed the equinox ten days out of place by the sixteenth century and prompted the Gregorian reform of 1582 — but for the astronomer's bookkeeping unit, the clean quarter-day is a virtue rather than a bug.