Sidereal Year
| Value | 31,558,149.8 s |
| Status | Conventional / typical value |
| Source | IERS Conventions (2010); epoch J2000 mean value |
| Categories | Astronomical |
| Planck time | 5.8535901e+50 tP |
| attosecond | 3.1558150e+25 as |
| femtosecond | 3.1558150e+22 fs |
| picosecond | 3.1558150e+19 ps |
| nanosecond | 3.1558150e+16 ns |
| shake | 3.1558150e+15 shake |
| microsecond | 31,558,150,000,000 µs |
| millisecond | 31,558,150,000 ms |
| second | 31,558,150 s |
| minute | 525,969.16 min |
| hour | 8,766.1527 h |
| sidereal day | 366.2564 sd |
| day | 365.25636 d |
| week | 52.17948 wk |
| fortnight | 26.08974 fn |
| month | 12.000455 mo |
| year | 1.0000174 yr |
| decade | 0.10000174 dec |
| century | 0.010000174 cent |
| millennium | 0.0010000174 mil |
| megayear | 0.0000010000174 Myr |
| eon | 1.0000174e-09 eon |
Learning zone
The sidereal year is the true orbital period: the time for Earth to return to the same point relative to the distant stars. It is the number to use in Kepler's third law and in any dynamical calculation. It differs from the year the calendar cares about because Earth's spin axis precesses like a slow top, dragging the equinox westward around the ecliptic once every 25 800 years and shortening the season-to-season year by 20 minutes.
Hipparchus discovered that precession around 130 BC by comparing his own star positions with Babylonian records more than a century old — one of the earliest cases of a scientific result emerging purely from an archive. The same drift is why the zodiac signs no longer line up with the constellations that named them, and why the celestial pole star changes over millennia.