Sidereal Day
| Value | 23.9344695833 h (86,164.0905 s) |
| Status | Reference value; the source states no uncertainty |
| Source | IERS Conventions (2010); Earth rotation is variable at the millisecond level |
| Categories | Astronomical |
| Unit | Value |
|---|---|
| hour | 23.93447 h |
| second | 86,164.091 s |
| minute | 1,436.0682 min |
| day | 0.99726957 d |
| sidereal day | 1 sd |
| week | 0.14246708 wk |
| fortnight | 0.07123354 fn |
| month | 0.032765176 mo |
14 more time units
| Planck time | 1.5982219e+48 tP |
| attosecond | 8.6164091e+22 as |
| femtosecond | 8.6164091e+19 fs |
| picosecond | 8.6164091e+16 ps |
| nanosecond | 86,164,091,000,000 ns |
| shake | 8,616,409,100,000 shake |
| microsecond | 86,164,091,000 µs |
| millisecond | 86,164,091 ms |
| year | 0.0027303753 yr |
| decade | 0.00027303753 dec |
| century | 0.000027303753 cent |
| millennium | 0.0000027303753 mil |
| megayear | 2.7303753e-09 Myr |
| eon | 2.7303753e-12 eon |
Learning zone
A sidereal day is one true rotation of the planet, measured against the stars. It is shorter than the solar day because Earth also advances about a degree along its orbit each day, so it must turn that extra degree, very nearly four minutes' worth, to bring the Sun back to the meridian. Over a year the difference accumulates to exactly one extra rotation: 366.24 sidereal days to 365.24 solar ones.
Observatories run on sidereal time because a star returns to the same position after exactly one sidereal day, and satellite operators care because a geostationary orbit must have this period, not 24 hours, to hang motionless over a point on the equator. Earth's rotation is not perfectly steady either: tidal friction lengthens the day by about 1.8 milliseconds per century, and seasonal winds and core motions shift it by a millisecond or two, which is why leap seconds exist.
Cite this page
formula.expert. (2026, October 16). Sidereal Day. https://formula.expert/constants/sidereal-day