sd
sidereal day
The sidereal day is one rotation of the Earth measured against the fixed stars rather than against the sun: 86,164.0905 seconds, about three minutes and fifty-six seconds short of the civil day. The difference exists because the Earth also moves along its orbit during a rotation, so it must turn slightly further than 360° to bring the sun back to the same place in the sky.
| 1 sd | 86,164.091 s |
Where the unit came from
Telescope drives are geared to sidereal rate, not solar, which is why a star tracked without correction stays put and the sun does not. The shortfall accumulates to exactly one extra rotation over a year, so a year holds one more sidereal day than civil days.
Solvers that use time