From Thesky6 programme I've calculated that the mean sidereal day is 86164.1207742 seconds. Not all numbers may be significant, but is there a more accurate calculation & from where?
You can calculate the mean sidereal day from the mean sidereal year.
The sidereal year is the time it takes the Earth to return to the same place with respect to the fixed stars. The mean sidereal year is
Each mean solar day the Earth rotates about the Sun by
A sidereal day is one complete rotation of the Earth about its axis with respect to the fixed stars. In a sidereal day the Earth rotates exactly 360° about its axis.
The mean solar day is 86400 seconds long. The mean sidereal day is therefore