Definition:Sidereal Day

Definition
A sidereal day is (approximately) the length of time it takes for Earth to rotate once on its axis relative to the stars.

The mean duration of a sidereal day is:
 * $23$ hours, $56$ minutes and $4.0916$ seconds
 * $23.9344699$ hours
 * $0.99726958$ (solar) days.

The duration of any given sidereal day is not constant because of various astronomical effects.

Also see

 * Definition:Day