\begin{definition}[Definition:Sidereal Day]
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.
\end{definition}