Sidereal month
= Sidereal month
{title2=$P_{\mathrm{sid}}$}
A <sidereal month> is the <Moon>'s <orbital period> relative to distant stars, about $27.32166$ days. It is shorter than the <synodic month> because the sunward direction changes as <Earth> travels around the <Sun>. The lunar spin period associated with <synchronous rotation> matches the <sidereal month> in the mean. https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html[Fred Espenak, NASA: the sidereal month]{ref}.