Sidereal time

Remarks

The hour angle of an object of right ascension $\alpha$ is $H = \mathrm{LST} - \alpha$. So $\mathrm{LST} = \alpha$ exactly for the objects on the local meridian. For an observer at east longitude $\lambda$, in hours, $\mathrm{LST} = \mathrm{GST} + \lambda$. Example: if $\mathrm{LST} = 6^{\mathrm h} 45^{\mathrm m}$, then Sirius, with $\alpha \approx 6^{\mathrm h} 45^{\mathrm m}$, is near the meridian. Non-example: the hour angle of the Sun plus $12^{\mathrm h}$ is apparent solar time, not sidereal time. The sidereal day is about $23^{\mathrm h} 56^{\mathrm m} 4^{\mathrm s}$ of mean solar time. "Mean" sidereal time uses the mean equinox, and "apparent" sidereal time uses the true equinox, which includes nutation.