Local Sidereal Time (GMST/LST)

The sidereal day runs 23 hours, 56 minutes and 4 seconds — almost four minutes shorter than the solar day. The difference is not a measurement error: while the Earth turns, it also advances along its orbit, so it has to turn a little more than 360 degrees to bring the Sun back to the meridian. Over a year that surplus adds up to exactly one rotation, which is why 366 sidereal days fit into 365 solar ones.

For anyone pointing a telescope, local sidereal time is the figure that matters: it tells you which right ascension is crossing the meridian at that instant. An object whose right ascension equals your LST is culminating now, at the highest point it will reach that night — and therefore looking through the least atmosphere. Planning an observing session is largely a matter of ordering targets by that comparison.

The page computes GMST, mean sidereal time at Greenwich, and adds longitude divided by fifteen to reach the local value. East longitude goes in positive. The word mean in the name means the nutation of the Earth's axis has been ignored: the gap between mean and apparent sidereal time reaches a little over a second, irrelevant for aiming an amateur telescope and relevant for astrometry.