Past exam of the mathematics course of the University of Cambridge / 2017 / iii / Paper 338 / 1 / a / iii / Solution

Local sidereal time is the right ascension on the upper celestial meridian. Since right ascension increases eastward while hour angle increases westward, their difference isOne hour of either coordinate is $15^{\circ }$. For a fixed stellar direction, $h$ increases at the sidereal rotation rate. Mean sidereal time should be used with mean right ascension, or apparent sidereal time with apparent right ascension, for the same reference equinox.