It is analogous to celestial longitude. Sidereal time - The right ascension of the meridian. Zenith - The point directly above the viewer. All astronomical positional data is given in terms of declination and right ascension. This coordinate system allows the plotting of stellar data independent of the rotation of the Earth.
Declination is a celestial latitude.
Right ascension is the celestial latitude. Thus, as the Earth rotates, the right ascension of the zenith changes. In one hour of rotation, the right ascension changes by 1 hour as well. Thus, these expensive computations are not done at every display. Thus, there are approximately 60,000 points that are converted upon initialization.