The Greenwich Masters CELESTIAL Instruction Manual page 8

4. Features continued...
The local sidereal time display lets you easily determine the location of the
constellations. (Local sidereal time can be read off the right ascension
graduation on the constellation dial opposite the correct date on the date
scale).
The twilight indicator lets you identify the hours of increasing darkness,
especially convenient for astronomical observations. (Astronomical
°
twilight line: use the -18 altitude on the transparent dial to determine the
beginning and ending times for the hours of astronomical twilight).
The pole star hour angle display lets you align an astronomical telescope to
the polar axis. (Right ascension values for the pole star between the years
2000.0 - 2050.0 are marked on the constellation dial for every ten years).
Hour angle is the angle measured counterclockwise from the meridian on
the transparent dial to the point marking the value for the right ascension
the present pole star.
The celestial sphere at the centre of the rotating constellation dial is not
displayed owing to the attachment of the watch hands.
7.