The angle formed at the pole, or its arc on the celestial equator, between the hour circle of a celestial object and the observer's celestial meridian measured in hours, minutes, etc.: it represents the elapsed time since the object passed directly over the observer.
If we now attach to the polar axis a graduated circle D D, called the" hour circle,"of which the microscope or vernier R reads o h when the declination axis is horizontal, we can obviously read off the hour angle from the meridian of any star to which the telescope may be directed at the instant of observation.
If the local sidereal time of the observation is known, the right ascension of the star becomes known by adding the observed hour angle to the sidereal time if the star is west of the meridian, or subtracting it if east of the meridian.
Through the eyepiece of the bent 1 telescope E' another hour circle attached to the lower end of the polar axis can be seen; thus an assistant is able to direct the telescope by a handle at H to any desired hour angle.
The local hour angle of an object is the local apparent sidereal time minus the apparent right ascension.
When the instrument has been set up and levelled (either with aid of the cross level d, or the levels k and 1), the reading of the circle p for the meridional position of the telescope is determined either by the method of transits in the meridian (see Transit Circle), or by the observation of the azimuth of a known star at a known hour angle.