Hour circle Sentence Examples
The hour circle is also read by microscopes, and the instrument can be used in both positions (tube preceding and following) for elimination of the effect of flexure on the position angles.
It is equal to the angle at the pole between the hour circle through the body and the meridian, but is usually expressed in time.
I i shows the whole instrument on a small scale with the telescope directed to the pole, and the hour circle set 6" from the meridian.
The hour circle has two toothed circles cut upon it, one acted upon by a worm screw mounted on the pier and driven by clockwork, the other by a second worm screw attached to the polar axis, which can be turned by a handle in the observer's hand and thus a slow movement can be given to the telescope in right ascension inde FIG.
But when much larger instruments are required the hour circle becomes inaccessible from the floor, and means have to be devised for reading both circles from the eyeend.Advertisement
The observer's eye is applied to the small telescope E, which (by means of prisms numbered I, 2, 3, 4) views the vernier attached to the cross-head simultaneously with the hour circle attached to the upper end of the polar axis.
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 lamp near the eye-end illuminates the field or the wires at pleasure, as well as the position circle of the micrometer and the declination circle; a separate lamp illuminates the hour circle.
A new feature in this instrument is the platform at the lower end of the polar axis, where an assistant can view the hour circle by one eyeFIG.
An hour circle attached to E P and a declination circle attached to the box containing the mirror N, both of which can be read or set from E, complete the essentials of the instrument.Advertisement
Other accessories are an hour-circle, around the north pole, a compass placed beneath the globe, and a flexible quadrant used for finding the distances between places.