As to adjustments 1 and 3, in default of circles the ordinary astronomical methods are not available, but a pretty close approximation can be made by levelling. A good machinist’s level is quite sensitive and reliable. The writer has one picked out of stock at a hardware shop that is plainly sensitive to 2′ of arc, although the whole affair is but four inches long.
Most mounts like the one of Fig. 168 have a mark ruled on the support of the polar axis and a latitude scale on one of the cheek pieces. Adjustment of the polar axis to the correct altitude is then made by placing the level on the declination axis, or any other convenient place, bringing it to a level, and then adjusting the tripod until the equatorial head can be revolved without disturbing this level. Then set the polar axis to the correct latitude and adjustment number 1 is complete for the purpose in hand.
Lacking a latitude scale, it is good judgment to mark out the latitude by the help of the level and a paper protractor. To do this level the polar axis to the horizontal, level the telescope tube also, and clamp it in declination to maintain it parallel. Then fix the protractor to a bit of wood tied or screwed to the telescope support, drop a thin thread plumb line from a pin driven into the wood, the declination axis being still clamped, note the protractor reading, and then raise the polar axis by the amount of the latitude.
Next, with a knife blade scratch a conspicuous reference line on the sleeve of the polar axis and its support so that when the equatorial head is again levelled carefully you can set approximately to the latitude at once.
Now comes adjustment 3, the alignment of the polar axis to the meridian. One can get it approximately by setting the telescope tube roughly parallel with the polar axis and, sighting along it, shifting the equatorial head in azimuth until the tube points to the pole star. Then several methods of bettering the adjustment are available.
At the present date Polaris is quite nearly 1° 07′ from the true pole and describes a circle of that radius about it every 24 hours. To get the correct place of the pole with reference to Polaris one must have at least an approximate knowledge of the place of that star in its little orbit, technically its hour-angle. With a little knowledge of the stars this can be told off in the skies almost as easily as one reckons time on a clock. Fig. 169 is, in fact, the face of the cosmic clock, with a huge sweeping hour hand that he who runs may read.
Fig. 169.—The Cosmic Clock.
It is a clock in some respects curious; it has a twenty-four hour face like some clocks and watches designed for Continental railway time; the hour hand revolves backward, (“counter-clockwise”) and it stands in the vertical not at noon, but at 1.20 Star Time. The two stars which mark the tip and the reverse end of the hour hand are delta Cassiopeæ and zeta Ursæ Majoris respectively. The first is the star that marks the bend in the back of the great “chair,” the second (Mizar), the star which is next to the end of the “dipper” handle.