One or the other is above the horizon anywhere in the northern hemisphere. Further, the line joining these two stars passes almost exactly through the celestial pole, and also very nearly through Polaris, which lies between the pole and δ Cassiopeæ. Consequently if you want to know the hour-angle of Polaris just glance at the clock and note where on the face δ Cassiopeæ stands, between the vertical which is XXIV o’clock, and the horizontal, which is VI (east) or XVIII (west) o’clock.
You can readily estimate its position to the nearest half hour, and knowing that the great hour hand is vertical (δ Cassiopeæ up) at Ih 20m or (ζ Ursæ Majoris up) at XIIIh 20m, you can make a fairly close estimate of the sidereal time.
A little experience enables one to make excellent use of the clock in locating celestial objects, and knowledge of the approximate hour angle of Polaris thus observed can be turned to immediate use in making adjustment 3. To this end slip into the plane of the finder cross wires a circular stop of metal or paper having a radius of approximately 1° 15′ which means a diameter of 0.52 inch per foot of focal length.
Then, leaving the telescope clamped in declination as it was after adjustment 1, turn it in azimuth across the pole until the pole star enters the field which, if the finder inverts it will do on the other side of the center; i.e. if it stands at IV to the naked eye it will enter the field apparently from the XVI o’clock quarter. When just comfortably inside the field, the axis of the telescope is pointing substantially at the pole.
It is better to get Polaris in view before slipping in the stop and if it is clearly coming in too high or too low shift the altitude of the polar axis a trifle to correct the error. This approximate setting can be made even with the smallest finder and on any night worth an attempt at observation.
With a finder of an inch or more aperture a very quick and quite accurate setting to the meridian can be made by the use of Fig. 170, which is a chart of all stars of 8 mag. or brighter within 1° 30′ of the pole. There are only three stars besides Polaris at all conspicuous in this region, one quite close to Polaris, the other two forming with it the triangle marked on the chart. These two are, to the left, a star of magnitude 6.4 designated B. D. 88 112, and to the right one of magnitude 7.0, B. D. 89 13.
The position of the pole for the rest of the century is marked on the vertical arrow and with the stars in the field of the finder one can set the cross wires on the pole, the instrument remaining clamped in declination, within a very few minutes of arc, quite closely enough for any ordinary use of a portable mount. All this could be done even better with the telescope itself, but it is very rare to find an eyepiece with sufficient field.
Fig. 170.—The Pole among the Stars.
At all events the effect of any error likely to be made in these adjustments is not serious for the purpose in hand, since if one makes an error of a minute of arc in the setting the resulting displacement of a star in the field will even in the most unfavorable case reach this full amount only after 6 hours following. I.e. with any given eyepiece an error of adjustment equal to the radius of the field will still permit following a star for an hour or two before it drifts inconveniently wide of the center.