If we allow an object to pass right across the field of view (the telescope being fixed), the apparent direction of its motion is the exact reverse of the true direction of the star's motion. This will serve as a guide in shifting the alt-azimuth after a star has passed out of the field of view.
The following technical terms must be explained. That part of the field of view towards which the star appears to move is called the _preceding_ part of the field, the opposite being termed the _following_ part. The motion for all stars, except those lying in an oval space extending from the zenith to the pole of the heavens, is more or less from right to left (in the inverted field). Now, if we suppose a star to move along a diameter of the field so as to divide the field into two semicircles, then in all cases in which this motion takes places from right to left, that semicircle which contains the lowest point (apparently) of the field is the _northern_ half, the other is the _southern_ half. Over the oval space just mentioned the reverse holds.
Thus the field is divided into four quadrants, and these are termed _north following_ (_n.f._) and _south following_ (_s.f._); _north preceding_ (_n.p._), and _south preceding_ (_s.p._). The student can have no difficulty in interpreting these terms, since he knows which is the following and which the preceding _semicircle_, which the northern and which the southern. In the figures of plates 3 and 5, the letters _n.f._, _n.p._, &c., are affixed to the proper quadrants. It is to be remembered that the quadrants thus indicated are measured either way from the point and feather of the diametral arrows.
Next, of the apparent annual motion of the stars. This takes place in exactly the same manner as the daily motion. If we view the sky at eight o'clock on any day, and again at the same hour one month later, we shall find that at the latter observation (as compared with the former) the heavens appear to have rotated by the _twelfth part_ of a complete circumference, and the appearance presented is precisely the same as we should have observed had we waited for two hours (the _twelfth part_ of a day) on the day of the first observation.
Our survey of the heavens is supposed to be commenced during the first quarter of the year, at ten o'clock on the 20th of January, or at nine on the 5th of February, or at eight on the 19th of February, or at seven on the 6th of March, or at hours intermediate to these on intermediate days.