Of course in many cases this is impracticable, because a phenomenon we wish to watch may occur when an object is not situated near the meridian.

But in examining double stars there is in general no reason for selecting objects inconveniently situated.

We can wait till they come round to the meridian, and then observe them more comfortably.
Besides, most objects are higher, and therefore better seen, when due south.
Northern objects, and especially those within the circle of perpetual apparition, often culminate (that is, cross the meridian, or north and south line) at too great a height for comfortable vision.

In this case we should observe them towards the east or west, and remember that in the first case they are rising, and in the latter they are setting, and that in both cases they have also a motion from left to right.
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.