This information is very valuable to the telescopist.

In the first place, as we shall presently see, it shows him what planets are well situated for observation, and secondly it enables him to map down the path of any planet from day to day among the fixed stars.

This is a very useful exercise, by the way, and also a very instructive one.

The student may either make use of the regular maps and mark down the planet's path in pencil, taking a light curve through the points given by the data in his almanac, or he may lay down a set of meridians suited to the part of the heavens traversed by the planet, and then proceed to mark in the planet's path and the stars, taking the latter either from his maps or from a convenient list of stars.[9] My 'Handbook of the Stars' has been constructed to aid the student in these processes.

It must be noticed that old maps are not suited for the work, because, through precession, the stars are all out of place as respects R.A.and Dec.