Half-hours with the Telescope eBook

Richard Anthony Proctor
This eBook from the Gutenberg Project consists of approximately 98 pages of information about Half-hours with the Telescope.

Lastly, the observer may turn to the stars [gamma]_{1} and [gamma]_{2} Draconis towards the north-west about 40 deg. above the horizon (they are included in the second map of Plate 2).  They form a wide double, having equal (fifth-magnitude) components, both grey. (See Plate 5.)

CHAPTER VI.

HALF-HOURS WITH THE PLANETS.

In observing the stars, we can select a part of the heavens which may be conveniently observed; and in this way in the course of a year we can observe every part of the heavens visible in our northern hemisphere.  But with the planets the case is not quite so simple.  They come into view at no fixed season of the year:  some of them can never be seen by night on the meridian; and they all shift their place among the stars, so that we require some method of determining where to look for them on any particular night, and of recognising them from neighbouring fixed stars.

The regular observer will of course make use of the ‘Nautical Almanac’; but ‘Dietrichsen and Hannay’s Almanac’ will serve every purpose of the amateur telescopist.  I will briefly describe those parts of the almanac which are useful to the observer.

It will be found that three pages are assigned to each month, each page giving different information.  If we call these pages I. II.  III., then in order that page I. for each month may fall to the left of the open double page, and also that I. and II. may be open together, the pages are arranged in the following order:  I. II.  III.; III.  I. II.; I. II.  III.; and so on.

Now page III. for any month does not concern the amateur observer.  It gives information concerning the moon’s motions, which is valuable to the sailor, and interesting to the student of astronomy, but not applicable to amateur observation.

[Illustration:  PLATE VI.]

We have then only pages I. and II. to consider:—­

Across the top of both pages the right ascension and declination of the planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given, accompanied by those of two conspicuous stars.  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

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