# Half-hours with the Telescope eBook

This eBook from the Gutenberg Project consists of approximately 116 pages of information about Half-hours with the Telescope.

The four maps are miniatures of Maps I., IV., VII., and X. of my
‘Constellation Seasons,’ fourth-magnitude stars, however, being omitted.

Plates ii., III., IV., and V., illustrating Chapters ii., III., IV., and V.

Plates ii. and iv. contain four star-maps.  They not only serve to indicate the configuration of certain important star-groups, but they illustrate the construction of maps, such as the observer should make for himself when he wishes to obtain an accurate knowledge of particular regions of the sky.  They are all made to one scale, and on the conical projection—­the simplest and best of all projections for maps of this sort.  The way in which the meridians and parallels for this projection are laid down is described in my ‘Handbook of the Stars.’  With a little practice a few minutes will suffice for sweeping out the equidistant circular arcs which mark the parallels and ruling in the straight meridians.

The dotted line across three of the maps represents a portion of the horizontal circle midway between the zenith and the horizon at the hour at which the map is supposed to be used.  At other hours, of course, this line would be differently situated.

Plates iii. and V. represent fifty-two of the objects mentioned in the above-named chapters.  As reference is made to these figures in the text, little comment is here required.  It is to be remarked, however, that the circles, and especially the small circles, do not represent the whole of the telescope’s field of view, only a small portion of it.  The object of these figures is to enable the observer to know what to expect when he turns his telescope towards a difficult double star.  Many of the objects depicted are very easy doubles:  these are given as objects of reference.  The observer having seen the correspondence between an easy double and its picture, as respects the relation between the line joining the components and the apparent path of the double across the telescope’s field of view, will know how to interpret the picture of a difficult double in this respect.  And as all the small figures are drawn to one scale, he will also know how far apart he may expect to find the components of a difficult double.  Thus he will have an exact conception of the sort of duplicity he is to look for, and this is—­crede experto—­a great step towards the detection of the star’s duplicity.

Plates VI. and vii., illustrating Chapters VI. and vii.

The views of Mercury, Venus, and Mars in these plates (except the smaller view of Jupiter in Plate vii.) are supposed to be seen with the same “power.”

The observer must not expect to see the details presented in the views of Mars with anything like the distinctness I have here given to them.  If he place the plate at a distance of six or seven yards he will see the views more nearly as Mars is likely to appear in a good three-inch aperture.

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