DESCRIPTION OF PLATES.
PLATE [I.]—Frontispiece.
This plate presents the aspect of the heavens at the four seasons, dealt with in Chapters [II.], [III.], [IV.], and [V.] In each map of this plate the central point represents the point vertically over the observer's head, and the circumference represents his horizon. The plan of each map is such that the direction of a star or constellation, as respects the compass-points, and its elevation, also, above the horizon, at the given season, can be at once determined. Two illustrations of the use of the maps will serve to explain their nature better than any detailed description. Suppose first, that—at one of the hours named under Map I.—the observer wishes to find Castor and Pollux:—Turning to Map I. he sees that these stars lie in the lower left-hand quadrant, and very nearly towards the point marked S.E.; that is, they are to be looked for on the sky towards the south-east. Also, it is seen that the two stars lie about one-fourth of the way from the centre towards the circumference. Hence, on the sky, the stars will be found about one-fourth of the way from the zenith towards the horizon: Castor will be seen immediately above Pollux. Next, suppose that at one of the hours named the observer wishes to learn what stars are visible towards the west and north-west:—Turning the map until the portion of the circumference marked W ... N.W. is lowermost, he sees that in the direction named the square of Pegasus lies not very high above the horizon, one diagonal of the square being vertical, the other nearly horizontal. Above the square is Andromeda, to the right of which lies Cassiopeia, the stars β and ε of this constellation lying directly towards the north-west, while the star α lies almost exactly midway between the zenith and the horizon. Above Andromeda, a little towards the left, lies Perseus, Algol being almost exactly towards the west and one-third of the way from the zenith towards the horizon (because one-third of the way from the centre towards the circumference of the map). Almost exactly in the zenith is the star δ Aurigæ.
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.