For many of their purposes astronomers find no use for the constellations, preferring to identify the stars by their position in right ascension and declination (equivalent to longitude and latitude), and in the great modern _Durchmusterungs_, or star catalogues, this plan is universally followed. Still, the constellations afford a very convenient classification of the stars, and probably they will never be abandoned even by professional astronomers; while from another standpoint they never can be abandoned, because they are among the most ancient and precious of human documents, valuable for history and for the understanding of mythology, and resistlessly charming in their poetic associations. But, to return to the description of the charts, the reader should be informed as to the meaning of the lines shown upon them, and of the indications found round their borders. In the four circular charts the closed curve crossing the sky from right to left represents the equator of the heavens, which is directly over the equator of the earth; the vertical line through the centre shows the meridian, or north and south line, which, so to speak, follows the observer wherever he may go, occupying the same place in the sky, _at the same hour of local time_, in all longitudes; and the dotted curve is the ecliptic, or the apparent annual path of the sun through the sky. The crossing points of the equator and the ecliptic are respectively the Vernal and the Autumnal Equinox, where the sun is at the two dates in the year when day and night are of equal length; and the farthest northern and southern points of the ecliptic are respectively the Summer and the Winter Solstice, where the sun is at the times of the longest and the shortest days in our hemisphere. These four fundamental points are all shown on the charts. Around the border the hours of right ascension are indicated by Roman numerals. Each hour corresponds to 15° of space, or one twenty-fourth of a circle of the sphere. The hours begin at the Vernal Equinox, which is graphically described as the “Greenwich of the Sky.” In the larger-scale charts at the end of the book the hours of right ascension are indicated at the bottom, and the degrees of north and south declination (the sign + standing for north and - for south) are shown at the side. In both cases the declination is reckoned from the equator. The four oblong charts of this series, taken together, represent the entire circuit of sky between 40° north and 40° south declination, and the two semicircular charts, taken together, show the stars within 50° of the north pole. Thus the entire set of six charts exhibits the complete dome of the heavens from the north pole to 40° south declination. In passing from the oblong to the semicircular charts it is only necessary to bring the hours of right ascension into accord. In the semicircular charts the hours will be found round the curved borders.