=3. The Constellations.= The division of the stars into constellations constitutes the uranography or the “geography of the heavens.” The majority of the constellations are very ancient, and their precise origin is unknown, but those which are invisible from the northern hemisphere have all been named since the great exploring expeditions to the south seas. There are more than sixty constellations now generally recognised. Twelve of these belong to the zodiac, and bear the same names as the zodiacal signs, although the precession of the equinoxes has drifted them out of their original relation to the signs. Many of the constellations are memorials of prehistoric myths, and a large number are connected with the story of the Argonautic expedition and with other famous Greek legends. Thus the constellations form a pictorial scroll of legendary history and mythology, and possess a deep interest independent of the science of astronomy. For their history and for the legends connected with them, the reader who desires a not too detailed résumé, may consult _Astronomy with the Naked Eye_, and for guidance in finding the constellations, _Astronomy with an Opera-glass_, or _Round the Year with the Stars_. The quickest way to learn the constellations is to engage the aid of some one who knows them already, and can point them out in the sky. The next best way is to use star charts, or a star-finder or planisphere. A considerable number of the brighter and more important stars are known by individual names, such as Sirius, Canopus, Achernar, Arcturus, Vega, Rigel, Betelgeuse, Procyon, Spica, Aldebaran, Regulus, Altair, and Fomalhaut. Astronomers usually designate the principal stars of each constellation by the letters of the Greek alphabet, α, β, γ, etc., the brightest star in the constellation bearing the name of the first letter, the next brightest that of the second letter, and so on. The constellations are very irregular in outline, and their borders are only fixed with sufficient definiteness to avoid the inclusion of stars catalogued as belonging to one, within the limits of another. In all cases the names come from some fancied resemblance of the figures formed by the principal stars of the constellation to a man, woman, animal, or other object. In only a few cases are these resemblances very striking.