There is no reason why everybody should not know the principal planets at sight nearly as well as everybody knows the moon. It only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. No telescope, or any other instrument whatever, is required for the purpose. There is but one preliminary requirement, just as every branch of human knowledge presupposes its A B C. This is an acquaintance with the constellations and the principal stars--not a difficult thing to obtain. Almost everybody knows the "Great Dipper" from childhood's days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. Some know Orion when he shines gloriously in the winter heavens. Many are able to point out the north star, or pole star, as everybody should be able to do. All this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens. If you are fortunate enough to number an astronomer among your acquaintance--an amateur will do as well as a professor--you may, with his aid, make a short cut to a knowledge of the stars. Otherwise you must depend upon books and charts. My Astronomy with an Opera-Glass was prepared for this very purpose. For simply learning the constellations and the chief stars you need no opera-glass or other instrument. With the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. You need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention. Learn by observation at what seasons particular constellations are on, or near, the meridian--i.e., the north and south line through the middle of the heavens. Make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. The importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[21] How to tell which planet it may be, it is the object of this chapter to show you. As an indispensable aid--unless you happen already to possess a complete star atlas on a larger scale--I have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter.