The student will find it useful to make a map of the heavens, or rather make several representing their condition at different times in the year. On this plot he should put down only the stars whose places and names he has learned, but he should plot the position of the planets at different times. In this way, though at first his efforts will be very awkward, he will soon come to know the general geography of the heavens. Although the possession or at least the use of a small astronomical telescope is a great advantage to a student after he has made a certain advance in his work, such an instrument is not at all necessary, or, indeed, desirable at the outset of his studies. An ordinary opera-glass, however, will help him in picking out the stars in the constellations, in identifying the planets, and in getting a better idea as to the form of the moon's surface--a matter which will be treated in this work in connection with the structure of the earth. CHAPTER IV. THE EARTH. In beginning the study of the earth it is important that the student should at once form the habit of keeping in mind the spherical form of the planet. Many persons, while they may blindly accept the fact that the earth is a sphere, do not think of it as having that form. Perhaps the simplest way of securing the correct image of the shape is to imagine how the earth would appear as seen from the moon. In its full condition the moon is apt to appear as a disk. When it is new, and also when in its waning stages it is visible in the daytime, the spherical form is very apparent. Imagining himself on the surface of the moon, the student can well perceive how the earth would appear as a vast body in the heavens; its eight thousand miles of diameter, about four times that of the satellite, would give an area sixteen times the size which the moon presents to us. On this scale the continents and oceans would appear very much more plain than do the relatively slight irregularities on the lunar surface.