The same kind of analysis applies to the stars, no matter how far away they may be, so long as they give sufficient light to form a spectrum. And in this way it has been found that the stars differ somewhat from the sun and from one another in their composition, and thus a classification of the stars has been made, and it has been possible to draw conclusions concerning their relative age, which show that some stars are comparatively younger than the sun, others older, and others so far advanced in age, or evolution, that they are drawing near extinction. Many dark bodies also exist among the stars, which appear to be completely extinguished suns. It only remains to add on this subject that, according to prevailing theories, the earth itself was once an incandescent body, shining with its own light, and at that time it, too, would have yielded a spectrum showing of what substances it consisted. =4. The Moon.= The earth is a satellite of the sun, and the moon is a satellite of the earth. The mean, or average, distance of the sun from the earth is about 93,000,000 miles; the mean distance of the moon is a little less than 239,000 miles. This distance is variable to the extent of about 31,000 miles, owing to the eccentricity of the moon's orbit about the earth. That is, the moon is sometimes nearly 253,000 miles away, and sometimes only about 221,600. The diameter of the moon is 2163 miles. Its bulk is one-forty-ninth that of the earth, but its mass is only one-eightieth, because its mean density is only about six-tenths as great as the earth's. The moon appears to travel in an orbit round the earth, but in fact the orbit is always concave toward the sun, and the disturbing attraction of the earth, as the two move together round the sun, causes it to appear now on one side and now on the other. But we may treat the moon's orbit as if the earth were the true centre of force, the attraction of the sun being regarded as the disturbing element. According to a mathematical theory, which has been largely accepted as probably true, but into which we cannot enter here (see Prof. George Darwin's _The Tides_, or Prof. R. Ball's _Time and Tide_), the moon was thrown off from the earth many ages ago, as a consequence of tidal “friction.” As it moves round in its orbit the moon keeps the same face toward the earth. This fact is also ascribed to tidal influence. [Illustration: =Spiral Nebula in Cepheus (H IV 76)= Photographed at the Lick Observatory by J. E. Keeler, with the Crossley reflector. Exposure four hours. Observe that the central portion is only of stellar magnitude. ] [Illustration: =Nebulous Groundwork in Taurus= Photographed at the Yerkes Observatory by E. E. Barnard with 10-inch Bruce telescope. Exposure six hours twenty-eight minutes. Prof. Barnard has suggested that some of these dark lanes in rich regions of stars are non-luminous nebulæ. ]