I have been thus particular in unfolding somewhat fully the present state of our knowledge concerning the twinkling of the stars, because it is evident that there are many interesting points connected with it, which may be studied by any patient and attentive star-gazer, and which do not need the instrumental appliances and technical refinements which are only to be found in fully-equipped public and private observatories. It should be mentioned in conclusion that the planets twinkle very little, or, more often, not at all. This is mainly due to the fact that they exhibit discs of sensible diameter and therefore that there is, as Young puts it, “a general unchanging average of brightness for the sum total of all the luminous points of which the disc is composed. When, for instance, point A of the disc becomes dark for a moment, point B, very near to it, is just as likely to become bright; the interference conditions being different for the 2 points. The different points of the disc _do not keep step_, so to speak, in their twinkling.” The non-twinkling of planets because they possess sensible discs is often available as a means for determining when a planet is looked for, which, of several objects looked at, is the planet wanted and which are merely stars. CHAPTER VI. THE MOON. The Moon being merely the satellite of a planet, to wit, the Earth, it should, according to the plan of this book, be included in the chapter which deals with its primary; but for us inhabitants of the Earth the Moon has so many special features of interest that it will be better to give it a special chapter to itself.