At present it would seem impossible to speak with certainty as to the precise history of the solar system. Meanwhile, we may console ourselves with the closing words of Professor Jeans’ paper, to the effect that “no difficulty need be experienced in referring existing planetary systems to a nebulous or meteoric origin on the ground that the configurations of these systems are not such as could have originated out of a rotating mass of liquid.” An investigation by Sir George Darwin, which has furnished inspiration to such hypotheses as that of Jeans, brings us nearer the immediate subject of this essay, since it treats of one of the last acts in the great drama of planetary existence, and attempts to derive the earth and moon from a common origin in a single rotating sphere. [Sidenote: Why the Day is Growing Longer] It is well known that, owing to the frictional effects produced by the tides, the earth is being gradually slowed down as it rotates upon its axis. Thus the day is constantly getting longer, so that in a few millions of years it will have increased in length from twenty-four to twenty-five hours. On the other hand, in past time it must have been shorter than at present: a few millions of years ago it was only twenty-three hours in length, and many millions of years earlier it was still less, only some five hours or so. At that time the earth was hotter than it is now, less rigid, more yielding, and, owing to its rapid rotation, less stable. The action on the moon of the tides produced in it by the earth is similar, and the rotation of the moon has been so far diminished by them that its day has become as long as the month--_i.e._, our satellite only turns once round on its axis in the time that it takes to revolve once round the earth; it is for this reason that our satellite keeps always the same face turned towards us. [Sidenote: The Moon Was Part of Our Sphere] The retardation of the earth in its rotation has, however, a very remarkable effect on the revolution of the moon; it involves--by the principle of the conservation of moment of momentum--a corresponding acceleration of the moon in its orbit, and, as a consequence of this, an enlargement of this orbit--that is, the moon is pushed away from us, as it were, and thus becomes more remote. But if so, the moon must have been nearer to us in times past. It is possible to trace the approach of the moon to the earth as we go backwards in time till the distance between them was only two and a half terrestrial radii instead of the sixty radii which now separate them. Mathematics do not take us farther back than this. But it is difficult to resist the suggestion that in the immediately preceding stage of development the earth and moon formed together a single sphere.