The date of this event cannot be fixed even approximately, but we know that it must have been rather recent, as astronomical events go, since the Earth must have been already condensed from a huge gaseous mass to a comparatively small solid or liquid form, not far from its present bulk. Indeed, the Moon must be one of the younger members of the solar system. At the same time, its age cannot be less than fifty million years, and may be much greater.
At its first appearance the shape of the Moon was not spherical, as the Earth would not have permitted so large a body to retain this shape so near to its own surface, but its present form was probably assumed after it had escaped to a distance of a few thousand miles.
The question may now occur to the reader, if the Earth gave birth in this manner to the Moon, why should not Venus and Mercury also have been supplied with companions? The explanation probably lies in the intensity of the solar tides, which would be many times greater upon those planets than upon the Earth and would thus effectually prevent their attaining a sufficiently high speed of rotation to throw off any satellites.
Looking forward now into the future, we find that the distance of the Moon must continue to increase, and the length of the terrestrial day also, but only up to certain limiting figures. When the distance of the Moon has increased by 100,000 miles, or to about 350,000 miles, the length of the lunar month will be fifty of our present days. At that remote epoch our day will have increased also to fifty of our present days in length; and the Earth and Moon will turn constantly the same face to one another as they did in the beginning. Thus the Moon will cease to rise and set, there will be one spot on the Earth where the Moon will be constantly overhead, and on the other side of the Earth the Moon will never be visible. This is assuming that at that remote time we still have seas and tides; but if we do not, which now seems far more probable, this latter condition of affairs will never be reached. It merely represents a goal toward which we are at present aiming.
Many persons while studying geography have been struck by the fact that the Atlantic coast lines of the eastern and western continents are strikingly similar, especially South America and Africa. Indeed, if we were to draw a straight line from Cape Sable, Nova Scotia, to Dutch Guiana, and fill the region to the west of it, a glance at a terrestrial globe will show that the western continent if carried eastward 2,500 miles would exactly fit into the eastern one from Behring's Strait nearly to Cape Horn.
