Page:The Moon (Pickering).djvu/25

Rh much as a wheel might revolve between two brakes. The result of this action is that the rotation of the Earth upon its axis is being gradually retarded, so that each day is a little longer than its predecessor. This action is so slight, however, that the change in the length of the day since the date of the Christian era is only a small fraction of a second. In the early geological times, on the other hand, when the interior of the Earth was molten, the effect was much more marked than at present, as an internal tide was then produced, distorting the surface much as our present tides distort an ice floe. In these early days the terrestrial day was much shorter than it is at present.

But if the Earth's rotation is being retarded by this action, since action and reaction are equal and opposite, the motion of the Moon in its orbit must be correspondingly accelerated. In fact, we have here a force which is constantly pulling the Moon forward. But if we increase the speed of a body in its orbit, its centrifugal force is increased, with the result that the body tends to fly away from its centre of revolution. As the result of this tidal action, therefore, the Moon is constantly moving away farther and farther from the Earth, and thus constantly traveling in a larger orbit. But if the Moon's orbit will be larger to-morrow than it is to-day, to-day it must be larger than it was yesterday, and if we carry our reasoning back into past geological times we come to a period when the distance of the Moon was only one-tenth of what it is to-day, and still further back it must have been almost touching the surface of the Earth. At that time our day was only about three of our present hours in length.

Now it can be shown that if our Earth were to revolve on its axis in rather less than three hours, instead of once in twenty-four, portions of it would be liable to fly away by centrifugal force. As the original Earth cooled and contracted from its nebulous form its rate of rotation upon its axis must have steadily increased. This increase was reduced somewhat by the powerful solar tides then in existence, but in spite of them the period finally shortened to about three hours. The force of gravity at the equator thus became less and less, and the solar tides in consequence higher and higher, until one day a catastrophe occurred, a catastrophe of such magnitude as has never been seen upon the Earth before or since—five thousand million cubic miles of material left the Earth's surface never again to return to it. Whether it all left at once or whether the action was prolonged we do not know, but we may try in vain to imagine the awful uproar and fearful volcanic phenomena exhibited when a planet was cleft in twain and a new planet was born into the solar system.