The past and future our satellite has been tne subject of interesting re searches during the last 30 years, principally undertaken by Prof. George Howard Dar win. The chief influence in its life history is assigned by him to tidal action, an idea first broached in 1754 by the philosopher Emmanuel Kant. Imagining that the earth, unattended by any satellite, was 'in a molten condition—the most probable supposition in view of its nearly spherical form and the nature of its surface— Professor Darwin inquires under what condi tions its rotation should be so rapid that, like a bursting fly-wheel, portions of matter should be thrown off. He finds that this would happen if the earth were rotating about its axis in some thing like two hours. He has also shown that pear-shaped and dumb-bell shaped forms will result from a rapid rotation and from the ac tion of the sun in producing tides; the moon might have been evolved from such a figure by the thinning and breaking of the handle. The earth would then have a satellite revolving very close to its surface in the same time that the earth took to turn on its axis. Either of these hypotheses seems to be allowable.
Past From this point, rigorous argument takes the place of speculation. The period of the satellite's revolution (a °month') begins to be slightly longer than that of the earth's rotation on its axis (a °days). The moon raises tides on the earth and the friction of these, like a break on a fly-wheel, gradually lengthens the day; the reactionary effect on the moon is to increase the length of the °month' also, but more rapidly than the May.' This goes on until the number of oclayss in a is 29. After this date the month and day still go on getting longer, but at different comparative rates, so that the number of days in a month diminishes until it is 27 1/3— the present condition. The time in which all these changes have taken place must be reckoned in millions or tens of millions of years and though both bodies have grown cool and solid, the water on the earth's surface has in the later portion maintained the process at a slower rate.
All this time the tidal friction has also had the effect of sending the moon further and further away from the earth.
In the future, the same processes will continue. The lengths of the month and that of the day will continue to increase, the latter overtaking the former until they are both over 1,300 hours or about 55 of our present days. Then, not only will the moon turn al ways the same face to the earth, but the earth will turn the same face to the moon and they will move together as if strung on a rigid bar. The same causes have operated to cause the moon always to present the same face to the earth. When the satellite was in a molten condition and not far from the earth, the tides raised on it by the attraction of the earth must have been enormous. The friction of these tides passing over the surface of the moon then slowed down the rotation of the moon on its axis until it was in a position where no more friction could act: that is, high tide on the moon was always at the same place on the moon and just under the earth. The axis of the moon in the direction of the earth must from this cause be a little longer than the other axis, though the differ ence has not been certainly observed.
Besides the numerous text books on general astronomy, all of which con tain information, readers may consult Nasmyth, 'The Moon' (2d ed., 1903) ; Nelson. 'The Moon> (1873) ; Proctor, 'The (1876) ; the Theory' (1896) by the writer is a mathematical treatise on its motions. (The Tides,' by Darwin, contains the latest theories concerning its past and future history. A new photographic atlas and description by Pickering has just appeared. The Lick and Paris ob servatories are also gathering materials for a complete description and charts of the surface.