A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 3 



The story of the physical conditions of the moon had best be begun by 

 noting that the relation of our satellite to a larger sphere is not exceptional, but 

 the most characteristic of all the relations of one stellar body to another. Of the 

 planets in the solar system, all save the two nearest to the sun. Mercury and 

 Venus, have one or more smaller spheres circling about them. The relation of 

 the sun to the several planets in a larger way repeats this plan of grouping lesser 

 about greater orbs. 



It is generally believed by astronomers that the celestial spheres have been 

 formed by a process of condensation, due to gravitation, of matter which was 

 originally widely diffused ; that our solar system, before it was organized into the 

 sun and lesser bodies, was in the form of a diffused nebulous mass of spheroidal 

 form which extended beyond the orbit of the outermost planet. As this matter 

 gathered towards the center, the material now in each of the planets and its satel- 

 lites parted from the parent body, probably at first in the form of a nebulous 

 ring, or spiral, which in time broke and gathered into a spheroidal mass. In 

 that detached portion of the parent nebula the process of concentration was 

 repeated, with the result that satellites, or, as we may term them, secondary 

 planets, were formed substantially as the greater spheres were set off from the 

 sun. There are many questions and doubts concerning the details of this nebular 

 theory, but that the evolution of our solar system and probably of all stellar sys- 

 tems took place in substantially the manner indicated appears to be eminently 

 probable ; it is, indeed, fairly well established by what we know of the distant 

 nebulje and by the rings of Saturn, which apparently contain the material which 

 normally should have formed one or more of its satellites, but which for some 

 unknown reason have remained unbroken. 



It is not certain at just what stage in the concentration of a nebula a planet 

 or a satellite may be set off from the parent body ; nor can the present distance 

 of the satellite from the main sphere be assumed as that at which the parting took 

 place. It is possible that the concentration of the parent body had gone so far 

 that the diffused or nebulous stage of its materials had been passed by and the 

 more advanced stage of igneous fluidity entered on. It is, however, more likely 

 that in all cases the separation occurred while the particles of matter were di- 

 vided as they are in a gas or vapor. As soon as the two spheres are separated 

 from one another, and so long as they remain in any measure fluid, the difference 

 in their gravitative attraction on the nearer and more remote part of their masses 

 induces tides, and the effect of these tidal movements, as has been shown by 

 Professor George Darwin, is necessarily to impel the two bodies farther apart. 

 It seems certain that before the earth and the moon became essentially rigid, as 

 they now are, the effect of these tides in driving them apart must have been 

 o-reat enough to account for a considerable part of the interval which now 



separates them. 



In the present condition of the moon, it is a sphere having a computed 

 diameter of 2159.6 miles and its mean distance from the earth 238,818 miles. So 

 far as has been determined, the moon exhibits no trace of flattening at the poles 



