44 2 



NA rURE 



[August 31, 1905 



theory of which I shall have next to speak claims to trace 

 the gradual departure of the moon from an original position 

 not far removed from the present surface of the earth. If 

 this view is correct, we may suppose that the detachment 

 of the moon from the earth occurred as a single portion 

 of matter, and not as a concentration of a Laplacian ring. 



If a planet is covered with oceans of water and air, or 

 if it is formed of plastic molten rock, tidal oscillations 

 must be generated in its mobile parts by the attractions of 

 its satellites and of the sun. Such movements must be 

 subject to frictional resistance, and the planet's rotation 

 will be slowly retarded by tidal friction in much the same 

 way that a fly-wheel is gradually stopped by any external 

 cause of friction. Since action and reaction are equal 

 and opposite, the action of the satellites on the planet, 

 which causes the tidal friction of which I speak, must 

 correspond to a reaction of the planet on the motion of the 

 satellites. 



At any moment of time we may regard the system com- 

 posed of the rotating planet with its attendant satellite 

 as a stable species of motion, but the friction of the tides 

 introduces forces which produce a continuous, although 

 slow, transformation in the configuration. It is, then, 

 clearly of interest to trace backwards in time the changes 

 produced by such a continuously acting cause, and to 

 determine the initial condition from which the system of 

 planet and satellite must have been slowly degrading. We 

 may also look forward, and discover whither the trans- 

 formation tends. 



Let us consider, then, the motion of the earth and moon 

 revolving in company round the sun, on the supposition 

 that the friction of the tides in the earth is the only 

 effective cause of change. We are, in fact, to discuss a 

 working model of the system, analogous to those of which 

 I have so often spoken before. 



This is not the time to attempt a complete e.xposition 

 of the manner in which tidal friction gives rise to the 

 action and reaction between planet and satellite, nor shall 

 I discuss in detail the effects of various kinds which are 

 produced by this cause. It must suffice to set forth the 

 results in their main outlines, and, as in connection with 

 the topic of evolution retrospect is perhaps of greater 

 interest than prophecy, I shall begin with the consider- 

 ation of the past. 



.At the present time the moon, moving at a distance of 

 240,000 miles from the earth, completes her circuit in 

 twenty-seven days. Since a day is the time of one rota- 

 tion of the earth on its axis, the angular motion of the 

 earth is twenty-seven times as rapid as that of the moon. 



Tidal friction acts as a brake on the earth, and there- 

 fore we look back in retrospect to times when the day 

 was successively twenty-three, twenty-two, twenty-one of 

 our present hours in length, and so on backward to still 

 shorter days. But during all this time the reaction on 

 the moon was at work, and it appears that its effect must 

 have been such that the moon also revolved round the 

 earth in a shorter period than it does now ; thus the month 

 also was shorter in absolute time than it now is. These 

 conclusions are absolutely certain, although the effects on 

 the motions of the earth and of the moon are so gradual 

 that they can only doubtfully be detected by the most 

 refined astronomical measurements. 



We take the " day," regarding it as a period of variable 

 length, to mean the time occupied by a single rotation of 

 the earth on its axis; and the " month," likewise variable 

 in absolute length, to mean the time occupied bv the 

 moon in a single revolution round the earth. Then, 

 although there are now twenty-seven davs in a month! 

 and although both day and nionth were' shorter in the 

 past, yet there is, so far, nothing to tell us whether there 

 were more or fewer days in the month in the past. For if 

 the day is now being prolonged more rapidly than the 

 month, the number of days in the month was greater in 

 the past than it now is ; and if the converse were true, 

 the number of days in the month was less. 



Now it appears from mathematical calculation that the 

 day must now be suffering a greater degree of prolongation 

 than the nionth. and accordingly in retrospect we look 

 back to a time when there were more days in the month 

 than at present. That number was once twenty-nine, in 

 place of the present twenty-seven ; but the epoch of twenty- 

 VO. 1870, VOL. 72] 



nine days in the month is a sort of crisis in the history 

 of moon and earth, for yet earlier the day was shortening 

 less rapidly than the month. Hence, earlier than the time 

 when there were twenty-nine days in the month, there was 

 a time when there was a reversion to the present smaller 

 number of days. 



We thus arrive at the curious conclusion that there 

 is a certain number of days to the nionth, namely twenty- 

 nine, which can never have been exceeded, and we find 

 that this crisis was passed through by the earth and moon 

 recently ; but, of course, a recent event in such a long 

 history may be one which happened some millions of years 

 ago. 



Continuing our retrospect beyond this crisis, both day 

 and month are found continuously shortening, and the 

 number of days in the month continues to fall. No change 

 in conditions which we need pause to consider now super- 

 venes, and we may ask at once, what is the initial stage 

 to which the gradual transformation points? 1 say, then, 

 that' on following the argument to its end the system may 

 be traced back to a time when the day and nionth were 

 identical in length, and were both only about four or five 

 of our present hours. The identity of day and month 

 means that the moon was always opposite to the same 

 side of the earth ; thus at the beginning the earth always 

 presented the same face to the moon, just as the moon 

 now always shows the same face to us. Moreover, when 

 the month was only some four or five of our present hours 

 in length the moon must have been only a few thousand 

 miles from the earth's surface — a great contrast with the 

 present distance of 240,000 miles. 



It might well be argued from this conclusion alone that 

 the moon separated from the earth more or less as a single 

 portion of matter at a time immediately antecedent to the 

 initial stage to which she has been traced. But there e.xists 

 a yet more weighty argument favourable to this view, for 

 it appears that the initial stage is one in which the stability 

 of the species of motion is tottering, so that the system 

 presents the characteristic of a transitional form, which we 

 have seen to denote a change of type or species in a 

 previous case. 



In discussing the transformations of a liquid planet we 

 saw the tendency of the single mass to divide into two 

 portions, although we failed to extend the rigorous argu- 

 ment back to the actual moment of separation ; and now 

 we seem to reach a similar crisis from the opposite end, 

 when in retrospect we trace back the system to two masses 

 of unequal size in close pro.ximity with one another. The 

 argument almost carries conviction with it, but I have 

 necessarily been compelled to pass over various doubtful 

 points. 



Time is wanting to consider other subjects worthy of 

 notice which arise out of this problem, yet I wish to point 

 out that the earth's axis must once have been less tilted 

 over with reference to the sun than it is now, so that the 

 obliquity of the ecliptic receives at least a partial explan- 

 ation. Again, the inclination of the moon's orbit may be 

 in great measure explained ; and, lastly, the moon must 

 once have moved in a nearly circular path. The fact 

 that tidal friction is competent to explain the eccentricity 

 of an orbit has been applied in a manner to which I shall 

 have occasion to return hereafter. 



In my paper on this subject I summed up the dis- 

 cussion in the following words, which I still see no reason 

 to retract : — 



" The argument reposes on the imperfect rigidity of 

 solids, and on the internal friction of semi-solids and 

 fluids; these are verae causae. Thus changes of the kind 

 here discussed must be going on, and must have gone on 

 in the past. And for this history of the earth and moon 

 to be true throughout it is only necessary to postulate 

 a sufficient lapse of time, and that there is not enough 

 matter diffused through space materially to resist the 

 motions of the moon and earth in perhaps several hundred 

 million years. 



" It hardly seems too much to say that granting these 

 two postulates and the existence of a primeval planet, 

 such as that above described, then a svstem would neces- 

 sarily be developed which would bear a strong resemblance 

 to our own. 



" .\ theory, reposing on vciac causae, which brings into 



