92 



KNOWLEDGE. 



March, 1913. 



the Moon close to the Earth, and each permanently 

 presenting the same face to the other. Now, 

 when this period was reached, both day and 

 month would have occupied some time between 

 three and five hours. It has been previously 

 mentioned that the speed of rotation necessary 

 to render the Earth unstable, owing to centri- 

 fugal force, would be about two and three-quarter 

 hours, so we have now arrived at an epoch when our 

 planet would barely be able to hold together. It is 

 assumed that the earth must at this time have been, 

 if not actually in a molten condition, at least 

 considerably more plastic than it is at present. 

 Even now the tidal action of the Sun is shown by 

 the difference between spring and neap tides, and the 

 more plastic the condition of the Earth the more 

 effective would this be. It is thought probable that 

 when our globe was rotating at this speed the forced 

 period of the tide would be in close agreement with 

 its free period, which would have the effect of increas- 

 ing the height of the solar tides sufficiently to render 

 it quite possible for the Earth to break up under the 

 combined strain, and thus give birth to the Moon. 



The principal difficulty which confronts us is our 

 inability to understand how it would be possible for 

 the latter to hold together under the strain imposed 

 upon it by the attraction of the Earth, and it is 

 suggested that the Moon was thrown off as a flock 

 of meteorites until it reached a sufficient distance to 

 allow it to condense into one bod} - . Nevertheless, 

 this suggestion is not very satisfactory, since the 

 meteorites would necessarily be at varying distances 

 from the Earth, and so would be travelling at 

 different velocities to preserve their equilibrium. 

 This would have the. effect of scattering them all 

 round their orbit, and so tend to nullify their 

 power of creating tidal friction. However, if this 

 difficulty is put aside, the facts brought out by 

 Sir G. H. Darwin's analysis are really remarkable, 

 and it seems more than difficult to ascribe this 

 wonderful coincidence to chance, especially as 

 the eccentricity of the Moon's orbit and the 

 obliquity of the ecliptic also harmonise with this 

 theory. If the Moon originated in any other way 

 the chances are enormously against the mutual 

 reaction of the two bodies being exactly as we find 

 them. To make this clearer let us assume the 

 present position of the Moon and the length of our 

 day to be unknown, but that we had evidence of our 

 satellite having originated in the way just described; 

 then, by calculating the force of tidal action, we 

 should find that this would be just sufficient to have 

 driven a body of the Moon's dimensions to a distance 

 of two hundred and thirty-nine thousand miles, and 

 reduced the rotation of the earth to its present period 

 of twenty-three hours fifty-six minutes. 



It is interesting to pursue this theory further, and 

 try to penetrate our planet's future in the same way 

 as we have endeavoured to trace its past history. 

 For, if no lunar energy is being lost by motion 

 against resistance, the length of the day must be 

 increasing more rapidly than that of the month. 



Consequently, we foresee a time when our day is twice 

 its present length, and the month containing only 

 eighteen of these days. Mathematical analysis 

 enables us to penetrate still further, until we reach a 

 period when the day and month are again equal, but 

 instead of being only four hours long, they extend to 

 about fifty of our present days. There is also this 

 difference that, whereas in the former case the Moon 

 was revolving in a state of unstable equilibrium, in 

 the latter it is dynamically stable. The time 

 required before the two bodies can reach this con- 

 dition is so vast that it passes human comprehension, 

 and it is more than likely that some third factor will 

 have interposed before the requisite period has 

 elapsed. However, if the system is still behaving in 

 its present orderly fashion, it is possible to forecast 

 the final act in this drama. We have reached a 

 time when the Earth and Moon are revolving as 

 if bound together by steel bars, and consequently 

 causing no tidal friction in each other. 



Nevertheless, the action of the Sun will continue 

 to affect the Earth, and the latter will consequently 

 commence to rotate more slowly than the Moon 

 revolves. This will cause a recurrence of tidal 

 friction between the two bodies, but with this 

 difference, that now the protuberance caused by the 

 Moon will be behind it instead of in front. This 

 will naturally have the opposite effect on the lunar 

 orbit and will gradually draw the satellite back in 

 ever decreasing spirals, accelerating the Earth's 

 rotation in the process, until it finally returns to the 

 surface of the latter. This time however there will 

 be no chance of the Moon's rebirth, as the earth will 

 have lost so much energy owing to solar friction 

 that even in the event of the Moon having 

 encountered no resistance in its celestial journey, 

 and so being able to return the whole of the energy 

 transmitted to it by the Earth, the latter's rate of 

 rotation will still be slower than at its birth. 



There is one more point to be considered, namely 

 the movement of Phobos, the inner satellite of Mars. 

 The motion of this satellite has frequently been 

 advanced as a proof of the former rapid rotation of 

 its planet, and so indirectly of the Earth. 



It was discovered in 1877 by Professor Hall, and 

 was found to revolve round its primary in seven hours 

 thirty-nine minutes at a distance of less than four 

 thousand miles from its surface. As the Martian 

 day exceeds our own by more than half an hour this 

 system appears to be absolutely unique, and its 

 discovery electrified the whole astronomical world. 

 It has been suggested, therefore, that this system 

 has been evolved by fission, and has already reached 

 its final stage with its day longer than its month ; 

 the rotation of Mars having been reduced to its 

 present period by solar tidal friction. It is unlikely, 

 however, that this can be the case, since the com- 

 parative weakness of the solar tides on the planet, 

 and the exceedingly small orbital momentum of the 

 satellite, present very considerable difficulties to the 

 supposition. Moreover, the phenomenon can quite 

 well be accounted for by the theory of capture in a 



