THE PROBLEM OF THE MOON'S ORIGIN 



By B. G. HARRISON, F.R.A.S., F.R.G.S. 

 {Continued from page 74). 



The system consists of seven principal planets, 

 besides our earth, in various stages of evolution, and 

 rotating at different speeds, hut the conditions 

 affecting each one are so diverse from those of our 

 own globe that no satisfactory analogy can be drawn. 

 It is true that the rotations of Jupiter and Saturn are 

 exceedingly rapid, but this is just what we should 

 expect if the theory of planetary formation previously 

 outlined is correct, since the larger planet would have 

 a larger moment of momentum than would be derived 

 from its extra mass alone, owing to the more rapid 

 circulation of particles caused by greater gravitative 

 force in its vortex during the planet's evolution. 



Another objection to the fission theory of the 

 Earth is the difficulty of understanding how it would 

 be possible for a concrete mass the size of the Moon, 

 torn away from the Earth by rapidity of rotation, to 

 hold together under the disruptive strain imposed by 

 the mutual attraction of the two bodies when in close 

 proximity. 



Thus it seems, so far as we are aware, that the 

 origin of the lunar terrestrial system by capture 

 offers none of the difficulties that we have to contend 

 with in the fission theory, and if we accept the 

 nebular hypothesis outlined earlier in this article, 

 would seem the most natural and concordant way of 

 accounting for the present position of the Moon. 



Nevertheless, weighty though the objections are 

 to the theory of the Moon ever having formed part of 

 the Earth, our satellite offers a very remarkable 

 piece of evidence in favour of this supposition.* 

 This evidence consists in the action of tidal friction. 

 The manner in which the Moon raises tides by 

 differential attraction is too well known to require 

 any explanation here. If no tidal friction existed, 

 high water would always be on those parts of the 

 Earth's surface directly under the Moon. As, how- 

 ever, water is not perfectly frictionless, the rotation of 

 our globe carries these aqueous bulges to a point some- 

 what in advance of the Moon's orbital position, and 

 they consequently act as a brake on terrestrial rota- 

 tion. Since it is impossible for one body to retard 

 another without a corresponding acceleration in its 

 own motion, it follows that the energy lost by the 

 Earth must be transmitted to the Moon. This has 

 the effect of increasing its orbital force, and under 

 the influence of centrifugal action driving it further 

 away from us. As, however, this force is only tan- 

 gential, and the action of recession involves motion 

 against the more powerful pull of direct attraction, 



the actual orbital velocity is decreased, and the addi- 

 tional energy transmitted becomes entirely potential. 

 Thus we see our day and lunar month both slowly 

 increasing in length, although not at the same rate. 

 Conversely, in earlier times both must have been 

 shorter. If this decrease had been a measurable 

 quantity in historic times, we should be able to detect 

 it from the record of the early eclipses ; for although 

 it is necessary to have the utmost accuracy in the 

 majority of astronomical observations if they are to 

 be of any value, in this case the knowledge of the 

 locality in which the eclipse was observed would to 

 a great extent compensate for the lack of precision 

 in the record of its actual time of occurrence. Yet 

 it appears that there is no conclusive information to 

 be obtained from this source, and its absence makes 

 us feel certain that retardation of rotation must now 

 be an exceedingly slow process. Nevertheless, this 

 is no proof of its non-existence, and since the loss of 

 energy by friction varies inversely as the sixth power 

 of the distance, any reduction of the Moon's orbit 

 would involve a vast increase in the retardation of 

 our axial rotation. 



It is possible, therefore, to imagine a time when 

 our day was only half its present length, and to 

 calculate, by deducting the energy which the Earth 

 has since that time transmitted to the Moon, what 

 the length of the month would have been at the 

 same period. At the present time the rate of increase 

 of the day is more rapid than that of the month, and, 

 consequently, if we work backwards, we find the 

 month shortening less rapidly than the day. This 

 relative progression continues until a time is reached 

 when there were twenty-nine days to the month, 

 instead of 27-3 as at present. This is a maximum, 

 and it has been mathematically demonstrated that 

 there could never have been more than this number 

 of days during one revolution of the Moon. Remote 

 though this period must actually have been, it 

 is yet comparatively recent when we consider 

 the vast time necessary for the entire process 

 required by this theory, and may be regarded 

 as a crisis in our satellite's history. Before this 

 epoch the month must have decreased more 

 quickly than the day, and, still travelling back- 

 wards, we can trace the Moon in the course of ages 

 moving ever faster in a huge spiral path. Rapidly, 

 now, though the acceleration of the Earth's rotation 

 must have proceeded, the Moon continued to gain 

 upon it until eventually both were revolving together, 



: ' The following theory is due to the researches of the late Sir George Darwin, F.R.S., from whose papers on the subject 



the figures and general particulars have been obtained. 



91 



