The Moon and the Earth.

Otago Witness, Issue 1581, 11 March 1882, Page 29

The Moon and the Earth.

We gave last week, an extract from the lecture delivered at the Midland Institute, Birmingham, by Professor Ball, dealing with the effect of the moon upon the tides of the earth. The following additional, extract from the same lecture gives the history (according to the Professor) of the relations of the moon and earth : — At the commencement of the history the two globes were quite close ; they were revolving rapidly, and the moon was constantly over the same locality on the primaeval earth. Ido not know where that locality was ;it was probably the part of the earth from which the moon had been detached. No doubt it was somewhere near the equator, but the distinction of land arid water had not then arisen. Around the primaeval earth the moon revolved in three hours ; the earth also revolved in three hours. This state of things formed what is known as unstable dynamical equilibrium. It could not last, Either, the moon must fall back again on the earth, and be reabsorbed into its mass, or the moon must commence to move away from the earth. Which of these two courses was the moon to take? The fact that the moon exists shows that it did not return to the earth, but that the moon adopted the other course, and commenced its outward journey. As the moon recedes, the period which it requires for a journey round the earth increases also. Initially that period was but three hours, and it has increased up until our present month of 656 hours. The rotation of the earth has been modified by the retreat of the moon. Directly the moon began to retreat the earth was no longer under an obligation to keep the same face thereto. When the moon waß at a certain distance the earth made two rotations for every revolution that the moon made. Still the moon got farther and farther away until the earth performed three, four, and more rotations for each of the moon's revolutions. Do not infer that the rate of the earth's rotation was increasing ; the contrary is the fact. The earth's rotation was getting slower, and so was that of the moon ; but the retardation of the moon was much greater than that of the earth. Even though the rotation of the earth was much more than the primitive three hours, yet that of the moon had increased to several times the rotation of" the earth. The moon receded still further and further, and at length a noticeable epoch was reached, to which I must call attention. At that epoch the moon was so far out that its revolution took 29 times as long as the rotation of the earth. The month was then 29 times the day. The duration of the day was less than the present 24 hours, but I do not believe it was very much less. The time we are speaking of is not very remote, perhaps only a veryfew million years ago. The month was then in the zenith of its glory. The month was never 29 timos as long as the day before. It has never been 29 times as long as the day since. It will never be 29 times as long as the day again. Resuming our history, we find the moon still continuing to revolve in an ever- widening circle, the length of the month and of the day both increasing. The ratio of the day to the month was still undergoing a change. When the moon was a little farther off the earth only revolved 28 times instead of 29 times in one revolution of the moon. Still the velocity of the earth abates until it only makes 27 revolutions in one revolution of the moon. This is an epoch of especial interest, for it is the present time. In the present order of things the moon revolves round the earth once while the earth rotates 27 times. This has remained sensibly true for thousands of years, and no doubt will remain sensibly true for thousands of years to come, but it will not remain true indefinitely. Wondrous as are the changes which have occurred in times past, not less wondrous are the changes whicn are to occur in time to come. The tides have guided our gropings in the past ; they will continue to guide our researches to make a forecast of the future. Farther and farther will the moon retreat, and more and more slowly will the earth revolve. But we shall not pause at intervening stages ; we shall try to sketch the ultimate type to which our system tends. In the dim future, many millions of years distant, the final stage will bo approached. Ac; this stage draws nigh, the rotation of the earth will again ap-

proach to .equality witn 1 thfc revolution of the moon. From the present irrOWth 1 of 27 days we shall pass to a month of 2© days, ol 25 days, and so on, until eventually we shall reach a month of two days, and lastly a; month of one day. When this state has been attained ttie earth will cotatantly turn the same region towards the moon. I oo .not know whatisthe locality on theearth which is destined for this distinction. Here you see that the! test' state and the last state of the earth-moon history are in ono sense identical. In each case fee' same? face of the earth is constantly directed towards tttef pioCn; In another way now •different are the' first 1 stage and the last! At the beginning the day ita'OMle month Were both equal, and they were eaoh three' hours, At the end the day and the month Will be again equal, but they will each be 1400 hours.^ The moon will then go round the earth in 1400 hours, while the earth will rotate on its axis in the same time. In other words, the day is destined in the veTy remote future to become as long as 57 of our days. This epoch will assuredly come if the universe ksts long enough. When it has come it will endure for' countless ages. It would endure for ever if the earth and the moon could be isolated from all external interference.

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