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DOES THE MOON ROTATE ON ITS OWN AXIS ?
CONTROVERSY on this subject in the Times was lately opened 1 by a letter from Mr. Jelinger Symons, inquiring the grounds on which the received opinion rested; the subject was certainly not taken up in the philosophical manner that might have been expected by the correspondents whose replies were published, and we are therefore induced, from the importance of the question, to give it a large place in our pages.
We may state the point at issue thus :-It is granted on both sides, that the Moon revolves round the Earth in 27 days 7 h. 43 m., or a lunar month, presenting always the same hemisphere to us; and that the Earth and Moon together revolve round the Sun in a year; but there is likewise attributed to the Moon a rotation on her own axis, performed in exactly the same period as her revolution round the Earth, an assumption which is considered necessary to explain the well-known fact that the same parts of the Moon are always presented to us. This would, indeed, if true, be a curious coincidence ; still, of course, it is not for this cause that we are disposed to doubt the truth of this elaborate hypothesis, but rather because the actual appearance of the Moon is best explained otherwise, and, indeed, because, were it true, the Moon would present to us every side in the course of a month. · Mr. Symons, in his letter to the Times, suggests that the Moon“ performs precisely the same motion in relation to the Earth that a point on the tire of a wheel does to the box or axle, or that the round end of the minute hand of a watch does to the pivot in the centre.” Other illustrations might be multiplied ; such as an inkstand near the rim of a round table, the table revolving round its centre; or a ball held in the hand at arm's length, while we turn round upon our heel. Of course the observer's eye must be within the circle, or near the point that represents the Earth ; for to an observer without the circle, as in the Sun or a fixed star, every side of the Moon will present itself in the course of a revolution, or a lunar month.
The following will give an experimental illustration of the subject :
Let A be a ball representing the Earth, B a smaller ball, representing the Moon, and C a hand connecting them, and moving freely round on a pivot, through the centre of A. Let B also be fixed to the extremity of C, by another similar pivot. Mark the point in B nearest and opposite to A. Move the hand C entirely round, so as to complete the revolution round A, without touching the ball B. The point X will remain opposite to A, and complete its transit round it without varying that position, presenting the same face to A. This is precisely how the Moon revolves round the Earth, presenting the same face always to it. Now, turn the ball B on its pivot whilst it is making the same revolution as before, round A (as the school-book astronomers st tell us the Moon turns), and we find that it successively presents every portion of its own surface to A; nor will this effect of its totation on its axis be in the least degree affected by its revolution round 4. Ito. rotation will present precisely the same changes of surface whether it', stands still or revolves round A. The motion of the Moon and its relation to the Earth are simply centrifugal ; no other force acts on the Moon such as would be requisite to create rotation.
We can certainly explain satisfactorily as above the appearance actually presented to us by the Moon in her course ; and, therefore, we do not see why we should take into consideration the absolute motion in space of a point on the Moon's surface : our question is merely with our model, whether the lesser ball is to rotate on its own axis or no. Indeed there is no authority for assuming that the Moon has any axis at all ; we only know that its section towards us is circular, though, without doubt, if it rotated, and had its axis perpendicular to its orbit (both which facts are, in works on astronomy, laid down and taken for granted), we should expect to be able to observe its form to be spheroidal. We find it stated, likewise, that all the secondary planets, such as the satellites of Jupiter, rotate upon their axes in exactly the same period in which they revolve round their primaries ; whether this statement is based upon observation, or merely presumed from analogy, let astronomers tell.
The librations and the inclination of the Moon's orbit to the ecliptic present no difficulties in our theory, more than in that commonly received ; nor will it make any difference whether we consider the Moon to revolve round the Earth, or both Earth and Moon to revolve jointly round their common centre of gravity.
It would almost seem as though the rotation of the Moon on her axis must have been taken for granted and looked upon as a necessary thing to begin with, and then the current explanation as to her time of rotation adopted as an apparent means of reconciling facts.
The following are a few of the Letters written on this vexed question, most of which have not been previously published :
THE MOON'S MOTION.
TO THE EDITOR OF THE “ TIMES." SIR,- This subject excites so much interest, and I have received so many communications on both sides of the question, that I very reluctantly trespass once more on your courtesy, praying to be allowed to support my statement more fully, that the
Moon has no rotation on her axis. My opponents produce elaborate proofs of the palpable fact that she revolves round the Earth, turning the same hemisphere towards it. This is not axial rotation, such as the astronomical books attribute to the Moon. Nor is the difference one in terms only, but in dictinct kinds of motion, governed by different laws, and producing widely different results and phenomena. It is one with which every mechanic is familiar,
The rotation of a body round a centre distant from it, like the governor-balls of a steam-engine, cannot be performed without progressive motion round its own orbit; but rotation on its own axis can. Arrest the Earth in its revolution round the Sun, and it will still rotate on its axis, for it has axial rotation. Arrest the moon in her revolution round the Earth, and she will be motionless, because she has no axial rotation. To put this question to a practical test, let an orrery be constructed so that the Moon shall rotate on her axis, as we are told she does, in the same period in which she revolves round the Earth. Let this period be ten minutes. I affirm that at the expiration of five minutes after the orrery is put in motion, the Moon will present to the Earth exactly the opposite hemisphere of her surface to that which she presented to it at starting. If I am right in this, the question is settled, and the Moon has no axial rotation ; for at whatever rate she rotates a similar result will ensue.
The practical effects of the moon having no axial rotation seem to be chiefly these -and they are not unimportant :-The projectile force of the centifugal motion of the Moon will affect that side which is the most remote from us, causing her convexity on that side, together with the ascertained fact that her centre of gravity is on the other side of the centre of her circumference, and the probability that there are humidity, atmosphere, vegetation, and, possibly, inhabitants there. If the Moon had axial rotation the projectile force would take equal effect on the whole of her surface; she would be a spheroid oblate at the Poles, and these phenomena, discovered by the German astronomers, would be deprived of the support they derive from her axial immobility.
If I am in error in my humble effort to correct what appears to me to be a fallacy well worth removing, I shall be grateful to any one who sets me right by relevant reasoning ; but, at present, I confess that I am conscious only of the mistake of supposing that a question of science might now, even in England, be treated with candour and debated with temper.-I am, Sir, your obedient servant, April 12.
JELINGER SYMONS. P.S. Mr. Hopkins apprises me that he has already supported the same view as myself in his well-known work on Geology.
38, THURLOE SQUARE, BROMPTON, LONDON,
8th April, 1856. SIR, I was much pleased to see in the Times of to-day the question of the Moon's rotation brought forward by you. It is too bad that such glaring errors should be propagated in our schools.
In my work “ On the Connection of Geology and Magnetism,” I have noticed this error, as you will observe by the accompanying extract. The question was also discussed last year in the Mechanics' Magazine, but it is difficult to eradicate old prejudices. I shall watch the discussion with great interest, and probably join in it should the Editor encourage it. -I am, Sir, yours respectfully,
Extract from “ The Connection of Geology with Terrestrial Magnetism,” Second Edition,
“ It is said that the Moon turns on her own axis, notwithstanding that she always presents the same face towards the Earth. That a body revolving round a centre, and continuing to present the same face to that centre, should rotate on an axis situated parallel to the revolving axis, is physically impossible. Yet it is maintained, and absolutely stated in our astronomical works, that the Moon rotates on her axis.
• Suppose a balloon were to float around the earth, it would continue to present the car towards the earth, or centre of revolution, in the same manner as the keel of a vessel sailing round the earth ; yet neither can be said to have rotated on their own axis in making the circumnavigation.
" The governor-balls of a steam-engine make analogous movements, and it will be at once perceived that they merely revolve around the centre of revolution, and do not rotate on their respective axis."
38, THURLOE SQUARE, BROMPTON,
April 12th, 1856. SIR,-In reply to your favour of yesterday's date, I beg to state that I wrote two letters to the Editor of the Times in support of your argument, that the Moon does not rotate on an axis situated within herself, and that she only revolves round the earth as her centre of revolution.
I regret to observe, that the Editor has not only declined inserting my letters, and others which have been forwarded to him in your favour, but has allowed letters to be inserted which were unworthy of the argument, and a disgrace to common sense and men of science.
I recommend “ E. E. D.” to complete his demonstration, by forming a ring of boys, and make them walk round his chalk centre, and then I put the question to him if it was possible for the boys to rotate individually on their own axis ? Were one of the boys to be taken from the ring, and placed on the chalk centre, and keep his face constantly to one of the boys in the ring while they were walking round, the boy in the centre would rotate on his own axis, but those around him only revolved round his centre.
A correspondent asked, “Where rotation ended and revolution began ?” I answered, When the axis left the body and became situated without.
Again, I said that a cog-wheel, as a whole body, rotated on its own axis when it moved; but in speaking only about the teeth, then we say the latter revolve round the central axis, whilst the teeth continue to present the same parts towards the centre ; a tooth can only rotate on its own axis by taking it out of the rim of the wheel and fix it in the centre.
Again, place three balls,- one in the centre of motion, the second in the crank-pin, and the third on the connecting-rod, near the latter. The central ball will rotate on its own axis ; the ball in the crank-pin will revolve like a portion of a ring ; but the ball on the connecting-rod, although it moves round the centre, and presents different faces to that centre, yet it only oscillates like the connecting-rod. It is most extraordinary to me that such a simple question should cause difference of opinion amongst men who profess to know the difference between rotation and revolution.
I have disputed this, and many other errors which are propagated in our elementary works, for many years.
One of my supporters (Mr. Mushet) and the Cambridge men occupied the columns of the Mechanics' Magazine on the same subject, last year, for several months. The letters intended to support the Moon's rotation were as offensive and ridiculous as those which have appeared this week in the Times. .
You are perfectly at liberty to make use of my observations on the subject. It is high time that such unaccountable errors should be removed.—I am, Sir, yours truly, JELINGER SYMONS, Esq.,
EVAN HOPKINS. Her Majesty's Inspector of Schools,
April 8th, 1856. Mr. Roebuck remarked in the House on Friday evening, that few persons write letters to the Times without having cause afterwards to repent of it.
Mr. Jelinger Symons, in his letter to-day, headed “The Moon has no Rotary Motion," seems very likely to afford the earliest illustration of the truth of the observation,
Can that gentleman be aware that the difficulty he has propounded has furnished, time out of mind, a standing dish for discussion on small science ? that it has been ever the Pons Asinorum of tyro astronomers ?
· Can he not perceive that he himself has written a very sufficient answer to himself ? -in these words:
“ If the Earth moved round the Moon, then the rotation imputed to her in the same period as the Earth's revolution round her, would be necessary to her
presenting the same face to us." Nothing more true! and is it not so ? does not the Earth move round the Moon ?
In which of the school-books that come under the official inspection of Mr. Symons, has he found that the centre of rotation between the Earth and Moon is coincident with the centre of the Earth ?
Can he not perceive that no alteration in the situation of his pivot, along his slip of wood and two balls, can possibly alter the fact with respect to any of its parts !
ROTA TOTA. [A Copy of this communication is sent to Mr. Symons, lest the Times might not extend to it the same favour as to his own.]
GREENOCK, 12th April, 1856. SIR, I have seen the Times, and from the inclosed * you will easily conceive that I feel very much interest on the subject of the Moon's rotation. Your position in society, and clear view of the matter, must soon, in my opinion, put the scientific world to rights on the point at issue, despite the sneers of the ignorant.--I am, Sir, yours most respectfully,
ROBERT WILSON, D.D.. JELINGER SYMONS, Esq., Her Majesty's Inspector of Schools.
9, BURY COURT, ST. MARY AXE,
9th April, 1856. SIR,—Will you kindly allow me, through your Journal, to say a few words on the controversy about the Moon's rotation ?
The gentlemen opposed to the proposition of Mr. Symons seem to confound progressive motion with rotary progression, and I do not think I can more briefly and better illustrate the difference than by the wheel of a carriage. If drawn or propelled forward, by whatever force, the wheel will rotate round its own axis ; put the drag on to the wheel, and let the carriage be going round the Earth ; the wheel, like the Moon, will then always present the same part to the Earth; and arriving again at the point from which it started, it will have made a revolution round the Earth, round the Earth's axis, but not round its own.
A ship sailing round the world presents to the fishes always the same face as the Moon does to us. Coming home again, it will surely not be said that the ship has performed a revolution—a rotation round her own axis, but round the axis of the Earth, round an axis lying within the centre of her orbit, exterior to herself. The ship, like the Moon, has a floating, but not a rotating progressive motion. To the Editor of the Times.
12, HANWAY STREET, OXFORD STREET, LONDON. SIR,—On the day that the ribald letters relative to your remarks about the Moon's rotation appeared in the Times, I wrote a short letter to the Editor, requesting that he would insert my brief observations on the too severe animadversions of his correspondents ; and asserting that the Moon did not rotate in the same sense as the Earth, or other than that in which every body revolving round a centre must rotate ob id ipsum.
True he inserted, the next day, your very terse and luminous letter, which quite superseded mine, or any other, on the matter ; but I think, that after inserting half a dozen letters, all in the same strain, he might have given you the benefit of the humble support I proffered—I really mean humble, for an Oxford First in Mathematics is entitled to little weight.