The expanse of heaven: a series of essays on the wonders of the firmament

Contents

PAGH

A Dream that was not all a Dream .... 1 The Sun 11

Life in the Ruddy Planet 66

II

I have hitherto considered the moon with reference chiefly to the services which she renders to our earth. I showed that, as a subordinate light-giver, as a measurer of time, as chief ruler of the tides, and, lastly, as a celestial index which shows the experienced seaman where he is on the wide expanse of ocean, the moon subserves most important purposes to the inhabitants of the earth. I propose presently to consider the moon in another aspect ; to enquire into her actual condition as a member of the solar system, and to describe briefly the interesting circumstances which have been ascertained by means of the telescopic scrutiny of her surface. Before passing, however, from the former branch of my subject, I would make a few remarks on the convenience of the present arrangement as compared with that which the great mathematician and astronomer Laplace suggested as better suited for our requirements.

It will be remembered that Laplace showed how a moon might so circle around the earth as to be at all times exactly opposite the sun, and therefore always 'full,' rising also always when the sun set and setting when the sun rose. I mentioned in my last paper certain difficulties attending this arrangement even when considered with reference to the particular result which it was designed to "bring about. I also touched on the inconvenience of some of the results which would have followed from it, and particularly on the loss we should sustain if we never had a perfectly dark night. But it is even more important to notice how seriously all the other services rendered by the moon to ourselves would have been affected if the moon's motions had been those devised so ingeniously by Laplace.

Let it be remembered that to be always opposite the sun the moon must circle round the earth in the same time as the sun, or once a year. Thus the moon would no longer afford any subordinate time-measures. It is manifest that this would be a decided loss.

Again, the sun and moon at present join in controlling the tides, in such a way that when the moon is opposite the sun or on the same side as the sun, we have a tide resulting from the combined action of the sun and moon ; while when the moon is half-way between these positions, as at first and third quarters, we have a tide which is merely the excess of the lunar tide-wave over the solar tide-wave. If the wave produced under the former circumstances be represented by the number 7, then the wave under the latter circumstances will be represented by the number 3 ; for the solar and lunar tide-waves are as 2 to 5, and they always coalesce so as to produce a single but variable tide-wave. Now it is clear that this variation of the tide-wave adds importantly to the service which the tides render to man. It is extremely convenient to have occasional very high tides, while it would manifestly be inconvenient to have the range of the tidal wave always at its greatest value. Now, if Laplace's arrangement prevailed we should have the moon always co-operating with the sun, for they work together equally whether they be on the same or on opposite sides of the earth. Thus we should always have ' flood ' tides, or rather we should know of no such differences as now distinguish flood-tides from neap-tides. This would be a second serious loss.

And lastly, as to the service which the moon renders to seamen by her motions among the stars, it is easily shown that she would at once lose a great part of her value iu this respect if she circled round once a year instead of once a month. She would move then with less than a twelfth of her present rate of motion (for there are more than twelve lunar months in a year). Now this would correspond almost exactly to the case of a clock which had lost its minute hand, so that we had to determine the time from the position of the hour hand. Wc 4 know that owing to the very slow motion of the hour hand we could never tell the time within several minutes by its means. We judge always by the minute hand. So with the moon. Her comparatively rapid motion enables the sailor to determine the time (not the time of day, but what may be called 'earth-time') very closely 5 whereas if she moved with only one-twelfth part of her present motion, she would be practically useless as a time-indicator. Nor let the reader imagine that the difference of her value in this respect would be insignificant in its effects. Time in this case means position; an error of time means an error of position ; and an error of position means danger. The danger is greater or less according as the error is likely to be greater or less. Now, unfortunately, a very small error of time corresponds to an error of distance quite sufficient to involve very great danger. Let us suppose, for instance, that a ship is in latitude sixty degrees — that is, somewhat farther northwards than the north of Scotland — and that she is making her way towards the American coast ; now suppose that observations of the moon have given the time very nearly right — say only half a minute wrong — and let us enquire how far wrong the estimated place of the ship would be. A degree in sixty north latitude contains half as many miles as at the equator, where, as we know, each degree contains sixty ' knots,' or nautical miles, or about sixty-nine common miles. Hence in latitude sixty north, each degree contains about thirty-four and a half miles. The earth turns through fifteen degrees each hour, and therefore through one degree in four minutes. So that an error of four minutes in the time would be equivalent to an error of thirty-four and a half miles in the determination of the ship's place. Half a minute would give an error, therefore, of about four and a half miles, quite enough to cast a ship upon some iron-bound shore when her captain judged that she was still at a safe distance. 1 It will be seen, therefore, how serious would be

1 That erroneous notions may not be suggested as to the way in which our seamen actually provide against such dangers as are here indicated, I must remark that in point of fact there is no reliable method for determining a ship's place at sea with such accuracy that a captain could approach shore confidently (at night, for instance) within a few miles. All that he can expect from observation is such an approach to accuracy that until he is at a moderate distance from shore, no special look-out for land need be maintained. Of course the more accurate his means of determining true earth-time, the shorter will be the period during which (if he is worthy of

34 Tlie Expanse of Heaven.

the mischief if all such errors as must inevitably arise in such cases were increased twelve-fold.

I cannot but think that the lesson we may derive from these considerations is a very instructive one. Let it be remembered that Laplace was a man of most remarkable powers. As a mathematical astronomer he comes next after Newton, of whom it has been justly said that he surpassed the whole human race in mental power. Laplace has not been judged to have spoken unadvisedly when, referring to Newton's work, he said that Newton was fortunate in being the earlier born, implying that he himself had the power to have discovered the great law of gravitation, had not Newton first accomplished the work. This, which would have been regarded justly as arrogance in another, has not been so imputed to Laplace even by those who question whether Laplace in Newton's place would

the trust reposed in him by crew, passengers, and employers) he will take no rest, and relax not one instant from watchfulness. To show what could be done if there were perfect means of determining the time, the following narrative may be cited : — When the ' Great Eastern ' is carrying a telegraph cable across the Atlantic, her captain of course knows the true Greenwich time within a single second, for it is flashed to him from Valentia. He can therefore determine his true place with great accuracy. Now it chanced that on one occasion the captain of the ' Great Eastern,' while thus in telegraphical communication with Greenwich through Valentia, had occasion to search for a buoy which had been left floating (attached to a sunk cable) in a particular latitude and longitude. He made for the spot according to his calculated latitude and longitude, and (according to the account) after the final directions had been given to the effect that the ship should follow a certain course for a certain time, he went below to examine a chart. "When the time came he was about to go on deck, hoping to have made his course so truly that the buoy would be in sight; but at that very instant the ship's side was struck by the buoy.

have done so much as Newton actually achieved. This is the highest praise which could be given to any astronomer short of saying that he was Newton's equal. Now see how the great mathematician failed when he employed his powers to show in what way the work of the Almighty might be improved. He showed how a certain advantage might have been obtained had a certain special arrangement been adopted. So far all was well. But he omitted to observe how much more would be lost than would be gained by the proposed alteration. His scheme was conceived in the spirit of the remark made by Alphonsus, king of Portugal, who, speaking of the system of the universe as understood in his day, said that if the Almighty had consulted him when the universe was about to be created, he could have given useful advice. Alphonsus was in one sense right, since his remark, ugly though it sounds, was really intended to imply no more than that, in his opinion, astronomers had not in his day discovered the true system of the universe. But had Laplace been consulted when the moon's position and path were designed (to use such words for want of better), and if his advice had been adopted, we should have had a moon which would have subserved but one out of four highly important services actually rendered to us by her. Well might Laplace have been answered by the Almighty, even as of old He .answered Job out of the whirlwind, 'Who is this that darkeneth counsel by words without knowledge ? Gird up now thy loins like a man ; for I will demand of thee, and answer thou me. Where wast thou when I laid the foundations of the earth ? declare, if thou hast understanding. Who hath laid the measures thereof, if thou knowest ? or who hath stretched the line upon it ? Whereupon are the foundations thereof fastened ? or who laid the corner stone thereof; when the morning stars sang together, and all the sons of God shouted for joy ? . . . Hast thou commanded the morning since thy days ; and caused the dayspring to know his place. . . . Knowest thou the ordinances of heaven ? canst thou set the dominion thereof in the earth ? . . . Wilt thou also disannul my judgment ? wilt thou condemn me, that thou mayest be righteous ? '

I will draw this portion of my subject to a conclusion by calling attention to one feature of the moon, which, though it does not tend in any way to increase the comforts of the human race, has been of great importance so far as their acquisition of knowledge has been concerned. I refer to the near agreement in point of apparent size between the sun and the moon, two globes which differ so remarkably as to their real dimensions. The agreement is so close that as the sun and moon slightly vary in apparent size, according to their slightly varying distance^ the moon looks sometimes slightly greater and sometimes slightly less than the sun. Now, it is easily seen that if this relation had not existed — and it is in a sense merely fortuitous, not existing in the case of any other planet

The Qtceen of Night. 37

which has a moon — we should know very much less than we actually know about our sun. If the moon had a disc much smaller than the sun's there would never be a total eclipse of the sun, and all those wonderful objects which make their appearance when the sun is totally eclipsed — the coloured prominences and the sierra, the glowing inner corona, and the radiated fainter glory which lies outside the corona — would have been altogether unknown to us. But we should scarcely have learned more if the moon had had a disc much larger than the sun's. For in that case when a total eclipse began, all the region round the sun, except that close to the part of the sun's face concealed last, would be hidden by the moon's much larger disc. At the middle of totality, the red prominences and sierra, as well as all the brighter part of the corona, would be altogether concealed from view. And, lastly, at the end of totality the same state of things would prevail as at the beginning, only now it would be close to the part of the sun just about to appear, that for a moment or two the red prominences would be visible. Manifestly it would be quite hopeless under such circumstances to attempt to obtain any satisfactory observations of the solar surroundings. We now see during totality the complete ring of prominences for two or three minutes, and the whole of the corona is shown. Even as thus shown it has been sufficiently difficult to ascertain the nature of these objects. But with a moon much larger than ours we could have learned scarcely anything respecting them, and with a moon much smaller we should have known absolutely nothing of the solar appendages.