In the High Heavens/Chapter 4

by Robert Stawell Ball

CHAPTER IV.

THE GREAT ECLIPSE OF 1893.

 

HE total eclipse of the sun which took place on April 15th—16th, 1893, is in some respects the most remarkable event of the kind in the present century; certainly no other like phenomenon occurring within the next decade will equal it in the presentation of exceptionally favourable conditions.

It is obvious that there are two criteria by which we may judge of the suitability of an eclipse of the sun for the purposes of the astronomer; the first relates to the astronomical conditions, and the second to those of a merely geographical character. Of course it must be understood that any eclipse which would disclose information sufficient to justify despatching an expedition for thousands of miles must be total. There is but little to be learned from any observations at a place from which the disc of the sun appears only partly obscured by the interposition of the moon. Such an opportunity may, indeed, enable accurate determinations of the relative positions of the sun and the moon to be obtained, and these are often of service in our efforts to improve the tables by which, the movements of the moon are calculated. But this object is of very slight importance compared with those which chiefly occupy our attention during a total eclipse. The primary question in determining the astronomical value of a total eclipse relates to the duration of the phase in which the obscurity is total.

Tested by this standard, the phenomenon which we are now to consider is one of exceptional value. The phase of "totality" lasted for 4 minutes 40 seconds on the east coast of Brazil. This may seem, indeed, but a short time in which to commence and complete an elaborate series of observations and measurements; but by skilful organization of the work it is now possible for a corps of experienced observers to effect, even in this very limited time, an amount of careful work that would greatly surprise anyone who was not acquainted with the resources of modern scientific methods. Indeed, on former occasions many successful eclipse observations have been made when the period of totality has been much less than that just stated. Even in the event which we are now considering, other stations at which the duration of totality was much below the maximum have been occupied with much advantage. Thus in Chili totality lasted for 2 minutes 56 seconds. It was 9 seconds longer in Argentina. It reached the maximum for available terrestrial statistics on the east coast of Brazil; but the actual maximum duration of 4 minutes 48 seconds would be observed from a point some hundreds of miles off in the Atlantic. On the west coast of Africa, at Senegal, the duration was 4 minutes 10 seconds.

To realise the conditions under which an eclipse is produced we must remark that, wherever the moon may happen to be, it bears at all times a long conical shadow projected behind it, sometimes into space, sometimes towards our globe. The cone tapers to a point at a distance which varies somewhat, but is about a quarter of a million miles from the moon. For the production of a total eclipse of the sun it is necessary that the eye which observes should be somewhere within the cone of shadow. Even when the moon does come in between the earth and the sun it will sometimes happen that the shadow-cone is too short to touch the earth, in which case an annular eclipse will result. Sometimes, however, owing to the varying distances of the sun and the moon from the earth this cone does extend far enough to reach the earth, and then observers who happen to occupy any spot in the shadow will have a total eclipse presented to them.

About 1 p.m., Greenwich time, on Sunday, 16th April, the sun was rising in the Pacific Ocean in a state of total eclipse, the moon casting a deep black shadow on the shining waters around. There does not happen to be any island lying near enough to the critical position for its inhabitants to have witnessed this interesting phase of the spectacle: Juan Fernandez was too far to the south and St. Ambrose too far to the north. The region of complete obscuration was at first oval in form, and the shortest diameter extended some ninety miles north and south. The black patch then commenced its great eastward journey, and presently reached land on the coast of South America. The local time was then about half-past seven in the morning at the point of arrival on the coast of Chili, in 30° south latitude. Professor Pickering was among the first of an ardent corps of astronomers ready to greet the total eclipse and to utilise to the utmost the advantages of an early station. Then the shadow began its journey across the South American Continent.

With a speed of something like 3,000 miles an hour, far swifter than any rifle bullet ever moved, the silent obscurity sweeps across wide deserts in the interior, and then over the noble rivers and glorious forests of Brazil, to quit the land after barely an hour has been occupied in the transit. Along its track it has been watched in two or three places by interested observers armed with spectroscopes, photographic cameras, and the other paraphernalia of the modern astronomer. Doubtless the sudden gloom caused no little dismay to many a tribe of savages in the deep interior of tropical America. We may also conjecture that other creatures besides man had their share of astonishment. Darwin and Bates have charmed all readers by their exquisite delineation of these virgin forests of Brazil, where organic nature is developed with a luxuriance which those whose rambles have been confined to sterner climes can hardly realise. Probably in Brazil, as elsewhere under similar conditions, tender plants evinced their belief that night had prematurely arrived. Beautiful flowers no doubt closed their petals as they are wont to do after sunset. Other flowers, again, which open out at night to solicit the attention of moths, to whom the darkness is congenial, doubtless began to expand their charms. With the advancing gloom such plants as emit their delicious perfume only when the glory of the day has vanished were likewise deceived, as they have been known to be on other occasions of a like kind.

We can also speculate on the amazement which the total eclipse must have produced among the various races
Fig. 11.—Path of the Moon's Shadow and Penumbra upon the surface of the Earth during the total Eclipse of the Sun, April 15th—16th, 1893.

of animals. The great flocks of Brazilian macaws must have wondered why the time for going to roost had indeed arrived again so soon. The chattering monkeys and the skulking jaguar would have been sorely puzzled; while the marvellous nocturnal insect life which Mr. Bates has so forcibly described would have been deceived into temporary vitality. For some minutes, it may be reasonably assumed that the forest depths resounded with those myriad notes of crickets and grasshoppers which appear to be one of the most striking features of night in the tropics.

Quitting the east coast of America, the lunar shadow took an Atlantic voyage. It crossed the ocean at perhaps its narrowest part, and may have buried in its gloom many a vessel, whose crew gazed with astonishment at the unwonted spectacle. Here the conditions of good observation, so far as celestial requirements are concerned, would have been of the most desirable nature. The sun would have been overhead and the fervid glories of the equatorial noon would have been suspended for the space of nearly five minutes. Splendid indeed must have been the view of the corona obtained by those who were fortunate enough to be in the right position on the ocean, with a clear sky overhead. But from the astronomer's point of view the observations which can be made on board ship are of but little importance; the deck does not offer the stable foundations that are required for elaborate photographic or spectroscopic apparatus.

For the space of an hour, therefore, while this ocean passage was in progress, there were but few opportunities, if indeed any, for valuable contributions of facts to illustrate our theories of the corona. The speed with which the shadow traversed the sea happened to be not so great as that with which it crossed South America. The consequence was, that rather more than an hour was occupied by its journey from the American to the African coast. This ocean distance is only about half as long as the track pursued across the South American continent; but owing to the curvature of the earth the incidence of the shadow makes it travel more quickly at the beginning or the end than in the intermediate stages, so that in consequence of the decline in speed about the middle of the eclipse, the time required by the ocean journey was about the same as that needed for the previous land journey.

A few minutes after half-past three, Greenwich time, on Sunday, 16th, the shadow reached land again on the African coast near the River Gambia, about north latitude 15°. Here the eclipse was destined to receive a cordial welcome from the bands of astronomers who were ready to receive it. Sweeping onwards with a pace which had now begun again to accelerate, the shadow advanced into the interior of Africa, keeping below the parallel of 20°, and gradually curving southwards. At four o'clock on Sunday afternoon, the position from which totality was to be observed had gone to the east of the meridian of Greenwich. The end of the phenomenon was now rapidly approaching; the last glimpse that could be had of it from this earth would have been from the desert of Sahara, where, just at the moment of sunset, the phase of totality was reached. At a quarter past four, Greenwich time, the eclipse ceased to be total anywhere, but an hour longer had yet to elapse before the partial eclipse had vanished from the globe. It will, of course, be understood that at any particular locality the total eclipse only lasted for the time that the shadow occupied in passing over that locality. Thus the duration at any particular station was only about as many minutes as the hours during which the total eclipse required for its terrestrial journey.

It is plain that the best sites, so far as astronomical conditions are concerned, must be those where the duration of totality is as long as circumstances permit. To secure this, on the occasion now before us we had to occupy sites which lay as nearly as possible along the middle of a strip eighty miles wide, extending from the South Pacific to the middle of the Sahara. It fortunately happens that on this occasion those localities where the astronomical conditions were favourable also turned out to be those where the geographical conditions were suitable and comparatively convenient. At Chili, in Argentina, in Brazil, and on the African coast, astronomers were able to obtain a series of admirable pictures, not often paralleled in eclipse observations. One special advantage offered by this chain of observing stations should be particularly noticed. It is a question of considerable importance to examine the nature of the changes which take place in the corona. It has sometimes been thought that such changes frequently occur with extreme rapidity. No doubt, when we remember the scale of the objects involved, it will hardly be imagined that in the brief interval of four or five minutes, during which the eclipse lasted, any variation in the corona should have taken place considerable enough to be recognised from the distance at which we are placed. We thus have the opportunity of carefully examining whatever changes may have taken place in the corona in the interval between the time of totality in Chili and the time of totality in Africa. As we have pointed out, this period is no less than two and a half hours. In this respect, the advantage offered by the eclipse of 1893 is almost unique, for though on other occasions observations of totality may have been possible for a number of seconds greater than those at either of the stations we have named, yet the circumstance of having in the same eclipse two occupied stations so widely separated as the western coast of North Africa and the western coast of South America is quite an exceptional advantage.

And now as to the problems which astronomers have proposed to themselves to solve when undertaking the observations of this as indeed of certain other recent eclipses of the sun.

The history of modern astronomy makes it plain that a remarkable change has taken place in the nature of the questions which specially demand attention during such phenomena.
Fig. 12.—Total and Annular Eclipses.

Twenty-four years ago a total eclipse was regarded as of special value, as it afforded the opportunity of investigating those remarkable prominences or coloured flames round the sun's margin which were then considered to be not otherwise visible than by the occurrence of a total eclipse. Attention was no doubt also directed in the earlier eclipses to the silvery corona which stretched from the sun to such a vast distance into the surrounding space. The corona, though a permanent append age of the sun, was only to be recognised when by the direct interposition of the moon the light of the sun was cut off, and in the gloom thus arising the radiance of the corona became readily and even brightly discernible. But the memorable discovery made by Janssen and Lockyer, independently, in 1868, showed that the prominences could be observed without the help of an eclipse, by the happy employment of the peculiar refrangibility of the rosy light which these prominences emit. This improvement in observational astronomy revolutionised the method of utilising eclipses. We are now so well acquainted with the forms of the prominences by the spectroscopic method that the eclipses have but little to teach us on that matter.

Of course it will be admitted that there are many circumstances with regard to these objects as to which we at present know but very little; however, we do not look in any considerable degree to eclipses for their solution. Quite recently a further extension has been given to the spectroscopic method of studying solar prominences by the beautiful invention of Professor Hale of Chicago. He has employed a very elaborate apparatus by which he is able, as it were, to sift out from the sunlight the beams of that particular refrangibility which astronomers would denote by saying that it belonged to the H line of the spectrum. With the light thus chosen Professor Hale obtains a photograph. It so happens that in the light of this particular hue—an invisible hue, it may be added, only perceptible to the peculiar sensibility of the photographic plate—the prominences are peculiarly rich. It follows that when all other light is withdrawn, as Professor Hale's method enables him to withdraw it, the ordinary solar light remaining has become so much weakened that it can no longer overpower the beams from the prominences, and hence an image is printed on the photographic plate. Thus we can now obtain, not as heretofore merely isolated views of special prominences through the widely opened slit of the spectroscope, but we are furnished, after a couple of minutes' exposure, with a complete photograph of the prominences surrounding the sun. In Professor Hale's remarkable pictures, not only is every large prominence exhibited with ample detail, but the incandescent region of the chromosphere from which these prominences arise is also recorded with accuracy.

It may therefore be said that with this admirable process available the eclipse is no longer of much account for the purpose of instructing us as to the prominences. No doubt a pleasing picture of these objects may be afforded. Professor Pickering, indeed, describes them as of much interest on the recent occasion; but the attention of the eclipse observer in the present day is almost wholly otherwise directed.

For the corona is still only known to us by such opportunities as eclipses present. No doubt attempts have been made by photographic methods of various kinds to enable the corona to be brought within our scrutiny under ordinary circumstances. Up to the present, however, success cannot be said to have rewarded these efforts. The sunlight is so intense that if it be reduced sufficiently by any artifice, the coronal light also suffers so much abatement that, owing to its initial feebleness, it ceases altogether to be visible. We are therefore wholly dependent on eclipses for accessions to our knowledge of the corona, so it will not be a matter of surprise that on the recent occasion the attention of the different parties was almost entirely concentrated on the minute scrutiny of the corona by every device which was likely to explain its nature.

The astronomers of Great Britain had as usual taken a leading part in organizing plans for the purpose of observing this eclipse. A joint committee of the Royal Society, and of the Royal Astronomical Society, had general charge of the arrangements. The sinews of war were chiefly provided from that liberal grant of £4,000 a year which the State places at the disposal of the Royal Society for furthering the interests of science in such ways as may seem most advantageous.

Assistance of other kinds was also forthcoming. In many cases the owners of valuable instruments placed them at the disposal of the observers. The Admiralty provided such facilities of transport as were needed to attain out-of-the-way places. The comity of nations was also illustrated by the readiness with which the authorities of the French and Brazilian Governments respectively complied with the requests made to them. They afforded accommodation and extended courtesies to the parties on the coast of Brazil and in the French territory on the African coast to which the two British expeditions were despatched.

A careful study of the meteorological conditions of the different localities was a necessary preliminary to the choice of stations. For it need hardly be said that, however suitable a station may have appeared to be from the astronomical facts of long duration and of high altitude of sun, yet if the locality in question were one likely to be obscured by clouds it would be somewhat improvident to despatch an expedition to a place where the chances of success would be so greatly jeopardised.

Perhaps the most elaborate study of the meteorological conditions bearing on the question was contained in a paper contributed by Prof. David P. Todd to the Meteorological Journal. In this he has brought together a mass of information collected from divers authentic sources. The inhabitants of Chili were able to report that the observations of any celestial phenomena which take place in April are almost certain to be made in a cloudless sky. Indeed it appears that in the mountainous regions of that favoured climate the atmospheric conditions are almost ideally perfect for the purposes of the astronomer. The course of the shadow then lay through Argentina, where the residents assure us that April is the best month of the year for clear atmospheres and light skies, and that it could only be through some exceptional misfortune that the observers would meet with disappointment. In Paraguay, which the shadow next traversed, it seems that meteorological zeal has not yet been kindled. No accurate information as to the clouds or weather to be expected in April was forthcoming in response to Mr. Todd's urgent inquiries. In despair of being able to offer climatic inducements to the expedition he expressed a hope that any eclipse party despatched thither might include zealous naturalists. The attractions offered by pumas, jaguars, cobras and cross vipers in abundance might in that case suffice to "offset the possible loss of the corona to the astronomer." But it may well be doubted whether the enthusiasm of the astronomer, who studies with much interest Serpens and Draco in the skies, would be sufficient to induce him to journey all the way to Paraguay in expectation of becoming acquainted with their terrestrial representatives, possibly on closer terms than he could desire.

In Brazil, where the astronomical conditions are of the best, the risk of clouds was considerable. It seems that about half of the days in April on the coast at Para Cura are likely to be obscured. Fortunately, however, the observers were favoured with good weather. Pains had also been taken to determine the chances of cloudiness at this season along the Atlantic track followed by the shadow. They showed that the probability of a clear sky at mid- day in April at any point along the track followed by the eclipse from Ceara, where the central line leaves the coast in Brazil, to Gambia, on the other side of the Atlantic, is about one half.

It was not possible to obtain any very definite information as to the extent of April cloudiness in the interior parts of Africa which were passed over by the lunar shadow ere it finally quitted the earth. It seems, however, impossible to doubt that an expedition might have been despatched to some locality in the far interior of Senegal or the Sahara, where the atmospheric conditions would have been excellent. A continuous chain of observations of the corona would then have been available from the time the sun was rising on the coast of South America to the time of sunset in Sahara. The great advantage of such an expedition would have been that it would have afforded an opportunity for testing in the completest manner whether the corona submitted to these rapid changes in the few hours to which we have already referred. The eclipse now under consideration was admirably suited for this investigation, for the terrestrial conditions were such as to enable the observations to be made both near the beginning and the end of the phenomenon. Further, as the sun spots were at the time very abundant, it might be presumed that the sun was in a condition of exceptional activity, and consequently it seemed reasonable to suppose that, in sympathy with what was going on below, the corona would be in a disturbed state. Unfortunately, however, it was not found practicable to make use of the extreme end of the track of the shadow.

The English Brazilian party, consisting of Messrs. Taylor and Shackleton, were stationed at Para Cura. The African party was organized on a somewhat larger scale. Professor Thorpe was placed in command of it, and he was accompanied by Lieutenant Hills, R.E., Sergeant Kearney, R.E., and Messrs. Fowler, Gray, and Forbes, from the Royal College of Science. They were despatched to Bathurst, thence to make their way to a station in French Senegambia only a few miles south of the central line of totality.

As the pictures of the corona vary so much with the instrument employed, it is clearly desirable to have some means of discriminating between the actual changes which may have taken place in the structure of the corona itself between one eclipse and the next, and those differences in the representation of it which merely arise from instrumental causes. There is no means of attaining this end so simple and so secure as to provide that the same photographic apparatus shall be used on each occasion. For this reason the corona was photographed in Africa on Sunday, the 16th instant, with the same 4-inch lens of 60 inches focus which was used in Egypt in 1882, in the Caroline Islands in 1883, in Granada in 1886, and in the Salut Islands in 1889. We have thus a connected series of pictures of the corona, taken as far as possible under similar conditions, extending over a period of eleven years.

Particular interest will be attached to the department of work assigned to Mr. Fowler in Africa. He photographed the spectrum of the corona, produced by placing a glass prism in front of an object-glass of six inches aperture. The peculiar advantage of this method of observing is that for each source of light of special refrangibility in the corona a distinct image of the corona will be impressed on the plate. If, for example, the coronal light was of that strictly monochromatic type of which the light of certain nebulae appears to be, then the coronal photograph as produced through the prism would represent the details of the structure in a single definite picture. If, however, as seems much more likely, the corona diffused light of two or more refrangibilities, then separate pictures of it would be obtained in distinct positions on the plate, in correspondence with each of the constituent rays. The several pictures thus obtained would be indications of the different kinds of light of which the corona was composed. So far as these various simulacra can be discriminated and interpreted they will afford indications of the material constituents of the luminous substances from which they originate. It need not be expected that these several pictures will resemble each other. If the different parts of the corona contain different elements in their constitution, as is certainly most probable, then the several pictures will evidence this by their difference in outline. No doubt the different photographs may to some extent overlap, but though this will interfere with the pictorial effect, it will not prevent their interpretation in the sense that it is instructive to the astronomer.

One of the most remarkable features in the structure of the corona is the presence of streamers or luminous rays extending from the north and south poles of the sun. These rays are generally more or less curved, and it is doubtful whether the phenomena they exhibit are not in some way a consequence of the rotation of the sun. This consideration is connected with the question as to how far the corona itself shares in that rotation of the sun with which astronomers are familiar. I should perhaps rather have said—that rotation of the sun's photosphere, which, as the sun-spots prove, is accomplished once every twenty-five days. Even this shell of luminous matter does not revolve as a rigid mass would do. By some mysterious law the equatorial portions accomplish their revolution in a shorter period than is required by those zones of the photosphere which lie nearer the north and south poles of the luminary. As to how the parts of the sun which are interior to the photosphere may revolve, we are quite ignorant. Nor does there seem much likelihood of any discoveries being made which will clear up this matter. We have no means of knowing to what extent the corona shares in the rotation. It would seem certain that the lower parts which lie comparatively near the surface must be affected by the rapid rotation of the photosphere. But it is very far from certain that this rotation can be shared in to any great extent by those parts of the corona which lie at a distance from the sun's surface as great as the solar radius or diameter.

The spectroscopic testimony forms of course an exclusive source of information as to the nature of the elementary bodies present in the corona. It must be admitted that our knowledge on this subject is rather of a negative character. The spectroscope has hitherto mainly afforded us indications of elements which seem to be undeterminable by our knowledge of terrestrial chemistry. Professor Schuster, after a careful discussion of the evidence afforded by other eclipses, has come to the conclusion that it is not at present possible to identify the lines specially characteristic of the coronal spectrum with those of any known terrestrial substances. Indeed, the corona presents a curious green line that seems to denote some invariable constituent in the sun's outer atmosphere; but the element to which this green line owes its origin is wholly unknown. It has been conjectured that it is due to some body present in the sun which is unknown to terrestrial chemists. The elucidation of this question is from every point of view one of the most interesting problems in solar physics.