Dal 21 marzo al 21 giugno 2016 a Firenze, Roma, Napoli, Palermo, Padova

The lesson of the eclipse

THE LESSONS OF THE ECLIPSE.

we are now beginning to receive the reports, with a little more detail than the telegraph could give, of the various expeditions sent out, with assistance from the English Govenment, to observe the Eclipse of 1870. If we may trust the accounts which reach us from one at least of these parties, the great question of solar physics which it was the special object of this yea.r’s efforts to solve has been settled once for all. The Sicilian expedition, which, it will be remembered, was under the charge of Mr. Lockyer, claims to have fairly run down, after a long and exciting chase, the much-disputed Corona. If ever it was possible to feel pity for a material object supposed to be some millions of miles big, any one with a spark of feeling would have felt it during these last few years for that particular phenomenon of a total eclipse of the sun which we have just named. It has been hunted from the sun to the moon, from the moon to the earth’s atmosphere, and from the atmosphere back again to the sun, until in the end there were those even among the most soberminded of our men of science who be to doubt and question whether there was any such thing as a corona at all. This time it was determined by universal consent that the thing should give no more trouble for the future; if the sky would but remain clear on the 22nd of December for just one minute and a half in the middle of the day, the corona should be marked down as well as telescopes, spectroscopes, polariscopes, and all the resources_ of philosophy could do it. With some hurry, but with an amazing amount of energy, three separate expeditions were arranged; an it is of one of these, the Sicilian mission, that we propose to give some account.

Roscoe&Bowenma_speq.

Roscoe e Bowen sull’Etna

The organization of this party was complete, as regards the special work to be done by each member, at the moment when, one fortnight before the eclipse, they started from Charing Cross; but all arrangements of position were reduced to chaos by the unfortunate accident which, exactly one week before the day of the eclipse, saw the entire corps of observers crowded together on the lava rocks under Mount Etna, congratulating themselves_on their personal safety, and engaged in the interesting speculation whether an object-glass which it cost months of polishing to bring into shape would be likely to be more safe when tossed by a sailor on to a rock, than the delicate spectroscope, with its endless. variety of screws and prisms, had been when it emerged, a pitiable object, from the chests and boxes which had accompanied its hasty transfer from the ship. The evening, however, saw the entire party safe at Catania, with but little damage done; and. it remained to form a fresh plan of attack. Very great assistance was rendered, at a rather trying moment, by the American party, who were already in force, and whose experience was invaluable ; and it was agreed that the proceedings of the observers should take an Anglo-American form, and that while the organization of each nation still remained intact, the observers should be to a considerable degree united, and the report published as a joint one. There were indeed few nations which were not represented in Sicily. M. Janssen, indeed, who had been expected in Sicily, took up his station in Algeria. The island, however, teemed with science. Each body of observers had its special function, and each savant his special detail; down to the very German professor of the moral sciences who, when asked what his particular work was to be, replied that it was his intention to remain on the line of totality, and allow the eclipse to enter into his soul. Every one, however, was profuse in offers of assistance to the Englishmen, who by their misfortunes seemed to have deserved a special amount of sympathy. At Catania the rooms and gardens of a splendid monastery on the outskirts of the town-one of the largest in Europe, now used as a college—were placed at their disposal; at Agosta there was a detachment of Engineers, sent by the Government from Malta, ready to receive them. A couple of days were enough to dispose the various parties. Four posts of observation were selected. At Syracuse the chief photographers of the expedition were placed, at Agosta the largest body of the polarizers, at Catania and on Etna the most important of the spectroscopists ; each division was rendered, however, as complete as circumstances would admit, and to each was attached one or more artists, whose business it was to make sketches of whatever phenomena presented themselves in the field of a telescope. So disposed, the little army of astronomers awaited the eventful day. It may serve as an illustration of the completeness with which the arrangements were made that, on the day before the eclipse, the varying solar “ prominences,” invisible to the eye or telescope, but discernible to the analysis of the spectroscope, were caught, mapped, measured, and in the possession of the observers at the chief centre of operations.

Solar eclipse observers, 1870. Astronomers using telescopes to observe the total solar eclipse from Syracuse, Sicily, on 22nd December 1870.

La spedizione inglese nel monastero dei benedettini a Catania

The week of expectation was fine, and was spent in setting up the instruments, and practising their use. The day came, the eclipse happened, as it happened on the same spot twenty-two hundred years ago, and by the evening the telegrams were coming in which told of the fate of each division. Some had seen all, some nothing. At Syracuse it had been fine, at Agosta moderately fine; at Catania the darkest of clouds had cut off every ray of light; on the mountain the eventful moment had passed in the middle of a whirlwind of snow. The Etna party was indeed to be pitied. A body of seven, with Professor Roscoe at their head, had on the preceding day successfully carried up on the backs of a dozen mules their baggage and instruments, food and fuel, to a point more than 5,000 feet above the sea. An attempt to push higher was defeated by the weather, and in a hut which was found upon the mountain they passed the night, wondering whether the storm, with its lightning and snow, which raged till morning, would yet give a chance of clear sky before two o’clock next day. It did clear at breakfast time; and far below, on the terrace of the monastery garden, might be seen with glasses the observatory of Mr. Lockyer. For some hours it was bright; all the precious telescopes were erected with cold fingers in the piercing wind; the batteries were charged, the adjustments made; and the commencement of the eclipse was noted just as the first floating clouds came up from the plain. As the sun darkened, the air thickened. When the moment of totality drew on, every one was at his post, even the one observer who had. been despatched to take his chance yet higher up in the snow; and as the single minute of darkness passed, a hailstorm of extraordinary intensity descended on the , almost blinding the eyes which were straining to catch a glimpse of the view which they had come a couple of thousand miles to see. Exactly eight minutes afterwards, the sky was clear again.
But it is time to describe what was actually seen by those of the expedition who were successful; and it is with great regret that we notice that among their number was not included Mr. Lockyer himself, to whose energy it was chiefly owing that success was achieved at all, and whose own observations would have been the most valuable, from his complete mastery of the science of spectroscopy, and tho light which, by means of it especially, he has been able to throw upon the physical side of astronomy. We shall not attempt to enter into a minute discussion of the results gained, but will rather point out their general bearing; and this will be perhaps assisted by a few words of explanation. In total eclipses the sun is seen to be surrounded, first by the ” chromosphere,” a bright rim of reddish light, with an outline moderately well defined, presenting generally the same phenomena, though sometimes hidden when the moon happens to be particularly near the earth; and there is no reason to doubt that this consists of a layer or layers of incandescent gas, chiefly hydrogen, arranged in order of density. Secondly, the coloured prominences, projecting here and there from the edge of the chromosphere. These now resent no difficult whatever. They are discernible at all times the Janssen-Lockyer method, and are known to be outbursts of heated hydrogen, many of them thousands of miles high, and constantly varying in position and magnitude. Thirdly the Corona. Of this sphinx of a phenomenon it is not only hard to say what it is, but oven to say what it looks like; for while some observers on previous occasions have noticed only a finer halo surrounding e chromosphere, others have extended this into well-defined and gorgeous shapes, have given it brilliant streamers extending heaven knows how many diameters of the sun in length, and even an elaborate organism with bundles of parabolic rays. The American astronomers at the last eclipse declared that they found iron in its composition, even in that of these mysterious rays or streamers. What then docs this eclipse reveal, as fur as the accounts have come to hand? In the first place, there is a corona – which it is some relief to hear – and this corona is solar. The halo of which we spoke as surrounding the atmosphere is in fact an apparently achromic continuation of it ; and it was observed by Professor Watson, well known in the United States as a patient and successful observer, to extend to about five minutes in height beyond the solar disc. He describes it as having the appearance of a shell, that well-known phenomenon of concentric layers which is resented by the nuclei of most comets which are near enough to e examined. Professor Watson also saw one of the “streamers” so often spoken of – and saw it disappear! It seemed to float away, he says, “ like a veil.” If, then, this observer is to be trusted – and there is no observer living who is more worthy of trust as regards a thing actually seen – the streamers are an atmospheric effect, and the corona, if we may continue to use the name, appears to be a solar envelope of gas surrounding the coloured gas of the chromosphere. Next come the observations of the polariscope, some of which have not yet reached us, but those which have at present comp to hand are distinct enough. Briefly stated, they are these : – The corona (or outer chromosphere) is strongly polarized; therefore it shines with reflected light. It is polarized in a plane different from that reflected from the moon’s surface at the moment of totality; therefore it is not atmospheric. It may hence be fairly considered to be a solar appendage, reflecting in an eclipse the light of the obscured sun.
Leaving further details, we turn lastly to the spectroscope ; for, as no photographs have as yet reached England, it is too soon to pronounce on the value of those which have been made. The most important spectroscopic observation was made by Mr. Burton an observer fully to be trusted, at Agosta. capodimonte_scheda_5dHe saw in the first p ace the ordinary spectrum of the chromosphere, including a certain line in the yellow art never before noticed; then the hydrogen lines, which were to e expected especially at the edge of these, and which simply show the comparative lightness of the substance which produces them; and lastly – a most important discovery – a clear green line by itself outside the part of the spectrum due to the chromosphere, and at about the same ‘ position as that noticed by the American astronomers last year. What is this green line? It cannot well be a hydrogen line, for, if it were, why wore not the other well-known lines of hydrogen present? It cannot be iron, for the same reason. It is like no substance in heaven or earth which is dreamt of in our philosophy. It is a gas – or shall we call it a metal? – which is so extremely light that it floats above the hydrogen, which is in a region of so low a temperature that it alone of the materials in its neighbourhood can yield any spectroscopic results, and which is green in colour. But for the fact that, as the polariscope shows, it shines chiefly by reflected light, this corona would, at all events as far as this particular is concerned, be green ; and as this is the very outside shell of all the shells of the sun hitherto discovered, we may even lay it down as an interesting fact in natural science that, as far as we know it, the sun is green on the outside. The only thing now left is that our chemists should produce this hitherto unknown substance in their laboratories, as they have already produced the similar thallium; or even perhaps the Janssen process may be repeated over again, and the workers with the spectroscope may not rest satisfied till they have traced this mysterious line in open day, and without the aid of an eclipse. Nay, what if it has been traced already? If this remote green line is the same which has been found in the aurora, and which is believed to have been found in the zodiacal light, what are we to say of the ranges of such a discovery? Have we in any sense, with any limitations, touched the edge of that cosmical ether, that unknown substance, which everything points to and nothing shows, which is yet perhaps revealed under certain magnetic conditions in the higher regions of our atmosphere; and can this mysterious gas be nothing but a zone of the pervading ether itself rendered luminous by the intense heat of the sun? Perhaps this may be a conjecture to which sober science has no right as yet to proceed; but, whatever the case may be, this green line in the spectrum of the outer chromosphere of the sun is the door by which those will for along time enter in who wish to search with success the regions of cosmical science as yet unexplored.
We may have dwelt too long upon the surmises to which these observations will give rise; but there is one point which ought not to be omitted, as with it is connected one of the most remarkable of the discoveries made. All that has now been made known was exactly in accordance with the predictions published beforehand. The instructions issued to the observers by the Organizing Committee point with extraordinary minuteness to the result which has been obtained. Read them with a change of tense, and they will almost serve for a history of the observations made. Even the height of the corona, five minutes in extent, by one of those happy strokes of luck which are always happening when men of real scientific genius take to predicting, 18 exactly what was tentatively predicted. But the most striking of all coincidences was this. Ve mentioned above that, on the day preceding the eclipse, observations of the invisible solar prominences were made by means of the spectroscope. The work was in the hands of one of the Catania arty, Mr. Seabrooke, who on the morning of the 22nd produced a map exhibiting their position and height. During the eclipse, Professor Watson, as we stated, sketched the corona carefully. The greater part of the next day was spent by him in making an exact drawing from his sketch, showing as accurately as possible the irregularities of its outline. In the evening this drawing was compared with the map of the prominences, and it was found that they exactly corresponded. The protuberances in the circle of the corona represented throughout the prominences which existed beneath, which were never seen at all, and which had been mapped beforehand in the way which we pointed out. Clearly the substance which gives rise to the corona was subject to the hydrogen storms beneath it, and bulged out in obedience to their pressure. Nor is this all; for the fact that the corona on this occasion was found by American observers to be smaller than that of last year is just what would have been expected by any one who noticed that, as Hr. Lockyer has shown, the prominences have been decidedly diminishing in extent during the last year.
We may hope to have another opportunity of recurring to the subject in connexion with the observations made by other expeditions sent out for the same purpose; but we have said enough to show that at any rate the Sicilian arty has done good work, and that the trouble and expense which it has taken to send them to their stations have not been thrown away.

The Saturday Review, 1871

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