130 THE ISSUES OF THE LATE ECLIPSE. BY J. CARPENTER, F.R.A.S., OF THE ROYAL OBSERVATORY, GREENWICH. THE eclipse expeditions of December last differed remarkably from others that had preceded them. They differed favourably as regards the elaborate character of the means and schemes of observation, and very unfavourably as regards the success of the observers' intentions. Upon the occasion of the first eclipse expedition, that of 1851, every observer went upon his own account to see what he could, without foreknowledge of what he would see, and without ideas upon the ultimate bearing of any observations he might have the good fortune to make. How successful the expeditionists were in viewing the phenomena may be judged from the fact that nearly a score of accounts of observations were given in the "Astronomische Nachrichten," from astronomers of repute located along the line of totality (which crossed Sweden and Norway), while as many more were included in a special volume of the "Transactions of the Royal Astronòmical Society." In the second expedition that for observation of the eclipse of July 18, 1860there was again a general vagueness of intentions: everyone was to observe what he could for himself; what little organisation was called for was discussed on board the Himalaya on her way to Spain. And how fortunate the observers were on this occasion is known to those who have had to refer to eclipse literature, and who have found records of bewildering extent of the observations made on that propitious day. How differently matters stood in relation to the last eclipse! There was but one subject of inquiry-the constitution of the corona. There were distinct points by which the question was to be attacked, and every observer had allotted to him a definite part of a well-considered scheme of observations. Instead of one man aiming to explain the whole phenomena of an eclipse by himself, as on previous occasions, every man was entrusted with one link in the chain of inquiry. The gazer was to sketch, the polariser was to study points precisely indicated, the spectroscopist the same, and the photographer was to expose his plates so as to seize upon critical appearances. These were the duties of principals; subordinate labours were as definitely arranged. Never before had there been so extensive and determinate an organisation; and it is quite conceivable that, had the weather favoured the observers, the one outstanding enigma of eclipse phenomena would have been so nearly solved that future eclipses would have possessed little interest for astronomers—a measure of success the desirability of which may perhaps be questioned. But this was not to be. Ill fortune met the observers at every turn. Deaf ears were turned to their prayers for Government aid; delays were forced upon them; one party was shipwrecked, and another nearly so; one detachment, of which the present writer was a member, had their observing tents and telescopes blown down by a gale, and their most valuable instruments saved from utter ruin as it were by a miracle; and, worst of all, the heavens frowned upon most of the expeditionists on the eventful day. And it is a curious, though an insignificant circumstance, that the weather misfortunes which befell the English observers scarcely affected their American confederates. But the few French and German astronomers who attempted observations were all unsuccessful. M. Janssen indeed deserves a martyr's fame. He escaped from Paris in a balloon, taking with him a silvered glass reflector of 13 inches aperture, and a small spectroscope, and landed at Savernay, whence he pushed on to Oran, arriving there some ten days before the eclipse and living like a hermit the while at his observing station, nine miles from the town, in a desolate and uncomfortable barrack : and all to no effect. The distribution of observing parties along the line of totality and their measures of success were as follows :— At Cadiz IN SPAIN. Lord Lindsay's party San Antonio English expedition detachment (under) small success Xeres Gibraltar Estepona At Oran Tunis the Rev. S. J. Perry) American party (under Professor Winlock) successful English expedition detachments (under IN AFRICA. no success insignificant success English expedition detachment (undery Vienna observers (Drs. Weiss and Oppolzer) no success At Terranova Carlentini Syracuse Augusta Catania Villamonda English detachment (under Mr. Ranyard) successful Etna no success In this tabulation the ill fortunes of the English observers are plainly apparent. Yet so far as securing observations is concerned there has been a good deal of success, and in the end it matters little who achieved it. How far the observations will go in settling the questions at issue we shall presently endeavour to examine. But, in the meantime, let us remember that a solar eclipse is an occasion for observations other than those relating to the sun's constitution and surroundings. When the moon passes between us and the sun, and appears as a black body on his disc, an opportunity is offered of making a determination of the moon's place at a critical part of her orbit. Ordinarily we cannot see the moon for a day or two on either side of conjunction, and the observations which are desirable for determining the errors of her predicted positions at such times are therefore wanting. The continuity of the watch which is kept upon the moon's intricate motions at a great observatory-Greenwich, for instance-is broken at this period of every lunation, except when an eclipse is visible from the observatory, and then the black moon can be observed upon the sun just as the bright moon is observed upon the sky. In less accurate times than the present this observation was simply made by noting the instants of beginning and ending of the eclipse; but there is so much uncertainty, depending chiefly upon the dimensions of the instruments employed, in noting these instants, that the data thus procured are rarely made use of. Occasionally an old Amateurs often betray great anxiety to secure the accurate times of first and last contacts in observing solar eclipses (the late eclipse offered abundant instances). Under the best of circumstances those times are of eclipse observation is available for fixing the moon's approximate place in the absence of better material, such as meridian determinations. An instance of such application presented itself a few months since, when Professor Newcomb, wishing to ascertain how, within wide limits of error, the present lunar tables represented the moon's place a century and a half ago, resorted to the contacts in the eclipse of 1715 observed by Flamsteed, Halley, and Pound. Such rough observations would be of little avail now. In order to extract useful data from the passage of the moon over the sun, the Astronomer Royal some years since devised a plan of measuring the cusps in their changing positions throughout an eclipse in such a manner as to bring out the errors of all the numerical elements concerned in the prediction of the phenomenon, the most important of which are the co-ordinates of the moon's position. This method was first put in operation in 1836, and again, with great instrumental power, during the eclipse of July 18, 1860, when the deduced errors of the "tabular places"-as the places predicted by calculation are called-were found to agree closely with those exhibited by observations of the moon which were procurable in the ordinary manner near to the time of conjunction, though there is necessarily a small incomparability from the virtual difference between the black moon on the bright sun, and the bright moon on the black sky; the effects of irradiation being reversed in the two conditions. These observations were repeated at Greenwich during the eclipse under notice, and with a similar resulting agreement between the observed and calculated data. It will be obvious that they do not require the eclipse to be total at the place of observation. One distinguished mathematical astronomer, however, Professor Newcomb, who is understood to be engaged upon the construction of new tables of the moon's motions, thought it desirable to make the cusp-measures directly upon the line of totality, and he came from the United States for the sole purpose of so making them. He stationed himself at Gibraltar, and saw enough of the eclipse in its partial stages (the total phase had no scientific interest for him) to secure what observations he desired. But he was baulked in another way. His measurements would be of no use without a very exact knowledge of the longitude of his station from a fixed observatory, or from Greenwich; without this, an essential datum, the astronomical time at which each was taken could not be obtained. Preparations of somewhat elaborate character were made to determine this longitude by the method of little value; and they are quite useless where the longitude of the observing station is not very exactly known, as is often the case. exchanging accurate time-signals by electric telegraph: Greenwich was to give its time to Gibraltar, and Gibraltar to return its time to Greenwich; Gibraltar local time being accurately determined at Professor Newcomb's temporary observatory. This exchange was to be made on several days before and after the eclipse through the medium of the Falmouth and Gibraltar cable, but, as may be remembered, the cable broke early in December, and it was not repaired till long after Professor Newcomb had left the Rock. His observations are consequently useless until another opportunity offers for effecting the longitude determination: then they can be made available. * The eclipse has therefore been of some import to metrical astronomy. Let us now take a glance at the materials which have been gleaned from it towards a solution of the physical questions at issue at the time of its occurrence, and to which it was appealed to decide. Of the phenomena revealed when the moon hides the photosphere of the sun, the corona only remains enigmatical. Baily's heads were long ago explained out of interest, and the red prominences now no longer need an eclipse to bring them under study. The object the most striking and the most anciently remarked is still the most bewildering. At the time of the eclipse four modes of observation were at hand to resolve the mystery of its nature. First, eye-sketches, with or without telescopic aid, to decide whether the corona is similarly depicted by observers near together and far apart. Second, photographic pictures, which would give the aspect of the corona free from personality (though they may include subjective appearances of photo-chemical character). Third, spectroscopy, to determine the gaseous or incandescent solid condition of the original source of the coronal light. Fourth, polariscopy, which it was hoped would show whether that source is in the corona itself or apart from Dr. Schmidt, of Athens, calls attention to the following account of an eclipse seen at Corfu in A.D. 968, in which the corona is very clearly described :-"Leon, the deacon, reports thus concerning the eclipse. The appearance of the eclipse was of this nature: December was carrying on its 22nd day, and in the fourth hour of the day, the sky being clear, darkness covered the earth, and the brightest stars appeared; and it was possible to see the disc of the sun obscure and without brightness, but with a certain radiance, faint and pale, in the manner of a fine band shining in a circle round the disc along its outer edge: and the sun, overlapping the moon a little (for she appeared directly intercepting him), sent out its own rays and filled the earth with light.'" The date curiously coincides with that of the last eclipse, though it is Old Style reckoning. The appearance of the stars seems to prove that the eclipse was total: it is marked so in the reliable list of Eclipses given in the Art de Vérifier les Dates des Faits Historiques. |