Abstract
BENGAL
Asiatic Society, June 7.—“Memorandum on the Total Eclipse of December í 1*and 12, 1871,“by Lieut.-Col. J. F. Ten-rant, K.E., F.R.S. In December of this year we have a Total Eclipse visible in Southern India. The duration is short, but in some respects the circumstances are very favourable, as the Line of central Eclipse passes over the Nilgherry Hills, where, I understand, fine weather may be confidentle expected. In order to be prepared, I have computed carefully the Central Line across India, and have added the extent to which errors of the Tabular place of the moon may be expected to shift it. I hope to have before the Eclipse a knowledge of what errors may be anticipated in the Tables, and thus be in a position to choose a central spot, if it is worth making a change. The figures, however, show that this is not probable, the principal result of an error in Right Ascension being to shift the centre of the shadow along its path, the deviation from which would be corrected by a small error in the declination which could hardly be foreseen. The duration of the Eclipse will be small. At the Nilgherries it will be about two minutes, but this cannot, so far as I know, be as yet accurately predicted, from uncertainty as to the real diameters of the sun and moon, when free from the enlargement by irradiation. If the value of ihe moon's diameter deduced by Oudemans from Eclipses, be used with that of the sun obtained in the Greenwich Transit Circle, then I find the duration in the Nilgherries just two minutes. The data of the Nautical Almanac give two minutes seven seconds, and if I may judge from the result I got in 1868 the real duration will fall between these. Short as this time is, it is enough with an adequate preparation to produce some results of value. It is long enough to allow photographs to be taken of the Corona, as to whose structure there is more to be discovered. There seems now no sort of doubt that the Corona is not only a solar appendage, but is, as I stated in my report on the Eclipse of 1868, the comparatively cold atmosphere of the sun. This should be further spectrosc-˜pically examined. Observers have differed about the number and position of the faint bright lines they have seen, but it does not seem that any one has connected the variations with the position of the part examined. To do this appears urgently necessary, and there have been additions made to the spectroscope which will allow more than one portion of the Corona to be examined, and its lines recorded during the short time it is visible. There isanother subject, too, of spectroscopic examination. Kirchhoff,in his theory of the solar constitution, supposed it surrounded by an extensive atmo-phere consisting of metallic and other vapours, as well as gases, by the absorption of which the dark Fraunhofer lines were produced. It has long been clear that there was no such extensive atmosphere, and some physicists have been satisfied that there is none such. Mr. Lockyer and his collaborateurs, though they have detected a great number of bright lines at the bases of the prominences, have never approached, so far as I know, the number of even the conspicuous dark lines, whose origin has, therefore, not been satisfactorily made out. At the Eclipse of December 22, 1870, however, Prof. Young, at the moment of obscuration, and for one or two seconds later, saw, as far as lie could judge, every atmospheric line reversed, and this was confirmed by Mr. Pye. I have but the scant information of this point given in the Royal Astronomical Society's Council Report, but it is sufficient to show me why this has not been seen before by observers looking out for it, and also to make me feel the importance of verifying the observation. To understand why it has not been seen before, it must be considered that ihe image of a bright object in the focus of a telescope when relieved against comparative darkness is enlarged by a phenomenon known as irradiation; the light encroaches on the darkness. The sun thus appears larger and the moon smaller than the real size. This continues till the real contact of the limbs internally; at this moment the thread of light, which previously had considerable width, appears suddenly broken and vanishes in a total ec'ipse; while in the transit of a planet or annular eclipse there appears the “black drop “of the observers of the Transit j of Venuin 1769. At page 16, vol. xxix, of the monthly notices of the Astronomical Society will be found some figures illustrating this phenomenon in a planetary transit. When we are dealing with so thin a stratum surrounding the true photosphere, we cannot see it in sunshine, as it is lost in the irradiation (t may be partly visible in very large telescopes where the irradiation s very small), and we are very ap: to lose it a the moment when the sun disappears, for it is found only between the places where a moment before the sun and moon's limb appeared, so that the observer following either of them might well miss it. In the search for and verification of this important observation, the duration of total phase can matter little. I have been in communication with the Home Secretary on the subject of observations of this eclipse, and my views, I may say, have btenmost cordially receivd. I am not yet in a position to submit a proposition officially, but I have great hopes of being able to do so in a few days.* I may just mention that in plotting the shadow track on a map it is necessary to allow for the error of its zero of longitude, a precision often forgotten. The longitudes of the G. T. Survey require a correction of 3'-2'7", and tho-e of the Atlas of India one of 4/-I1“to adjust them to the accepted longitude of Madras.
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Societies and Academies . Nature 4, 339–340 (1871). https://doi.org/10.1038/004339a0
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DOI: https://doi.org/10.1038/004339a0