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Nature volume 20, pages 512513 | Download Citation



BIELA'S COMET IN 1852.—In view of the probable approach to the earth's orbit of the two heads of Biela's comet in the present year, it is not without interest to recall the circumstances under which these bodies were last observed in the autumn of 1852. As soon as the calculated place of what was assumed to be the principal comet of 1846, according to Santini, was sufficiently removed from the sun's place to afford a chance of discovery, a search was commenced at several European observatories, notably by Secchi, at the Observatory of the Collegio Romano at Rome. The comet was not found in its computed position, and the cause of this is now known to have been the abandonment by Santini of his old semi-axis major, founded originally upon Damoiseau's calculation of the perturbations of mean motion between the appearances in 1805–6 and 1826, and the observations of those years and the substitution of a value deduced by Plantamour from the observations in 1845–46; had the original semi-axis been retained the comet would have been readily found by means of Santini's computations. Extending the limits of the search, therefore, Secchi detected a faint comet on the morning of August 26, 1852, some 6° from the calculated place, which Prof. Peters of Altona immediately pointed out as probably one portion of Biela's comet, from the rate and direction of its motion, as, indeed, it proved to be. (In Memorie dell' Osservatorio del Collegio Romano, anni 1852–55, the discovery is dated, by a misprint, August 16, civil reckoning, the first observation was made on August 25, at 16h. 14m. M.T.) This object was observed on several subsequent mornings, and on September 16 Secchi found the other head of the comet, following that previously observed about two minutes of time, and about half a degree to the south. With the great refractor at Pulkowa, M. Otto Struve found Secchi's comet of August 26, on September 18 (astronomical), or immediately after the notice reached him, and two mornings later, he observed both heads. Mr. James Breen, to whom Prof. Challis had intrusted the Northumberland equatorial at Cambridge for a search for the comet, found one portion of it on September 8, and observed it further on September 16 and 21. At Berlin one head was detected on September 17, and reobserved on September 22. M. Otto Struve, in his account of the Pulkowa observations, calls that head of the comet which was first observed by Secchi on August 25, A, and that found on September 15, using now astronomical dates, he calls B; the latter was the north-preceding comet, the former the south-following one. A discussion of the observations of both heads, twenty-two in number, showed that those at Cambridge referred to A on all three mornings, and those at Berlin to B; both nuclei were observed at Rome on September 19 and 20, and at Pulkowa on September 20, 23, and 25. The appearance of the two portions of the comet is best described in M. Otto Struve's memoir, which is also accompanied by two admirably executed drawings, depicting their relative aspect on September 20 and 25, B on September 18 was at least 30′ in diameter, with sensible brightening in the centre, but no decided nucleus, and the light of the comet was about equal to that of a star of Argelander's ninth magnitude. On September 20 A was easily seen with the finder of the large refract or B both heads were of about equal brightness, B might be a little the brighter, and exhibited a distinct nucleus; the nucleus of A was not so distinct as that of B, and there was a greater brightness of the nebulosity, as well as an extension of it in the direction of B; the apparent diameters about 1′ and 40″; the diameter of B, which was circular, was estimated 40″. On September 23 A was notably fainter than B, and without nucleus; the lengthened form of A was only seen with difficulty, but the sky was not quite transparent. On September 25 there was a remarkable change as compared with the relative appearance of the two heads five days before; A was materially fainter than B; the latter was very distinct in the finder, while the place of the former was hardly suspected; diameter of A about 30″, that of B from 50″ to 60″. A was round, B slightly oblong; the brightest part of A was not in the centre of the nebulosity, but in the direction of B, and the nucleus of B was in the opposite direction to A, the brightest part of the nebulosity unequally distributed about the nucleus of B being turned away from A; the position-angle of this direction was 286°. On September 28, the last day of observation, the moonlight was strong, and B only was seen with difficulty. We give these details, not remembering to have seen them reproduced in this country; but the description fails to convey the impression made by comparing M. Otto Struve's drawings of September 20 and 25; were it not that we know to the contrary, it might almost be inferred there from that one portion of the comet had revolved round the other to the extent of 180°; their relative appearance had been wholly interchanged, and it will be remembered that about February 12, 1846, the secondary comet much exceeded in brightness the primary one, though this continued only three or four days, when the latter resumed its previous decided superiority. There was thus, as M. Struve remarks, the same interchange of brightness between the two nuclei at both appearances, and this he is inclined to attribute to a mutual action. It may, however, be remarked that the distance between them in 1852 was, according to Hubbard, 0.0193, or about 1,750,000 miles, which seems to militate against such an explanation, and rather to induce an idea of action inherent in the separate comets, or of influence exercised upon them through their approach to the sun. At M. Struve's observations of September 20, using Hubbard's elements, we find the distance of A from the earth was 1.492, and that of B, 1.483; while on September 25, the distance of A was 1.525, and of B, 1.511; so that there was no marked change of distance between the dates of his drawings.

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