this appearance, the eye (which in fig. 1 is supposed to view the cloud in a direction perpendicular to the plane of the circle) must be so placed that the visual rays may be acutely inclined to the plane of the cloud; also that the cloud, if I may be allowed the use of an expression so awkward, must pass quite round on the other side of the moon; that is, on the side at the greatest distance from the earth; and this I hold to be impossible on the following grounds: Wherever there are clouds, there must be an atmosphere to support them. The atmosphere of the earth does not reach to the moon; and even allowing that it did, the clouds which surround this planet do not extend so far from its surface as the orbit of the moon is distant. The cloud observed did not exist in the atmosphere of the moon, inasmuch as no telescope that I ever heard of could detect such things, and this appearance was perceptible to the naked eye. From these considerations, we may, I think, infer, that the cloud did not exist on the further side of the moon; and, therefore, that there could be no such appearance as that described by Dr. Clarke. Let us now consider what would have been the appearance of the cloud, supposing it actually to encircle the moon in the direction of the horizon. The moon, as we may judge from Dr. Clarke's representation, was at the full, and the luminous appearance of the cloud was derived from her light. But the half of the moon most distant from the earth would not, in this situation of circumstances, have been illumined by the sun, and could, therefore, reflect no light on the adjacent part of the nebulous circle. The further half of the ring would thus not have been luminous; in other words, it would have been invisible. I have only further to observe, that if the moon had really been surrounded by a circle of cloud, and if it had been so viewed that the plane of the cloud produced passed through the eye, the appearance would have been as in fig. 3. Now nothing is more common than to see a strait band of cloud, such as in Mr. Howard's nomenclature is called a Cirrostratus, stretching across the disc of the moon. Such an appearance is described by Dr. Clarke himself, at p. 485, where he says, that the moon appeared as if divided into two parts. We are perfectly familiar with such appearances in this climate, yet nobody ever supposed that they are caused by circles of cloud viewed in a particular direction. Indeed if this is the case with the Doctor, he has made a most unwarrantable assumption, the grounds of which he can best explain. If this was not the appearance of the moon, as viewed by Dr. Clarke, I can only account for the phenomenon by supposing it such as is represented by fig. I. In either case, the Doctor must have taken the liberty of supposing the point of view altered in order to produce the appearance of Saturn, with a representation of which he has treated his readers in fig. 2. explanation of the passage than appeared to him necessary in a work intended for general circulation. I am far from wishing to insinuate that the Doctor allows himself the traveller's licence, but cannot help observing that the circumstance, as it is stated in his work, is calculated to try the faith of his readers to the The to which I allude is the account of the appearpassage ance of the moon,* as observed by him on the road from Tornea to Kiemi, at p. 487. After having described the oval appearance of the moon's disc, he proceeds: utmost. "This changeful scenery still continued, varying at every instant at last there ensued a more remarkable appearance than any we had yet witnessed. The vapours dispersed, and all the rolling clouds disappeared, excepting a belt collected in the form of a ring, highly luminous, around the moon, which now appeared in a serene sky, like the planet Saturn augmented to a size 50 times greater than it appears through our best telescopes. The belt by which the moon's rays were reflected, became, beyond description, splendid, and the clear sky was visible between this belt and the full fair orb which it surrounded. Certainly if the same phenomenon had been visible in England, the whole country would have been full of it from one end of our island to the other." In reading this passage nothing remarkable is observed; for it is easy to conceive that a circle of clouds may have been formed through which the orb of the moon was visible, as represented in fig. 1. But as descriptions in words whether "demissa per aurem or on paper "Segnius irritant animos Quam quæ sunt oculis subjecta fidelibus " (viz. drawings); and as Dr. Clarke has accompanied his description with a wood-cut, which places the matter in a point of view entirely different, I intend on this cut to found my objections. Figure 2 is a copy of the representation which the Doctor has given of this phenomenon. Now it is evident that to produce The application of the term "planet" to the moon, at p. 485, is, I think, of doubtful authority, and should have been rejected by the philosophic Clarke. According to this new nomenclature, the satellites of Jupiter, of Saturn, and of Uranus, are all planets; however, "de minimis non curat lex." this appearance, the eye (which in fig. 1 is supposed to view the cloud in a direction perpendicular to the plane of the circle) must be so placed that the visual rays may be acutely inclined to the plane of the cloud; also that the cloud, if I may be allowed the use of an expression so awkward, must pass quite round on the other side of the moon; that is, on the side at the greatest distance from the earth; and this I hold to be impossible on the following grounds: Wherever there are clouds, there must be an atmosphere to support them. The atmosphere of the earth does not reach to the moon; and even allowing that it did, the clouds which surround this planet do not extend so far from its surface as the orbit of the moon is distant. The cloud observed did not exist in the atmosphere of the moon, inasmuch as no telescope that I ever heard of could detect such things, and this appearance was perceptible to the naked eye. From these considerations, we may, I think, infer, that the cloud did not exist on the further side of the moon; and, therefore, that there could be no such appearance as that described by Dr. Clarke. Let us now consider what would have been the appearance of the cloud, supposing it actually to encircle the moon in the direction of the horizon. The moon, as we may judge from Dr. Clarke's representation, was at the full, and the luminous appearance of the cloud was derived from her light. But the half of the moon most distant from the earth would not, in this situation of circumstances, have been illumined by the sun, and could, therefore, reflect no light on the adjacent part of the nebulous circle. The further half of the ring would thus not have been luminous; in other words, it would have been invisible. I have only further to observe, that if the moon had really been surrounded by a circle of cloud, and if it had been so viewed that the plane of the cloud produced passed through the eye, the appearance would have been as in fig. 3. Now nothing is more common than to see a strait band of cloud, such as in Mr. Howard's nomenclature is called a Cirrostratus, stretching across the disc of the moon. Such an appearance is described by Dr. Clarke himself, at p. 485, where he says, that the moon appeared as if divided into two parts. We are perfectly familiar with such appearances in this climate, yet nobody ever supposed that they are caused by circles of cloud viewed in a particular direction. Indeed if this is the case with the Doctor, he has made a most unwarrantable assumption, the grounds of which he can best explain. If this was not the appearance of the moon, as viewed by Dr. Clarke, I can only account for the phenomenon by supposing it such as is represented by fig. I. In either case, the Doctor must have taken the liberty of supposing the point of view altered in order to produce the appearance of Saturn, with a representation of which he has treated his readers in fig. 2. If you do not consider the matter as too trivial to occupy a place in the Annals, I shall feel obliged by your affording me an opportunity of meeting with an explanation, and am, Sir, Your most obedient servant, S. XIV. Notice of an Annular Eclipse in the Thirteenth Century. By the Rev. James Yates, M.G.S. SIR, (To Dr. Thomson.) Birmingham, May, 1819. The learned and curious observations of Mr. Francis Baily upon the annular eclipse of the sun, which will take place in September, 1820,* induce me to think that the following notice of one, which was seen in the same quarter of the globe, may be interesting to some of your readers. The passage occurs in "The Norwegian Account of Haco's Expedition against Scotland," first published in the original Islandic, with a literal English version, by the Rev. James Johnstone, A.D. 1782. I extract both the original Islandic, and Mr. Johnstone's English translation. "á er Hakon Konongr lá i Rögnvalzvagi dró myrkr mikit á sólina, sva at lítill hringr var biartur um sólina utan, ok hellt þ ví nockora stund dags." Id est, "While King Haco lay in Ronaldsvo a great darkness drew over the sun, so that only a little ring was bright round the sun, and it continued so for some hours."-P. 44, 45. Haco invaded Scotland in the year 1263; he sailed with his navy into Ronaldsvo, which appears to have been the name of a bay or harbour in South Ronaldsay, one of the Orkney islands, some time "after St. Olave's wake;+" and he quitted Ronaldsvo on the day of St. Lawrence's wake." These two days correspond to July 29 and August 9, which fixes the time of the eclipse with considerable precision, and shows it to be the same, which is marked in catalogues as having happened on Aug. 5, 1263. Had this account been published at an earlier period, it might have supplied in some degree the long chasm remarked by Maclaurin, who says, that Ricciolus in his Catalogue mentions no annular eclipse from the year 334 to 1567. In the last edition of the "Art de verifier les Dates," A.D. 1783, this eclipse is marked as annular. The expression "it continued so for some hours," must be understood to mean only, that the obscuration of the sun continued for some hours. |