Astronomical Register: A Medium of Communication for Amateur Observers and ...

JUPITER'S SATELLITES.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir,—In reading any work on Astronomy, one cannot help feeling how short and unsatisfactory is the explanation of a phenomenon occurring sometimes in the transits of Jupiter's satellites, and yet spoken of as a curious fact—I mean the passage of one or other of them across the disc of the planet as a dark spot.

Now, as your subscribers must of necessity consist chiefly of persons who, like myself, take up the study as a recreation, and are therefore likely to be interested in questions which would be beneath the notice of, and perhaps waste of time with, professed astronomers, I may with the less presumption venture to offer a few remarks upon this question, and beg your insertion of them should they be deemed not unworthy to occupy a page or two in your Register.

Whatever other causes there may be for such an appearance, it would undoubtedly take place, if at the time of transit it were under eclipse by one of its superiors; and this under certain coincidences must happen, and under very favourable circumstances may be seen. Yet these coincidences are so far between, and these circumstances so seldom favourable, that we need not feel surprised, on reading in a work only half a century old, that "the satellites of Jupiter have never been known to eclipse one another"-though more recently it has been found that they sometimes do. Evidently this should happen most frequently to the first, less so to the second, very rarely to the third, and I might almost say never to the fourth.

Were all their orbits in the same plane with that of their primary, an eclipse of one by another would be a matter of very frequent Occurrence. Within the limits of those portions of their orbits in which only a transit can take place, the first would be eclipsed by the second in every other one of its revolutions fourteen or fifteen times in succession, and then pass an interval of nearly two hundred revolutions without undergoing any eclipse at all. It would be eclipsed by the third in every fourth revolution twice or three times consecutively, and then pass an interval of nearly sixty revolutions without any. The second would be eclipsed by the third in every other one of its revolutions five or six times consecutively, and then pass an interval of about one hundred and twenty without any. The limits within which a transit of the first can take place are about 42° of its orbit, of the second about 35°, of the third about 30°. The longest duration of any eclipse, from first immersion to complete emersion, would be, of the first by the second about 35 minutes, and by the third about 24; of the second by the third about 50. The duration of that part of the eclipse, of the first by the second, in which the whole of the shadow would take effect, would be a little over 3 minutes, and this when central would be annular, the shadow of the second being so much smaller than the body of the first; and consequently this satellite, when transiting under this condition, would appear still as a round spot, yet smaller than its own shadow, which would have just passed, or would be just about to pass, across the disc of the planet.

The duration of the total eclipse of the first by the second would be about 4 minutes, and of the total eclipse of the second by the third about 12.

But the orbits of the satellites are not in the same plane with that of their primary, being inclined at an angle of about 3°, and in consequence of the revolutions of their nodes are not often in the same plane with one another; and no eclipse of the first by the second can take place when the angle between their lines of nodes is greater than 9°, or by the third when the angle between their lines of nodes is greater than 12°, or of the second by the third when the angle between their lines of nodes is greater than 7°, even when both satellites are in the ascending or both in the descending. But if the lines of nodes be respectively within these limits, and at the same time within about 15 of the radius vector of the planet, then it is possible, under all the circumstances above stated, for an eclipse to take place during the time of transit. The limits here given of the angular distances of the lines of nodes are, however, for an eclipse where the shadow would only just touch and pass away immediately; whereas for such an eclipse as would have a strongly marked effect in causing the satellite to make the transit as a dark spot, the lines of nodes must be nearly coincident, and this coincidence of the lines of nodes of the first and second, or of the first and third, in the proximity of the radius vector of the planet, cannot occur within a less interval than six years-how much greater I am unable to say, as I do not know how nearly the time of revolution of nodes of the first satellite has been determined; that of the second and third in not much less than thirty; and that of the third and fourth in not less than one hundred and twenty. And even then many such periods may pass, and the occurrence take place without a possibility of observing it; for though under the above-named circumstances and conditions such an eclipse, and consequently such a transit of a satellite as a dark spot would take place, it might yet do so at a time of year when the planet is too nearly in conjunction with the sun, or at a time of day when it would be below the horizon, as regards any place where an observation could be taken-for I imagine it would require rather a powerful instrument to make it—or when the night was too cloudy and nothing whatever could be seen. I have not attempted to give the limits, &c. for the coincidences of the half revolutions of the lines of nodes, when one satellite is in the ascending and the other in the descending, as the difficulties in the way of the occurrence are so greatly increased, and the chances of seeing it, through the extreme shortness of its duration, are so greatly diminished-my object being to excite enquiry as to the times at which it may be seen. I have no means of ascertaining the exact present positions of the lines of nodes of the several satellites, and cannot at all, therefore, say when the phenomenon is likely to take place again; but some of your friends will be able to do this: and if anyone could be induced to think the question worth the trouble of working out, and to send the result to you, I doubt not it would prove acceptable to many of your readers. I am, Sir, faithfully yours,

E. J.

METEOR OF SEPTEMBER 24, 1865.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Through the kindness of Mr. Glaisher, I lately received a third observation of the meteor of Sunday evening, the 24th of September last, described in the Astronomical Register for October. ́I quote from his Meteorological Register at Greenwich, as follows:

"1865. September 24th, 7h. 48m. 45s. P.M. G.M.T.—A very bright meteor, larger and brighter than Jupiter, bluish white; duration two to three seconds; faint train; path 30 degrees in length, parallel to the ecliptic, from near y Aquarii to near a Capricorni.'

Comparing this observation with those of the same meteor already described in the Astronomical Register for October, the height of the meteor is found to have been

At commencement, 67 miles above the coast of France; N. lat. 49° 57', E. long. 2° 12′.

At disappearance, 38 miles above the English Channel; N. lat. 50° 26', E. long. 0° 23'.

Path, 91 miles in four seconds, directed from a point in R.A. 2o, N. Decl. 2°, close to the first point of Aries. Velocity, 23 miles a second.

The meteor was directed from the radiant-point in Cetus or Pisces, described in the Monthly Notices of the Astronomical Society for December 9, 1864. If ten marks are to be reckoned for the best examples of accordance, I would not value this result at less than six, as the meteor was three times observed.

I am, Sir, your obedient servant,
ALEXANDER S. HERSCHEL.

CHEL

[Mr. Herschel's letter was accompanied by a well-executed drawing, representing the path of the meteor as observed at Hawkhurst and Greenwich.—ED.]

NOTES AND GLEANINGS.

SOLAR SPOTS.-Some rather large spots have been visible on the surface of the sun during the past month, one or two of which were large enough to be distinguished without optical assistance. Our correspondent, Mr. Brodie, has written to the Times in relation to them at some length. Mr. Dawes has also communicated the following letter to the same journal:-" Sir,-There is now a fine solar spot distinctly visible with the naked eye, properly defended by a piece of coloured glass. A delicate train of small spots, visible with a small telescope, follows the large one. The visibility of spots on the sun by a good eye without optical aid is by no means so rare a phenomenon as seems to be generally supposed. Even by scientific writers of eminence, particular instances of such an occurrence have been often referred to, and the year and day and the observer quoted, as if it

were important that so rare a fact should be exactly specified. It would, therefore, scarcely be imagined that a year seldom, if ever, passes without such an occurrence. Last year, between the middle of May and the middle of June, five spots or groups of spots were visible with the naked eye, four of which were very obvious. I have occasionally seen two such on different parts of the sun's disc at the same time.-I am, Sir, your obedient servant, W. R. DAWES.-Hopefield Observatory, Haddenham, Bucks, Oct. 13." Mr. Adams, of Edgbaston, has sent us a sketch of the above spot, which was, he says, "remarkable for the clear and defined ring of light surrounding the middle of the umbra." This was at 3h. 30m. on the 15th Oct.

THE NEW PLANET (No. 84) has received the name of Clio. We hope for the future to be able to report an improved taste in the selection of these names, after such barbarities as Maximiliana, Nemausa, &c. The following are the elements of Clio's orbit, according to the calculations of Mr. Dolman of Durham :

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The figures of M. Valentiner, of Berlin, do not differ essentially from the above.

ENCKE. Our readers will learn with regret that this distinguished Prussian died at Spandau on Aug. 26, at the age of 74.

ASTRONOMY WITHOUT MATHEMATICS is the title of a new work by E. B. Denison, Q.C., of clock fame, just published by the Society for Promoting Christian Knowledge.

ECLIPSES IN OCTOBER.-The lunar eclipse of October the 4th has been photographed by Mr. De la Rue, and, the atmosphere and other conditions having been very favourable, an excellent result has been obtained. Mr. De la Rue has arranged a stereoscopic effect, by combining this new image with one of a former eclipse, the result of which is very striking. We are informed that Mr. A. Brothers, of Manchester, has also been very successful in obtaining photographs of this eclipse. The solar eclipse of the 19th was invisible in London, the weather being wet and overcast the whole day.

COMIC SCIENTIFIC PERIODICAL. Messrs. Trübner & Co. are about to issue a publication entitled Gammon and Spinach, on the ground that the boundary between the sublime and the ridiculous is as often overstepped by the savant as by others, and that such false steps offer a wide field of fun.

NEBULE.-No. 1541 of the Astronomische Nachrichten contains some interesting notes by M. Schultz, of Upsala, on 36 nebulæ selected from Sir J. Herschel's catalogue.

LORD WROTTESLEY.-An interesting lecture on Astronomy, by Lord Wrottesley, will be found in the Birmingham Daily Post of Sept. 26.

ADMIRAL SMYTH.-Some may be glad to know that a portrait and notice of the late lamented Admiral will be found in the Illustrated London News of Sept. 30.

Mr. W. C. BURDER, the well-known meteorologist, died at his residence at Clifton on the 18th of October, at the comparatively early age of 43. The Times adds: "He was the discoverer of the small but beautiful comet of March and April 1854, and also of the large comet of June and July 1861."

8 LIBRE.-Schmidt, some time since, announced that he had grounds for suspecting this somewhat conspicuous star to be variable. Writing from Athens, under the date of May 27, he reiterates the statement. We infer that he is in doubt about the period, though this would seem to be a little under 7 days.

COMETARY INTELLIGENCE.