« PreviousContinue »
readers whom I address is the Rev. T. W. Webb's, reduced from Beer and Mädler's. Undoubtedly, however, the most minutely accurate and elaborate lunar map yet made is the one of 7.67ft in diameter, by Schmidt of Athens, published at the expense of the German Government in 1878. Maps by Russell and by Blunt are in circulation, but they are not of much value as regards details.
The British Association for the Advancement of Science, through a sub-committee, began in 1866 the preparation of an entirely new map of the Moon, but this was eventually abandoned by the Association. The late Mr. W. R. Birt, however, continued it for a time.
A wax model of the whole lunar surface was executed many years ago by a Hanoverian lady named Witte, and Nasmyth has modelled in plaster of Paris several single craters. Photography, too, has been called in by De La Rue, Rutherford, and others, with good results.
In computing the places of the Moon the Tables of Burckhardt, published in 1812, were formerly used, but in 1862 the new and more perfect Tables of Hansen were introduced at the Nautical Almanac office; and these have entirely superseded Burckhardt's. Damoiseau, Plana, Carlini, Pontécoulant, Lubbock, and afterwards Delaunay, in addition to Hansen, did much to improve the theory of the Moon. Delaunay's labours earned for him a foremost place in the rank of geometrical astronomers. More recently still, Sir G. B. Airy has been treating the subject by a new method. His memoir entitled the “ Numerical Lunar Theory” was published in 1887. He is understood to be still investigating some points in it which need further elucidation b.
According to a recent determination by Stone the Moon's mass is giso that of the Earth.
To record a tithe of the influences ascribed to the Moon would be a herculean task; nevertheless (in addition to the tides) one
& Fig. 65 is from a photograph of one of these. But they are of little value, being very inexact.
h Month. Not., vol. xxxiv. p. 89. Jan. 1874.
deserves notice. Evening clouds at about the period of Full Moon will frequently disperse as our satellite rises, and by the time it has reached the meridian a sky previously overcast will have become almost or quite clear. I first observed this in 1857, and subsequently found that Sir J. Herscheli had made the same remark. The idea has been disputed", but I am firmly convinced of its truth. Humboldt speaks of it as well known in South America, and Arago indirectly confirms the theory when he shows that more rain falls at about the time of New Moon (cloudy period) than at the time of Full Moon (cloudless period according to the theory). According to Forster, Saturday new Moons result in 3 weeks of wet weather. He alleged that observations extending over 80 years showed this coincidence'. Bernadin asserts it as a fact that many thunderstorms occur about the period of New or Full Moon. With these possible exceptions it is safe to assert that “changes” of the Moon have no discoverable influence on the weatherm.
i Outlines of A st., p. 285.
Ellis, Phil. Mag., 4th Ser., vol. xxxiv. p. 61. July 1867.
Month Not., vol. ix. p. 37. Dec. 1848. m See Nasmyth and Carpenter, Moon, p. 180.
THE ZODIACAL LIGHT.
General description of it.-When and where risible.-Sir J. Herschel's theory.
Historical notices.- Modern observations of it.- Backhouse's Conclusions.
ASTRONOMICAL writers are not agreed as to the proper head A under which to describe and discuss the Zodiacal Light. I deal with it here, because, whatever its origin, it is a matter of terrestrial cognizance, and therefore a description of it may, without any serious incongruity, be associated with what has to be said about the Earth.
The Zodiacal Light is a peculiar nebulous light of a conical or lenticular formą, which may very frequently be noticed in the evening soon after sunset about February or March, and in the morning before sunrise about September. It extends upwards from the Western horizon in the spring and from the Eastern horizon in the autumn, and generally, though by no means always b, its axis is nearly in a line with the ecliptic, or, more exactly, in the plane of the Sun's equator. The apparent angular distance of its vertex from the Sun's plane varies, according to circumstances, between 50° and 70°; sometimes it is more; the breadth of its base, at right angles to the major axis, varies between about 8° and 30°. During its evening apparition it usually reaches to a point in the heavens situated not far from the Pleiades in Taurus. It is always so extremely ill-defined at
A Lens, a lentil.
6 Month. Not., vol. xxx. p. 151. Murch 1870, et infra,
the edges that great difficulty is experienced in satisfactorily determining its limits. In Northern latitudes the Zodiacal Light is generally, though not always, inferior in brilliancy to the Milky Way; but in the Tropics it is seen to far greater advantage. Humboldt said that it is almost constantly visible in those regions, and that he himself had seen it sufficiently luminous to cause a sensible glow on the opposite quarter of the heavens. In the winter of 1842-43 it was remarkably well seen in this country, the apex of the cone attaining a length of no less than 1059 from the Sund. Lassell also mentions having seen the light very conspicuous at Malta in January 1850 • No satisfactory explanation has yet been given of this phenomenon ; it is, however, very generally considered to be a kind of envelope surrounding the Sun, and extending perhaps nearly or quite as far as the Earth's orbit. Sir J. Herschel's opinion was " that it may be conjectured to be no other than the denser parts of that medium which we have some reason to believe resists the motion of comets ; loaded, perhaps, with the actual materials of the tails of millions of those bodies, of which they have been stripped in their successive perihelion passages [!!]. An atmosphere of the Sun, in any proper sense of the word, it cannot be; since the existence of a gaseous envelope propagating pressure from part to part, subject to mutual friction in its strata, and thereby rotating in the same, or nearly the same, time with the central body, and of such dimensions and ellipticity—is utterly incompatible with dynamical laws?.” In connexion with this speculation it may be mentioned that during the visibility of the great comet of 1843 in March of that year, the Zodiacal Light was unusually brilliant ; so much so, that by many persons it was mistaken for the comet.
The Zodiacal Light is of a reddish hue, especially at its base, < But on this point see Humboldt's xiv. p. 16, Nov. 1853. Observations by later statement on p. 145, post.
Burr and Webb will be found at pp. 45, & Detailed particulars will be found in 83, and 181 of the same volume; and see the Greenwich Obserrations, 1842. a paper by T. Heelis in Mem. of the Lit.
For observations by E. J. Lowe, see and Phil. Soc. of Manchester, 3rd Ser., Month. Not., vol. x. p. 124, March 1850; vol. ii. p. 437, 1865. vol. xi. p. 132, March 1871; and vol. ' Outlines of Ast., p. 658.
where also it is most bright, and where it effaces small stars. Undulations and likewise a sort of flashing have been noticed in it.
It has been suggested that the Zodiacal Light is identical with what Pliny and Seneca call the “Trabes 8,” but more likely this was the Aurora.
The Zodiacal Light was treated of by Kepler; afterwards by Descartes, about the year 1630; and then by Childrey, in 1659b; it was not, however, till J. D. Cassini, who saw it first on March 18, 1683, published some remarks on this phenomenon that much attention was paid to it i.
In the year 1855, the Rev. G. Jones, Chaplain of the U. S.. Steam-Frigate Mississippi, published some remarks on this phenomenon , as brought under his notice during a cruise round the world in the 2 preceding years. He stated :-“I was also fortunate enough to be twice near the latitude of 23° 28' North, when the Sun was at the opposite solstice, in which position the observer has the ecliptic at midnight at right angles with his horizon, and bearing East and West. Whether this latter circumstance affected the result or not, I cannot say; but I there had the extraordinary spectacle of the Zodiacal Light simultaneously at both East and West horizons from 11 to 1 o'clock for several nights in succession.”
Mr. Jones concluded his very interesting letter as follows :“You will excuse my prolixity in stating these varieties of observations, for the conclusion from all the data in my possession is a startling one. It seems to me that those data can be explained only by the supposition of a nebulous ring with the Earth for its centre, and lying within the orbit of the Moon 1."
On the publication of the foregoing, Humboldt transmitted to
6 Hist. Nat., lib. II. cap. 26.
h Natural History of England, 1659. Brit. Bacon., p. 183. 1661.
| Anc. Mém. de l'Acad. des Sciences, vol. viii. p. 121.
* Gould's Astronomical Journal, No. 84, May 27, 1855. In the Month. Not., vol. xvii. pp. 204-5, May 1857, are some
distrustful remarks on this communication, to which the reader should refer, and at p. 47 is some account of J. F. J. Schmidt's work on the Zodiacal Light.
See Jones's original memoir in vol. iii. of the 4to. ed. of the U.S. Exploring Expedition Narratire. (Washington, 1836.)