of 354h 22m, or 14a 18h 22m of our mean solar time; in other words, it is equal to half the period of the Moon's synodical revolution round the earth. As is the case on the Earth, the length of the longest day on the one hand and of the shortest on the other increases and diminishes according as the assumed place of observation approaches the lunar poles: so that at the selenographic latitude of 45° these times become 14d 21h 19m and 14a 15h 26m; and at the latitude of 88°, 18d 17h 28m and 10d 19h 16m respectively. By an observer placed on the Moon some astronomical phenomena would be witnessed under circumstances widely different from those under which we see them. The apparent diameter of the Earth would be about 2°, and its apparent superficial extent 13 times greater than the apparent superficial extent of the Moon as seen from the Earth. More than this: the Earth is almost a fixed object in the lunar heavens, only altering its place by the amount of the libration, or traversing backwards and forwards a space having an extent of 15° 8′ in longitude and 13° 6′ in latitude. The Earth exhibits to the Moon exactly the same kind of phases which the latter does to us, but in a reverse order. For when the Moon is full, the Earth is invisible to the Moon; and when the Moon is new, the Earth is full to the Moon. These remarks apply only to those parts of the lunar surface which are turned towards our globe; for a spectator on the opposite side would never see the Earth at all, and spectators located on the apparent borders of the lunar disc would only now and then obtain a glimpse of it in their horizon, for which they would be indebted to the librations in longitude and latitude already noticed. If the whole sky were covered with full Moons they would scarcely make daylight, for Bouger's experiments give the brilliancy of the full Moon as only 300000 that of the Sun. Wollaston's value is 801072 P, Zöllner's 1000, and G. P. Bond's 170990. The Moon's surface is supposed to be much heated, possibly, according to Sir J. Herschel, to a degree much exceeding that of boiling water, yet we are sensible of no heat at all, all probably being absorbed P Phil. Trans., vol. cxix. p. 27. 1829. G in the upper strata of our atmosphere. Melloni, however, at one time thought he had detected a sensible elevation of temperature by concentrating the rays of the Moon in a lens 3 feet in diameter, though subsequently he was led to doubt the accuracy of his observations. C. P. Smyth also thought that he obtained evidence on Teneriffe of the Moon's rays, but his instrumental means were not very perfect. More recently Prof. Tyndall states that his experiments seem to shew that the Moon imparts to us, or at least to the Professor's thermometric apparatus, rays of cold. From all this it is obvious that nothing certain is known on the subject. The first astronomer who paid much attention to the delineation of the Moon's surface was Hevelius, who in his well-known Selenographia, published in 1647, gave a detailed description of it, accompanied by one general and some 40 special charts; which, taking into consideration the inferior optical means at his disposal, were very creditable to the industry of the illustrious observer of Dantzig. Four years later Riccioli brought out a map of the Moon, having proper names assigned to many of the principal localities; and this nomenclature, improved and enlarged, is still in general use. J. D. Cassini and T. Mayer of Göttingen published charts in the years 1680 and 1749 respectively, the latter of which was the only one used by observers for many years subsequent to the opening of the present century. In 1791 Schröter published a large work entitled Selenotopographische Fragmente, in which are given diagrams of many of the principal spots. Schröter was an industrious observer, but his descriptions are not always satisfactory. In 1824 W. G. Lohrmann of Dresden published the first 4 of a series of 25 excellent lunar charts, but was prevented by failing sight from continuing the work. Beer and Mädler's elaborate Mappa Selenographica was published in 1837, and is undoubtedly the best of the kind yet published; but the most generally useful and also most generally accessible map for the class of readers whom I address is the Rev. T. W. Webb's, re r An Astronomer's Experiment, &c., p. 213. duced from Beer and Mädler's. Maps by Russell and 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, is now (1866) engaged in the preparation of an entirely new map of the Moon. A wax model of the whole lunar surface was executed some 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 and others, with good pictorial but not very useful results. In computing the places of the Moon the tables of Burckhardt, published in 1812, have hitherto been generally used, but in 1862 the new and more perfect tables of Hansen were introduced at the Nautical Almanac office; and these will eventually, no doubt, entirely supersede Burckhardt's. Damoiseau, Plana, Carlini, Pontècoulant, Lubbock, and lately Delaunay, in addition to Hansen, have done much to improve the theory of the Moon. Fig. 29 is from a photograph of one of these. CHAPTER VIII. MARS. ♂ Period, &c.-Phases.-Apparent motions.-Its brilliancy.-Telescopic appearance.— Its ruddy hue.-Polar snow. -Axial rotation. -The seasons of Mars. Its atmosphere.—Has Mars a Satellite?—Ancient observation of Mars.—Tables of Mars. MAR ARS is the first planet exterior to the Earth in the order of distance from the Sun, and, as we shall presently see, bears a closer analogy to it than do any of the other planets. Mars revolves round the Sun in 686a 23h 30m 41, at a mean distance of 139,312,000 miles, which an orbital eccentricity of 0'093 may augment to 152,284,000, or diminish to 126,340,000 miles. The apparent diameter of Mars varies between 4.1′′ in conjunction and 30.4" in opposition. The diameter at mean distance of the planet from the Earth being 7.28" (Le Verrier), the real diameter is somewhat less than 5000 miles. Very varying results have been arrived at as to the compression of Mars. Sir W. Herschel gave it at; Schröter contradicted this, and asserted that it must be less than ; Bessel merely decided that it was too small for measurement with his great heliometer at Konigsberg; Arago from Paris observations extending over 36 years (from 1811 to 1847) deduced. Hind considers that, and Main that is not very far from the truth. Mars exhibits phases, but not to the same extent as the inferior planets. In opposition it is perfectly circular, between this and the quadratures it is gibbous, and at the minimum phase, which occurs at the quadratures, the planet resembles the Moon 3 days a See his memoir in Ast. Nach., vol. xxxv. p. 351. |