Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/871

] equator to the ecliptic is 1 28 42. Mr Baily makes it 1 30 11&quot; 3. Since the descending node of the equator coincides with the ascending node of the orbit, it is evident that its plane lies on one side of a plane parallel to the plane of the ecliptic, while the plane of the orbit lies on the other. Thus the plane of the equator makes an angle of about 1 30 with the first, and of G 39 with the second, on the average. But these angles are slightly variable. The coincidence of the nodes of the lunar equator and orbit ranks among the most curious discoveries of modern astronomy. _ It was shown by Lagrange to be a necessary consequence of the attraction which the earth exercises on the lunar spheroid.

The various features of the moon s surface have been observed with great interest since the discovery of the telescope, and astronomers have been at much pains to determine their selenographic positions. On account of their number, it has been found necessary to distinguish them by particular names. Riccioli designated the most conspicuous of them by the names of astronomers and other eminent men. Hevelius gave them the names belonging to countries, islands, seas, and regions on the earth, without reference to situation or figure. The nomenclature of Riccioli is now universally followed. Delineations of the lunar disk have been given by Hevelius, in his tidenographia, by Cassini, liusseli, Schroter, Lohr- mann, and others. The subjoined engraving (tig. 36) givew ! a pretty accurate view of the appearance of the moon in her mean libration.

 

 .—The Moon in her mean Libration (telescopic or inverted view).

 

 That there are great inequalities on the surface of the moon is proved by looking at her through a telescope at any other time than when she is full ; for then there is no regular line bounding the dark and illuminated parts, but the confines of these parts appear as it were toothed and cut with innumerable notches and breaks ; and even in the dark part, near the borders of the enlightened surface, there are seen some small spaces illuminated by the sun s beams. It is impossible that this should be the case, unless these shining points were higher than the rest of the surface, so that the rays of the sun may illumine their summits before they reach their basis. Portions of con siderable extent are also perceived on the lunar surface, which are never brilliant like the other parts, but remain constantly obscure. These are thought by some to be old sea-bottoms ; they were formerly supposed to be seas, but this idea has been abandoned. Some of these cavities are upwards of four English miles in depth, and forty in circumference at the orifice. An insulated mountain is fre quently observed to rise in the centre of these enormoua pits or caverns, and they are surrounded by high annular ridges the masses of which would fill the enclosed cavities. 