moon, like all other satellites, as far as at present known, revolves round her own axis in precisely the same time that she revolves round the earth; she thus presents always the same face to us, and consequently, though her comparative proximity has enabled us to become better acquainted with her surface than with that of any other heavenly body, our knowledge is confined to one half of her surface, with the slight exception of the knowledge obtained from her libration (q.v.). To the inhab itants of the side of the moon next the earth—if the moon had inhabitants, which is very improbable—the latter would appear as a luminary about 2° in diameter, immovably fixed in their sky, or at least changing its position only to the extent due to the moon's libration. The earth would thus seem to them to have a disk about 15 times larger than that of the sun.
Physical surface of the moon, as seen from the earth, presents a most irregular grouping of light and shade. The dark portions were named by the earlier astronomers as seas, lakes, etc., and still retain these names, although there is strong. evidence against the supposition that the moon, or at least that portion of it presented to us, contains any water. The brighter parts of the moon are mountainous, as is proved by the fact of their casting shadows when the sun's rays fall upon them obliquely, and also by the ragged appearance presented by the interior illuminated border of the moon, an appearance which can only be satisfactorily accounted for on the supposition that the surface of the moon is not level, in which case the higher portions will be illumi nated some time before the light reaches the level parts; and it is observed that as the illumination proceeds, bright spots start up in advance of it, and when the moon is on the wane, these same spots continue to shine for some time after the surrounding sur face is immersed in gloom. The mountains occur either singly, when they are generally of a circular form, and are called craters, or in groups, which are mostly annular, and form a sort of wall inclosing a deep depression or plain, in which are situated one or more conical mountains. The craters are not unfrequently 8 or 10 m. in diaineter, and. some of the walled plains measure more than 100 m. across. The principal mountain range is the Apennines, which crosses the surface from n.e. to S.W., and attains, accord some authorities, an altitude of about 20,000 ft., though sir John Herschel gives about 2 m. as the probable limit of elevation above the moon's surface. The heights are estimated from a micrometric measurement of the length of their shadows, a method not, in this case, susceptible of much accuracy. The moon everywhere presents traces of volcanic agency, but no active volcanoes have yet been discovered, nor is there any sign of recent volcanic action. Seen through the telescope,' she presents a bleak, deso
late appearance, without indications of animal or vegetable existence. She appears to be devoid of an atmosphere, or, if one exists, it must be of exceeding rarity.
The influence of the moon in causing tides (q.v.) has long been well known, and there is some reason for supposing that she produces a similar effect on the atmosphere, combining with other causes in the generation of winds. Those winds which prevail about the time of new and full moon, and at the vernal and autumnal equinoxes, are particularly ascribed to her influence. On the supposition that the moon might also affect organic nature, experiments were instituted by Mead, Hoffmann, and others; but no certain results were attained. The periodicity which has often been noticed in certain • diseases, especially in insanity (hence called lunacy), was long supposed to have some connection with lunar influence, and this opinion is held to some extent at the present day. The chemical effects of the moon's rays are, so far as is at present known, fee ble. though in particular instances they exhibit an actinism as powerful as that of the sun. Decomposition of animal matter takes place more rapidly in moonshine than in darkness, and the moon's rays, when concentrated, have a sensible effect on the ther mometer.
The best map of the moon is the large and accurate one of MM. Baer and which presents a most minutely detailed Picture of her visible surface; the map is 3 ft. in dinnieter. SCO for further information the articles PERTURBATIONS; LIBIIA.TION; NODES; EVECTION; METONIC CYCLE; TABLES, LUNAR, etc.
Superstitions regarding the moon was anciently an object of woenip, and even in the 17th c. she was supposed, by the common people of England. to exercise great influence over human affairs. The times for killing animals for food, gathering herbs, cutting down wood for fuel, sowing seeds of various kinds, were all regulated by the "age" of the moon, and these set periods were considered to be a necessary part of practical knowledge, and ignorance or neglect of them to lie infallibly productive of loss. There were similarly defined periods for taking particular medicines, and attempting the cure of particular diseases. Many such superstitions prevailed until a recent period in the Highlands of Scotland, favorable or unfavorable consequences from any occurrence being predicted according to the age of the moon at the time it happened. Throughout Scotland, the waning moon was considered to have an evil influence, and full or new moon to be the most auspicious season for commencing any enterprise. The same opin ion was held in Scandinavia and Germany, and the history of all nations teems with similar superstitions. See the article Ecmessfs.