An eclipse of the moon is a universal phenomenon, since the moon actually loses her light. in whole or in part ; while in an eclipse of the sun, the moon hides the sun from one part of the earth, but not from another. The fanner can only take place when the conjunction (or sameness of longitude) of the moon and earth's shadow, that is, the opposition of the sun and moon, or the full moon, happens when the moon is near her node. [Ectirse; SUN ; &mos.) For the phennmenn of the occultation of n star by the moon, see OCCULTATION.
By the harms-moon is meant a phenomenon observed in our lati tudes at the time of the full moon nearest to the autumnal equinox. when it happens for a few days that the moon, instead of rising fifty. two minutes later every day, rises for several days nearly at the same time. Something of the Fame sort takes place always when the moon is near her node ; but the circumstance is most remarkable when it happens at the time of greatest moonlight.. The reason is that the increase of declination (which is most rapid when the moon is near the equator, which she must be when full moon conies nearly at the time of the equinox) compensates the retardation which would other• wise arise from her orbital motion. [SrnEeE.] Sec the treatise above cited, pp. 80, 81.
The discovery of the telescope, and the examination of the moon which followed. soon showed that the planet always turns the same face towards the earth, or very nearly. From hence it immediately follows that the moon must revolve round tin axis in the same time as that axis revolves round the earth. if any one should walk round it circle without turning himself round, that is. keeping his face always in the same direction he would present alternately his front and back to the interior of the circle. But if he desires to turn his face always inwards, lie must turn round in the same direction as lie walks round. [Merrioe, DinEcetoe OF.] If the moon moved uniformly round in her orbit, and had a uniform rotation of the same duration, then if her axis were perpendicular to the plane of the orbit, and the spectator were always at the earth's centre, the face of the moon would be always actually the same. None of these suppositions are true. 1. The motion in the orbit is irregular while the rotaticu is uniform and exactly the sidereal month : the consequence will be that when the moon is moving quicker than the average, a little of the western side will be mating into view, and a small portion of the eastern aide will bo disappearing, and rice reraei. 2. The axis of the moon is not per pendicular to her orbit, but is out of the perpendicular by an angle of 5 8' 49"; the consequence is, that as she revolves in her orbit, the north and south poles of the moon will alternately become invisible, each during half a revolution. 3. The spectator is in motion round the earth's axis, which will slightly vary the part seen of the moon in the course of the day. These effects are called libmtions : (1) the libration in longitude, (2) the libration in latitude, (3) the diurnal libration. The second will be elucidated in SEASONS, CHANar. OF, and
the third in PRECESSION and NUTATION.
The way in which we know that the face presented is always nearly the same, is by observation of that face, which is varied by numberless spots and streaks. The following cut represents the general appearance of the moon at full, being a view of its average face in the moan state of libration, that is to say, no part of the present edge is ever hidden by libration without as much of the opposite edge being hidden nt some other time.
It may here be mentioned that photography has been applied to the production of views of the lunar surface with great success. Among these, the beautiful photographs by Professor Bond in America, and Mr. Warren De La Rue in this country, hold a distinguished place. Indeed the stereoscopic views of the moon in different states by tho Letter gentleman, !cave but little to be done by succeeding photegm. Olen, their effect being such as to excite the surprise and admiration of all by whom they are viewed.
For particulars respecting the spots on the moon, the reader is referred to a work lately published by the Rev. T. W. Webb, entitled `Celestial Objects for Common Teleeeopes ' In this little work, which contains a vaet amount of information on practical subjects, a map of the moon Is given, exhibiting the places and delineations of 404 of the principal craters and other appearances on the lunar surface carefully reduced from the large map published by Beer and Madder, and for the most part by actual observation. Omni observers of the lunar surface must however bear in mind,Let c A be the ecliptic, and let it c be a part of the moon's orbit, with the node at D. It must be remembered that the whole takes place on a very small part of the sphere, an that all the portions of the orbits that the figure of a spot varies exceedingly according to the degree of its illumination. Thus many spots and appearances that are strongly marked in the increasing or waning moon are almost if not entirely invisible at the time of full moon. They also vary greatly at different periods of the lunation, arid the alteration in some is so great that they are with- difficulty recognised as the seine spots. Many astronomers are consequently at the present time engaged in following and delineating the appearance of certain spots through the whole period of a lunation, and thus it is hoped future observers may be enabled to verify them at any time during that period. This if fully carried out would be a most valuable boon to persons engaged in lunar investig utioas. as every variation arising from the before-mentioned causes would be readily recognised; and should any actual alteration take place in any of these craters, a thing that has repeatedly been suspected. it could not fail in being immediately detected.