Moon, the satellite of the earth, the nearest of the heavenly bodies to us. It is an opaque spheroid 2,159.8 m. in diameter, shining by reflecting the light of the sun. Situated at an average distance of 238,818 m., the moon revolves about the earth in 27.32106 days, this being her mean sidereal revolution. To this motion are due her monthly phases. The course of these, however, is only completed in a lunar month, or synodical revolution, the mean length of which amounts to 29.53059 days. For the phases depend on the moon's position with respect to the sun, which is constantly advancing in the direction of her motion; so that, after completing 360° of her orbit, she has the whole amount of the sun's monthly progress, which is an arc of about 29°, to pass over before she can complete her course of phases.

The former period is sometimes called the sidereal month, the latter the synodic month. When not eclipsed, she always presents to the sun an illuminated hemisphere; her phases depend on the amount of that hemisphere turned toward the earth. If the earth is directly between her and the sun, we see all of it; if she is between us and the sun, we see none of it; if she is midway between these positions, we see half of it. In the first position, she is said to be in opposition; in the second, in conjunction; in the third, in quadrature, or quarter; and her phases, in order, are known familiarly as new, crescent, half-moon, gibbous, and full. - From the constancy of the physical features of the moon's disk, it is evident that she always presents to us the same hemisphere. To do this she must turn upon her axis precisely once while making one revolution in her orbit. This appears to be the general law of the motions of the satellites. But it is not quite accurate to say that the moon constantly presents the same hemisphere to every observer upon the earth.

Her axis of rotation being inclined one degree and a half to her orbit, and maintaining the same general direction in space as she moves round the earth, she appears to nod backward and forward in an arc of about 13° in the course of every revolution, exposing to view the regions just beyond her N. and S. poles alternately. Nor is this all. As the moon's orbit, like that of every other planetary body, is an ellipse, her orbital velocity is not uniform, being most rapid when she is nearest the earth. Thus she sometimes gets ahead of her mean place, and sometimes lags behind it; and as her axial rotation is absolutely uniform, we are enabled to look over her edge, so to speak, now on the eastern and now on the western side. The arc through which she oscillates in this way amounts to more than 15°. And again, the constancy of the direction of her hither hemisphere is to be referred to the earth's centre, so that the observer, situated upon the extremity of the earth's radius, views her from an elevation of nearly 4,000 m.; and when she is in the horizon it is plain he can look over her elevated edge, as it were.

The oscillation thus occasioned is much smaller than either of the others, amounting only to about 2°. These several exposures are called the moon's libra-tions: the first her libration in latitude; the second her libration in longitude; the third her diurnal libration. The absolute maximum librations from the moon's mean position are as follows: libration in latitude, 6° 44'; in longitude, 7o45'; diurnal, 1° 1 1/2'. If the whole surface of the moon be regarded as equal to 10,000, then instead of seeing only 5,000 parts, as we should do if there were no libration, our range of view extends over 5,802 parts without taking the diurnal libration into account, and over 5,889 parts if diurnal libration be considered. So that only 4,111 parts of the moon out of 10,000 remain absolutely concealed from human ken. - To the casual observer the motions of the moon in different seasons of the year seem exceedingly irregular. She is sometimes seen, at the full, coursing along a circle which passes near the zenith in these latitudes, and sometimes, in the same phase, along an arc low down in the southern sky.

It is plain that this is mainly owing to the inclination of the earth's equator to the ecliptic; but there is a large residual effect which is due to the inclination of the moon's orbit to the plane of the ecliptic, amounting to 5° 8', so that during one half of her orbit she is south of the sun's annual path, and during the remaining half north of it. The points where she crosses the ecliptic are known as her nodes; that at which she passes from the southern to the northern side of the line is called her ascending node, the other her descending node. If the ecliptic were a line of light ever conspicuous in the sky, and the moon's path intersecting it also a conspicuous line of light, the place of crossing would be seen to be different every month, being removed further and further to the westward at intervals of about three diameters of the moon. This at least is the average rate of the motion; for the motion is not only not uniform, but is at times reversed. It is known as the retrograde motion of the nodes; the period of completing the whole circuit of the ecliptic is 18.5997 years. The orbit of the moon being an ellipse, having the earth at one of its foci, her distance varies in different parts of her monthly course.

The nearest point of her orbit is called perigee, the furthest apogee; the two are known as apsides. These points are not fixed, but move forward (on the whole) from west to east, occupying succes-sively every position in the circumference of the ellipse in the course of 8.8505 years. These two remarkable motions, viz., of the nodes and of the apsides, are due to the disturbing action of the sun. - The moon's surface has no obvious indications of water, nor of an atmosphere. Mr. C.B. Boyleof New York, however, who has long made a special study of the moon, maintains that she has a slight atmosphere, and that she has also water in the shape of numerous small ponds which for optical reasons are not always visible through the telescope, but have occasionally been noticed by astronomers as bright sparkling points. Sehroter (about 1800) claimed to have discovered indications of vegetation on the surface of the moon. These con-gist of certain traces of a greenish tint which appear and reappear periodically; much as the white spots covering the polar regions of Mars, supposed to be snow and ice, are observed to increase in the winter and waste in the summer of those regions of the planet.

As we are able, under the most favorable conditions, to use upon the moon telescopic powers which have the effect of bringing the satellite to within 150 to 120 in. of us. we should doubtless notice any such marked changes on her surface as the pas-• of the seasons produces, for example, on our own globe. In the most powerful instruments yet constructed the surface of the moon presents a scene of wildest desolation. In every direction are circular caverns or pits, many of enormous size; the floor of one is seen to be strewn with huge blocks. The inner walls are commonly Bteep, and their depth often frightful, being many thousand feet. They are surrounded by annular ridges, the masses of which would exactly till the enclosed cavities. In the centre commonly rises a conical mountain. All this plainly points to a volcanic origin. There decided alluvial of mountains 120 m in diameter. From these ranges shoot up stupendous peaks, one to the height of 16,000 ft. Isolated peaks here and there rise abruptly from extended plains to the height of 6,000 to 7,000 ft. These elevations are determined by calculations based on the height of the sun above the horizon of the lunar place under inspection, and the length of the shadows cast.

The most favorable time for observing these remarkable features is when the moon is about half full. Beyond the illuminated .hemisphere mountain peaks, rising miles above the average level of the surface, are then bathed in sunlight, while the intermediate space is veiled in darkness. Thus the peaks are at such a time seen as silver points detached from the bright crescent; or, if they form a chain stretching toward the rising sun, they may appear as ragged promontories of light jutting far out into the darkness. An admirable chart of the moon has been constructed by the eminent Prussian observers, Beer and Madler, whose work, Der Mond, must be consulted for a full account of the physical condition of our satellite. They place the height of one mountain at 23,823 ft. This, considering the relative magnitudes of the moon and the earth, is far more stupendous than any known elevation of terrestrial surface. More recently Schmidt of Athens, Greece, has made an elaborate series of observations, extending over the years 1839-'72. The diameter of the chart constructed from these observations is to be six Paris feet, and it is to be published in 25 sections.

The application of photography to the moon, though it has not yet resulted in giving maps comparable in accuracy of detail with those by Beer and Mad-ler, and by Schmidt, has yet given pictures of extreme value and interest. In 1840 Dr. J. W. Draper of New York first succeeded in photographing the moon. With a telescope 5 in. in aperture he obtained pictures on silver plates, and presented them to the lyceum of natural history of New York. Bond of Cambridge, Mass., made photographic pictures 2 in. in diameter with the refractor of the Harvard observatory in 1850. Since then, Secchi in Rome, Bertch and Arnauld in France, and Phillips, Hartnup, Crookes, De la Rue, and others in England, have made lunar photographs, some of those by De la Rue being admirable. Dr. H. Draper and Mr. Rutherfurd of New York have taken some of the finest photographic views yet produced. To one of the photographs by Rutherfurd (taken Feb. 27, 1871) I)e la Rue ascribes the palm of absolute superiority among all the lunar photographs yet taken. - The mass of the moon is not accurately known, though the most trustworthy determinations agree in placing it at about 1/81.4 part of the mass of the earth. The mass of the moon is intimately associated with her distance and motions.

It is best determined from the nutation of the earth's axis (see Nutation), and when determined must be added to the earth's mass in calculating the deflecting action of the mutual gravitation of the earth and moon, a reduction being made for the sun's perturbing influence. As the actual deflection is known, and can therefore be compared with the result thus theoretically determined, we have a means of testing the various determinations of the moon's distance. Prof. Colbert of Chicago considers that the lunar elements deduced balance each other most satisfactorily if we take the following values: mean equatorial horizontal parallax, 57' 0.67"; mean distance in miles, 238,973; mass of moon to earth's as 1 to 81.38; and thence he deduces: diameter of moon, 2,160.35 m.; volume of moon to earth's as 1 to 49.2; density, earth's as 1, 0.6044; distance of centre of orbit from the earth's centre, 13,121.5 m.; mean distance of centre of gravity of earth and moon from the earth's centre, 2,900.86 m. It may be remarked, however, that the various elements dealt with are not as yet determined so exactly that very much reliance can be placed on the method of testing here indicated.

It is to be noted, in passing, that the term lunar parallax as commonly used is applied (not quite correctly, however) in such a way that the earth's radius, instead of being to the distance as cosecant of the parallax, bears to it the ratio, arc: radius. (See Chauvenet's "Astronomy.") The faint apparition of the entire lunar disk at the time of new moon is considered to be due to the reflection of the light received from the earth, whose illuminated hemisphere is then turned toward her.

Full Moon, from Photographs taken by Prof. H. Draper, New York.

Full Moon, from Photographs taken by Prof. H. Draper, New York.

Moon at the First Quarter from Photographs taken by Prof.H.Draper, New York.

Moon at the First Quarter from Photographs taken by Prof.H.Draper, New York.