MOON. - Satellite of the Earth, from which it probably originated through the division of the still-fluid terrestrial mass (v. COSMOGONY). According to the theory of the astronomer G. Darwin (son of the naturalist Ch. Darwin), in the earliest times the Moon would have been very close to the Earth and would have completed its revolution around it in only five hours. Then it would have moved away as a result of the tides, and indeed it would still continue to move away, until reaching a distance equal to about one and a half times the present one, at which it will complete a revolution in fifty-five days. It is demonstrated that at that time the Earth too will complete a rotation in the same time, so that in the end the day and the lunar month will become equal to 55 present days. Probably also when the Moon solidified, its distance was less than the present one. Armellini indeed has demonstrated that this would exactly explain the slightly elongated ellipsoidal shape (similar to an egg) of the lunar globe. At present the Moon revolves around the Earth from west to east (direct sense), along an elliptical orbit having the Earth's barycenter as its focus, at a mean distance of 384,440 km., completing one revolution in 27° 7' 43' 11'.
This interval of time is called the sidereal revolution, not to be confused with the synodic revolution, which is the time the Moon takes to return to conjunction with the Sun (interval between two consecutive new moons = 29° 12' 44' 3'), which determines the lunar phases and the consequent movable feasts. The point of the lunar orbit closest to the Earth is called the perigee and is 363,000 km. away; the farthest is the apogee and is 406,000 km. away. The line joining the perigee to the apogee is called the line of the apsides and rotates on the plane of the lunar orbit from west to east in 8 years and 310 days. The plane of the lunar orbit cuts the plane of the ecliptic along a line called the line of the nodes, which rotates on the plane of the ecliptic from east to west in 18 years and 219 days. The Moon rotates while turning toward the Earth always the same face, which means that it rotates around its own axis in a time equal to the duration of the sidereal revolution. However, since the lunar orbit is slightly elliptical in shape, by the law of areas (v. REPLER), the speed of revolution is not uniform, whereas that of rotation is uniform, so that the lunar disk seems to undergo small oscillations (which astronomers call libration in longitude), showing and concealing regions near the edges over an amplitude of about 8°. Moreover, since the lunar axis of rotation is inclined at 83° 30' to the Moon's orbit around the Earth, there is a libration in latitude by which now one, now the other of the two lunar poles is glimpsed over an amplitude of about 6°. And further, because the observer is on the surface, and not at the center of the Earth, there is a third form of apparent oscillation, called diurnal libration, over an amplitude of about 1°. All these librations (discovered by Galileo) mean that about 6/10 of the lunar surface can be observed. In its motion of revolution around the Earth, the Moon is twice aligned with the Earth itself and with the Sun: once in conjunction (that is, between the Sun and the Earth), and then it shows its shadowed face and there is the new moon; while another time in opposition (that is, on the side opposite the Sun), and then it shows its illuminated face and there is the full moon. There are instead the quadratures (that is, the first and last quarter) when the Earth-Sun line forms a right angle with the Earth-Moon line. If the new moons or full moons occur near the line of the nodes, there may be an eclipse of the Sun or of the Moon.
The diameter of the lunar globe is about 3,470 km.; that is, slightly more than a quarter of the Earth's diameter, which is about 12,700 km. The lunar mass turns out to be about 1/81 of the Earth's mass, so that the mean density of the Moon relative to water turns out to be equal to 3.33, or notably lower than the Earth's density, which is equal to 5.52. Gravity at the Moon's surface is about 1/6 of gravity at the Earth's surface, so that a man weighing 72 kg. would weigh about 12 kg. on the selenic surface. The Moon has no atmosphere, or has traces so minimal that they cannot be detected by our instruments. This is shown through the telescope by the absolute absence of clouds, by the sharpness of the lunar relief even near the edges, by the absolute absence of refraction phenomena in solar eclipses, by the instantaneous disappearance without change of color of the same bodies when they are eventually occulted, and finally by the spectroscopic examination of the light reflected from the lunar surface. It is believed, however, that the Moon, given its probable common origin with the Earth, initially had an atmosphere similar to the Earth's, which, however, was soon dispersed into space, also because of the weak attractive force of the lunar globe. It follows that on the Moon there is an absence of water, which would otherwise evaporate forming an atmosphere of vapor.
The Moon reflects the light of the Sun, and modern photometric research has established precisely that its albedo (reflecting power) is about 0.07; that is, that the full Moon reflects about 7% of the light it receives from the Sun. The luminosity decreases very rapidly with the phases and with the increase of elongation (angular distance of the Moon from the Sun), which suggests a great ruggedness of the lunar soil. It has also been found that the surface temperature of the Moon, examined with the thermoelectric pile placed at the focus of large telescopes, varies from about 80° above zero during the lunar day, which lasts half a lunation, that is, 14 terrestrial days, to about 200° below zero and perhaps even less during the lunar night. This strong jump in temperature is explained by recalling that the lunar rocks are exposed to 14 days of continuous solar illumination not attenuated by the screen of the atmosphere and to 14 days of night, in which the radiation of the stored heat is not impeded by any atmospheric obstacle; as well as by recalling that the notable porosity of the planet's surface makes the conduction of absorbed heat slow, both toward the interior and toward the exterior.
Examined with the naked eye or with small telescopes, the Moon shows light regions of a citrine-yellow color, called lunar lands; and darker regions, of a gray color, which Galileo called lunar seas and which the ancients believed to be less dense regions, an opinion refuted by Dante (cfr. dark signs in Dante, Paradiso, II, 49 ff.).
The proximity to the Earth and the absence of atmosphere have facilitated the telescopic and photographic observation of the lunar surface, which modern means of investigation bring to a distance of about 50 km. But even modest telescopes are enough to discern the steep shape of the lunar mountains, whose height has been deduced from the shadow they cast and has been found in some cases to measure up to about 7,600 meters. And indeed the observation of the lunar surface appears exceedingly suggestive, both in the dazzling luminosity of the full moon that makes its craters shine like silver basins; and in the grazing light of the first and last quarter that brings out the mountains and the reliefs of the soil.
The lunar topography appears very different from the terrestrial one, since mountain chains are scarce, while craters are very numerous, partly similar to terrestrial volcanic craters, but extremely more numerous and more extensive and with different characteristics. In fact, while the largest terrestrial craters rarely exceed 12 km. in diameter, some lunar crater exceeds 200 km.
Moreover, these very numerous craters, often grouped together and superimposed, generally have their floor situated at a level lower than that of the surrounding lands (contrary to the generality of terrestrial volcanoes); they many times present a central cone, or even several cones; they are encircled by a ring of more or less steep rocks, and they are absolutely lacking in any trace of volcanic activity. Indeed, these characteristics make several astronomers doubt the volcanic origin of the lunar craters, which some (such as, for example, the American astronomer J. J. See) believe instead to have been produced by the fall of large meteorites.
The dark zones of the lunar surface, called seas by Galileo, are probably constituted by great expanses of cooled lava. This was confirmed by the research carried out at the Observatory of Rome, where, by means of a long series of photometric measurements, the albedo of the lunar lands and seas was determined, and it was found that, whereas the albedo of the lands was very similar to that of the lighter volcanic material (pumice or trachyte), the albedo of the seas could be compared to that of the basaltic lavas of Vesuvius. Later the Russian astronomer Barabascheff found that even the angle of polarization of the rocks constituting the lunar seas was almost equal to that of lavas and basalts, which confirms our opinion. It must be noted, however, that these expanses of cooled lava appear furrowed by deep crevices probably due to phenomena of contraction. Furthermore, from some lunar craters there often depart streaks that extend to a great distance, forming a kind of radial system. Characteristic is the radial system of streaks departing from the crater of Tycho, near the lunar north pole, which can be discerned by observing the full Moon even with a simple pair of binoculars.
On the lunar surface no manifestation of animal or vegetable life has ever been noted, which moreover would be impossible owing to the lack of water and atmosphere and to the very strong swings of temperature.
There exist various atlases reproducing the appearance of the lunar surface, both in freehand drawing and in photography. One of the first astronomers who drew a map of the Moon was Hevel of Danzig, who gave the lunar seas names drawn from the belief that the Moon influenced meteorological conditions and other phenomena. Thus on the Moon there are the Mare Serenitatis, the Mare Tranquillitatis, the Mare Imbrium, etc. To the lunar craters Father Riccioli gave the names of great astronomers and philosophers: Aristotle, Plato, Copernicus, Tycho, etc. To the few mountain chains were given names drawn from terrestrial orography: thus on the Moon there are the chain of the Apennines, of the Alps, etc.
Today the best lunar maps, executed visually with the telescope, are those of Schmidt of the Observatory of Athens and the older ones of the German astronomers Beer and Maedler. Among the modern photographic lunar atlases, the most important is perhaps that of the Observatory of Paris due to Loewy and Puiseux, to which may be added that of the Lick Observatory. At present attempts are being made to produce an atlas of the Moon with colored photographs, so as to reproduce exactly the tints of the different lunar regions. The attempts made so far, by means of light filters of different colors, seem rather encouraging. - See plate CXVI.
