Moon. – The Earth’s satellite, 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 its early stages the Moon would have been very close to the Earth and would have completed its revolution around it in just five hours. It would then have moved away due to tidal effects, and indeed it continues to recede, until it reaches a distance roughly one and a half times its current one, at which point it will complete a revolution in fifty-five days. It can be shown that at that time the Earth will also complete a rotation in the same period, so that eventually the day and the lunar month will both equal fifty-five current days. It is likely that when the Moon solidified, its distance was less than it is today. Armellini has even demonstrated that this would precisely explain the slightly elongated ellipsoidal shape (resembling an egg) of the lunar globe.
At present, the Moon revolves around the Earth from west to east (direct motion), along an elliptical orbit with the Earth’s barycentre as its focus, at an average distance of 384,440 km, completing one circuit in 27° 7′ 43″ 11‴. This interval is called the sidereal revolution, to be distinguished from the synodic revolution, which is the time the Moon takes to return to conjunction with the Sun (the interval between two consecutive new moons = 20° 12′ 44″ 3‴), and which determines the lunar phases and the resulting movable feasts. The point of the lunar orbit closest to the Earth is called perigee and is 363,000 km away; the farthest point is apogee, 406,000 km away. The line joining perigee and apogee is called the line of apsides, and it rotates in the plane of the lunar orbit from west to east in 8 years and 310 days. The plane of the lunar orbit intersects the plane of the ecliptic along a line called the line of nodes, which rotates in the plane of the ecliptic from east to west in 18 years and 219 days.
The Moon always turns the same face toward the Earth, which means that it rotates on its own axis in the same time as its sidereal revolution. However, since the lunar orbit is slightly elliptical, by Kepler’s second law the revolution speed is not uniform, whereas the rotation speed is uniform; hence the lunar disk appears to undergo small oscillations (which astronomers call librations in longitude), revealing and concealing regions near the edges over an amplitude of about 8°. Moreover, since the Moon’s rotational axis is inclined at 83° 30′ to its orbit around the Earth, there is a libration in latitude, so that one or the other of the two lunar poles is alternately visible over an amplitude of about 6°. A third form of apparent oscillation, called diurnal libration, with an amplitude of about 1°, arises from the fact that the observer is on the Earth’s surface rather than at its centre. These librations (discovered by Galileo) mean that about six-tenths of the lunar surface can be observed.
In its revolution around the Earth, the Moon is twice aligned with the Earth and the Sun: once in conjunction (i.e., between the Sun and the Earth), when it shows its dark side and a new moon occurs; and once in opposition (i.e., on the opposite side of the Sun), when it shows its illuminated side and a full moon occurs. The quadratures (i.e., first and last quarter) occur when the Earth–Sun conjunction forms a right angle with the Earth–Moon conjunction. If new or full moons occur near the line of nodes, a solar or lunar eclipse may take place.
The diameter of the lunar globe is about 3,470 km, i.e., slightly more than a quarter of the Earth’s diameter, which is about 12,700 km. The Moon’s mass is about 1/81 of the Earth’s, so that its mean density relative to water is 3.33, significantly less than the Earth’s density of 5.52. Gravity at the Moon’s surface is about 1/6 of that at the Earth’s surface, so that a person weighing 72 kg on Earth would weigh about 12 kg on the lunar surface.
The Moon has no atmosphere, or at most traces so minimal that they cannot be detected by our instruments. This is shown by telescopic observation: the complete absence of clouds, the sharpness of lunar relief even near the edges, the total lack of refraction phenomena during solar eclipses, the instantaneous disappearance without colour change of stars occulted by the Moon, and finally spectroscopic examination of the light reflected from the lunar surface. It is thought, however, that the Moon, given its probable common origin with the Earth, initially had an atmosphere similar to the Earth’s, which was soon dispersed into space, partly because of the Moon’s weak gravitational pull. Consequently, there is no water on the Moon, which would otherwise evaporate and form a vapour atmosphere.
The Moon reflects sunlight, and modern photometric research has established that its albedo (reflecting power) is about 0.07; i.e., the full Moon reflects about 7% of the light it receives from the Sun. Its brightness decreases rapidly with the phases and as the elongation (angular distance of the Moon from the Sun) increases, suggesting a highly rugged lunar surface. Measurements with thermocouples at the focus of large telescopes show that the Moon’s surface temperature varies from about +80° during the lunar day (which lasts half a lunar month, i.e., 14 Earth days) to about –200° or even lower during the lunar night. This sharp temperature swing is explained by the fact that lunar rocks are exposed to 14 days of uninterrupted, unattenuated solar illumination and 14 days of night, during which the emission of stored heat is unimpeded by any atmospheric screen; it is also explained by the considerable porosity of the planet’s surface, which slows the conduction of heat both inward and outward.
Examined with the naked eye or small telescopes, the Moon shows bright regions of a citron-yellow colour, called lunar terrae; and darker regions of a grey colour, which Galileo called lunar maria and which the ancients believed to be less dense—a view refuted by Dante (cf. the dark markings in Dante, *Paradiso*, 11.49 ff.).
The Moon’s proximity to the Earth and its lack of atmosphere have facilitated telescopic and photographic observation of its surface, which modern instruments can resolve to about 50 km. Even modest telescopes reveal the rugged form of lunar mountains, whose heights, deduced from their shadows, have been found in some cases to reach about 7,600 m. The lunar surface is especially striking both in the dazzling brilliance of full moon, when smaller craters shine like silver basins, and in the grazing light of first and last quarter, which highlights the mountains and relief of the terrain.
Lunar topography appears very different from terrestrial topography, since mountain ranges are scarce, while craters are extremely numerous, partly resembling terrestrial volcanic craters but far more numerous, extensive, and with different characteristics. Indeed, while the largest terrestrial craters rarely exceed 12 km in diameter, some lunar craters exceed 200 km. Moreover, these extremely numerous craters, often grouped and overlapping, generally have their floors situated at a lower level than the surrounding terrain (contrary to most terrestrial volcanoes), frequently display a central cone, and sometimes multiple cones, are encircled by a ring of more or less steep rocks, and show absolutely no trace of volcanic activity. Indeed, these features lead many astronomers to doubt the volcanic origin of lunar craters, which some (such as the American astronomer J. J. See) believe instead to have been produced by the impact of large meteorites.
The dark regions of the lunar surface, named "seas" by Galileo, are probably composed of large expanses of cooled lava. This was confirmed by research conducted at the Rome Observatory, where a long series of photometric measurements determined the albedo of lunar lands and seas, and it was found that while the albedo of the lands was very similar to that of 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 polarization angle of the rocks constituting the lunar seas was almost equal to that of lavas and basalts, which confirms our view. It must be noted, however, that these expanses of cooled lava appear riven by deep crevices, probably due to contraction phenomena. Furthermore, from some lunar craters there often extend streaks that stretch over great distances, forming a kind of ray system. Characteristic is the ray system of streaks emanating from the crater of Tycho, near the lunar north pole, which can be seen even with a simple pair of binoculars when observing the full Moon.
No manifestations of animal or plant life have ever been observed on the lunar surface, which would in any case be impossible due to the lack of water and atmosphere and the extremely violent temperature fluctuations.
There exist various careful representations of the lunar surface, both in freehand drawings and in photographs. One of the first astronomers to draw a map of the Moon was Hevelius of Danzig, who gave the lunar seas names drawn from the belief that the Moon influenced meteorological conditions and other phenomena. Thus we have on the Moon the Mare Serenitatis, the Mare Tranquillitatis, the Mare Imbrium, etc. The lunar craters were named by Father Riccioli after great astronomers and philosophers: Aristotle, Plato, Copernicus, Tycho, etc. The few mountain ranges have been given names drawn from terrestrial orography: thus we have on the Moon the Apennine range, the Alps, etc.
Today the best lunar maps, drawn visually with the telescope, are those of Schmidt of the Athens Observatory and the older ones by the German astronomers Beer and Mädler. Among modern lunar photographic atlases, perhaps the most important is that of the Paris Observatory by Loewy and Puiseux, to which may be added that of the Lick Observatory. At present an attempt is being made to produce an atlas of the Moon with colored photographs, so as to reproduce exactly the hues of the different lunar regions. The attempts made so far, using light filters of different colors, seem rather inconclusive. — See pl. CXVII.
