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Moon
The only natural satellite of the Earth, visible by virtue of reflected sunlight. It has a diameter of 3476 km, lies at a mean distance from the Earth of 384 400 km, and completes one orbit around the Earth in 27.322 days (see month; sidereal month). Its mass in relation to that of the Earth is 1 : 81.3, i.e. 0.0123. It is in synchronous rotation, i.e. it keeps the same face – the nearside – toward the Earth, although more than 50% of the Moon's surface can be seen as a result of libration. The same face is not however always turned toward the Sun, the length of the solar day on the Moon being equal to the Moon's synodic period, i.e. 29.530 59 days. During this period – the synodic month – the Moon exhibits a cycle of phases, reaching an apparent magnitude of –12.7 at full Moon. It can also undergo eclipses and produce occultations.The Moon's mean orbital velocity is 10 km s–1. Its orbital motion is however subject to secular acceleration and periodic inequalities due to the gravitational attraction of the Sun, Earth, and other planets (see also annual equation; evection; parallactic inequality; variation). In addition its perigee and nodes move eastward and westward with periods of 8.85 years and 18.61 years, respectively: this results in different lengths for the sidereal, anomalistic, and draconic months. Physical and orbital characteristics are given in the table.
Although Earth-based observations have revealed much lunar information, the Moon has now been carefully photographed, measured, and sampled by the Apollo spacecraft and by Ranger, Lunar Orbiter, Surveyor, Luna, and Zond spacecraft. It has an extremely tenuous atmosphere: a collisionless gas in which helium, neon, argon, and radon were detected by Apollo instruments. This near lack of atmosphere, together with the absence of an appreciable global magnetic field, exposes the regolith to extremes of temperature (–180 °C to +110 °C) and to the solar wind, cosmic rays, meteorites, and micrometeorites. Satellite magnetometer surveys and detailed analysis of lunar rocks have revealed that the Moon's magnetic field was originally considerably stronger and might have been generated internally by a fluid core rather than produced by some external means.
The Moon's interior structure is investigated from seismograms of moonquakes and meteoritic and artificial impacts. The temperature of the central core may reach 1500 °C but a metallic iron composition is precluded by the Moon's moment of inertia. The Moon's center of mass is displaced toward Earth by 2.5 km because of a thick farside crust.
The Moon was formed out of refractory (high melting point) materials about 4600 million years ago. Its outer layers were melted and differentiated to produce mafic cumulates (iron magnesium silicates) and a feldspathic crust (calcium aluminum silicates), below which radioactive elements were concentrated. The first 750 million years of lunar history constituted a period of intense bombardment, culminating in a cataclysm during which the basins and larger highland craters were formed. The eruption of radioactive basalts (KREEP), or their excavation during formation of the Imbrium Basin, resulted in a concentration of natural radioactivity in the western hemisphere, as measured by gamma-ray spectrometers. A zone of melting then moved inward toward the Moon's center to generate a sequence of basaltic lavas that produced the maria. Since then the Moon has been relatively quiescent.
The origin of the Moon is still debatable. Similarities in composition indicate that the Earth and Moon are related and argue against capture theories, which propose formation elsewhere and subsequent capture by the Earth's gravitational field. Differences in composition, including the Moon's relative enrichment in refractory material and its depletion in iron and volatile material, including water, eliminate any direct formation from the Earth by fission. The giant impact hypothesis (or big splash) now seems more likely. This proposes that the glancing high-speed collision of a huge, possibly Mars-sized body with the primitive Earth smashed what crust the Earth had, melting rock to a great depth. A spray of molten rock from impactor and Earth (mantle rather than iron-rich core material) was ejected and condensed to form a ring of orbiting debris, and the Moon coalesced from this.
See also Moon rocks; Moon, surface features.
Moon
Moon Symbols and Celebrations in the Ancient World
In the ancient world many cultures viewed the moon as a symbol of fertility, death, and rebirth. These concepts are illustrated in the moon's changing phases. Each month the moon waxes, wanes, disappears, and returns to the sky. Indeed, in the ancient Mediterranean world a number of deities associated with fertility, death, and the afterlife claimed the moon as one of their symbols. These deities include the Egyptian god Osiris, the Greek god Dionysus, and the Babylonian goddess Ishtar (or Innana). The moon was also frequently associated with water and with women, feminine principles, or goddesses. Perhaps this association came about because the length of the moon's cycle, 29.5 days, mirrors that of the menstrual cycle (for more on moon symbolism, see also Hare).
The communal celebrations and sorrows of ancient peoples (and other groups who live in greater contact with the natural world than we do today) often waxed and waned in accordance with the moon's cycle. These peoples frequently greeted the return of the new, crescent moon with relief and the arrival of the full moon with celebration. They often experienced the waning moon as a time of uneasiness and the dark of the moon as a fearful interlude.
The Jewish culture in which Jesus was raised fits this pattern of lunar celebration in part. The ancient Hebrews created a luni-solar calendar, which used both the lunar cycle of 29.5 days and the solar cycle of a year to measure time. Each month began on the day a new moon was sighted. This day was celebrated as a festival, Rosh Hodesh. This observance is mentioned in certain Bible texts (for example, Numbers 10:10, 28:14), which indicate that the ancient Hebrews commemorated each new moon with sacrifices and celebrations. Some Jews still observe Rosh Hodesh today. Several Jewish holidays, including Passover, were timed to occur at the full moon (the others are Sukkot and Tu Bishvat). According to the ancient Jewish tradition recorded in the Bible, the first day of Passover falls on the fourteenth day of the Jewish month now called Nisan (Exodus 12:6). This date places the festival close to the time of the spring equinox.
The Moon in the Easter Story
Christian scriptures indicate that Jesus was captured and killed at the time of Passover, therefore at the time of the full moon. Three of the four biblical accounts of Jesus'death, those recorded in the Gospels of Matthew, Mark, and Luke, agree that the Last Supper took place on the first day of Passover (for more on the Last Supper, see Maundy Thursday). That same night Judas Iscariot betrayed Jesus and handed him over to the servants and followers of Jerusalem's religious authorities. These texts imply that Jesus was tried and executed the following day. The Gospel according to John records slightly different dates, however. John declares that Jesus was captured on the night before Passover and that he was crucified on the following day, just as the Jews were making their preparations for the Passover meal that they would consume later that evening (for more on crucifixion, see Cross).
People listening to accounts of Jesus' death and resurrection in the ancient world, where the moon and its cycle were already well-known symbols of death and rebirth, might find it easy to apply further lunar symbolism to the Easter story. All Gospel accounts of Jesus' crucifixion declare that he was buried on the day of his death and remained in the tomb for the following day, which was the Sabbath (for more on the Sabbath, see Sunday). They also agree that he rose from the dead early in the morning on the day after the Sabbath. These two dark days in which Jesus was dead and sealed in the tomb find their parallel in the days of the month in which there is no moon. After this apparent "death," the moon comes back to life, reappearing to the naked eye as a slender crescent on the third day after it disappeared. Jesus also reappeared on the third day, emerging from the tomb fully alive on Easter Sunday. Centuries later an early Christian writer found the moon's monthly cycle to be an apt symbol of the Christian belief in resurrection. He noted:
Luna per omnes menses nascitur, crescit, perficitur, minuitur, consumitur, Innovatur. . . . Quod in luna per menses, hoc in resurrectione semel in toto tempore. (Eliade, 175)
[The moon during every month is born, grows, reaches its full size, diminishes, is consumed and is renewed. What happens to the moon throughout the months, happens in resurrection but once for all time. (Translated by Nancy Stork)]
The Timing of Easter
Christian authorities sometimes refer to feasts such as Easter, which do not always occur on the same date every year, as "movable feasts." Most of these movable feasts are related to Easter, and their timing is thus affected by the cycles of the moon. Indeed, Easter may be thought of as Christianity's great lunar festival.
The reason for the shifting date of Easter boils down to the fact that the solar cycle of one year is not divisible by an even number of lunar cycles. It takes the earth 365.2422 days to complete its revolution around the sun. We call this period of time a "year." It takes the moon 29.5 days to complete its cycle, going from dark to full to dark again. The Jews, and many other peoples, called this period of time a "month." Indeed, the word "month" comes from the ancient IndoEuropean root word meaning "moon." During the 365.2422 days that it takes the earth to complete one solar cycle, the moon completes twelve cycles and is 11.25 days into a thirteenth cycle. Because we do not begin a new lunar cycle on the same day we begin a new solar cycle, the various phases of the moon fall on different dates from year to year. The holidays attached to specific dates of the solar year, such as Christmas, fall on the same date each year. The date of Easter, however, is set by both the moon and the sun, and so it shifts about on the calendar.
Christian scripture asserts that Jesus was crucified and resurrected at the time of the Jewish Passover. When early Church authorities met at the Council of Nicaea (325 A.D.) they worked out a system of determining the date of Easter that would place it at approximately this time of year. They decreed that Easter should always fall on a Sunday and that it should not coincide exactly with Passover. Eventually Christian religious authorities agreed that Easter would fall on the first Sunday following the first full moon which occurs on or after the spring equinox (see also Easter, Date of). The spring equinox, the twenty-four-hour period in which day and night are of equal lengths, marks the end of the first quarter of the solar year. This rule means that in countries following the Western Christian tradition Easter may occur on any Sunday between March 22 and April 25. Of course, the timing of Easter affects the timing of the whole cycle of related Christian holidays and observances, including Ash Wednesday, Lent, Holy Week, Ascension Day, and Pentecost.
Moon Lore and Moon Science
For centuries folk beliefs have asserted that the moon affects the behavior of humans, plants, and animals. Most of this lore, while fascinating in and of itself, does not pertain to the Easter story. One bit of moon lore can be tied to the holiday, however. For example, much folklore insists that the full moon inspires irrational behavior in human beings. In fact, the Latin word for moon, luna, can be found in our English word "lunatic." Although today we use the word lunatic to describe a person whose behavior is irrational at all times, the word was originally used to describe people who "went crazy" at the time of the full moon. Several scientific studies support this supposed connection between the full moon and disorderly or erratic behavior. One of them found that murder rates in two major U.S. cities increased noticeably around the time of the full moon. Other studies have shown that hospital emergency rooms see more patients at the full moon than at any other time in the lunar cycle and that psychiatric units admit more patients at this time. The story of Jesus' betrayal, arrest, trial, punishment, and crucifixion, which took place at the time of the full moon, fits this pattern well. These violent events exploded out of an unstable mixture of human motivations and emotions, including fear, greed, anger, blame-shifting, and power-mongering. A believer in lunar folklore might not be surprised to find out that they took place at the time of the full moon.
Further Reading
Bellenir, Karen. Religious Holidays and Calendars. Second edition. Detroit, MI: Omnigraphics, 1998. Bram, Jean Rhys. "Moon." In Mircea Eliade, ed. The Encyclopedia of Religion. Volume 10. New York: Macmillan, 1987. Cain, Kathleen. Luna: Myth and Mystery. Boulder, CO: Johnson Books, 1991. Eisenberg, Azriel. The Story of the Jewish Calendar. New York: Abelard-Schuman, 1958. Eliade, Mircea. Patterns in Comparative Religion. Cleveland, OH: Meridian, 1963. Katzeff, Paul. Full Moons. Secaucus, NJ: Citadel Press, 1981. Niemann, Paul J. The Lent, Triduum, and Easter Answer Book. San Jose, CA: Resource Publications, 1998. Rees, Elizabeth. Christian Symbols, Ancient Roots. London, England: Jessica Kingsley, 1992.
Moon
(religion, spiritualism, and occult)"Now when Aradia had been taught, taught to work all witchcraft, . . . she said unto (her pupils): Whenever ye have need of anything, once in the month, and when the moon is full, ye shall assemble in some desert place, or in a forest all together join to adore the potent spirit of your queen, my mother, great Diana." This is found in Charles Leland's book of Italian witchcraft, obtained at the end of the nineteenth century. A later incantation to Diana, also in Leland's book, says, "Thou who art Queen of the sun and of the moon and of the stars. . . " The Witch Goddess was associated with the moon from earliest times. Even today, Witches will meet at the full moon to honor their deities. These meetings are called esbats. Doreen Valiente refers to the Bible's Book of Job and its reference to moon worship as indicative of the antiquity of such rites. Barbara Walker claims that moon worship was the religion of Adam and that the early Christians insisted that the full moon shone on Jesus's crucifixion. The Babylonians worshiped the moon as a queen, the ancestor of the sun, and exceeding the sun in power.
The moon has not always been regarded as feminine. There are many moon gods. In Eskimo legend the moon is a male who is lover to the feminine sun. Old English nursery stories speak of the Man in the Moon. In Japanese Shinto (meaning "the Way of the Gods"), tales are told of the sun goddess Ama-Terasu and her brother the moon god Tsuki-yomi. An argument between them led to the god being banished to never again see his sister face to face. In the Emergence Myth of the Navajo people, The First World was too small for First Man, First Woman and Coyote, so they traveled to the Second World, where there were two men who became Sun and Moon. Moon-Old-Man is found among the Pueblo Indians at Isleta, Jemez, Taos, and Tewa. In some areas, such as the Upper Amazon, there is a belief that if a girl stares at the moon she may become pregnant by him. The Semitic moon god Sin lived on top of Mount Sinai, while the Egyptian god Thoth was god of the moon.
The phases of the moon are always addressed when working magic—positive magic (to increase, strengthen, attract, for planting) being done during the waxing phase and negative (to decrease, rid, end, harvest) in the waning phase. The full moon is one of the best times for scrying and other forms of divination. In that the moon revolves around the earth in 28 days, there is a relationship to the menstrual cycle of women. More than that, the phases of the moon affect the spawning of various sea creatures such as crabs, oysters, mussels, and sea-urchins, and the very movement of the tides. Many farmers used to (and some still do) plant and sow, shear sheep, and slaughter swine according to the progress of the moon.
While some peoples have worshiped the sun and the moon as deities in themselves, Witches see the moon simply as a symbol of the goddess. In the Full Moon Esbat there is the ritual of "Drawing Down of the Moon," in which the High Priest calls upon the Goddess to descend into the body of the High Priestess and to speak through her to the assembled worshipers. The ancient Greeks used to speak of the witches of Thessaly "drawing down the moon," meaning that they called upon the power of the moon in their rites.
Moon
(pop culture)Because the vampire is a nocturnal creature, one might expect it to have a special relationship to the moon, as John Polidori certainly assumed in his original vampire tale, “The Vampyre,” published in 1819. Lord Ruthven, the vampire, was killed in the course of the story. However, he was taken out to the pinnacle of a nearby hill so that his body could be exposed to the “first cold ray of the moon that rose after his death.” The moon’s rays revived the vampire. This idea of the moon’s effect on a vampire was picked up by writers and dramatists who built on Polidori’s tale through the first half of the nineteenth century.
James Malcolm Rymer followed Polidori’s lead in Varney the Vampyre, and through the words of Chillingworth, a man wise in such matters, explained to his readers the nature of the vampire’s resurrection. In chapter 4 of the story Varney was shot, and mortally so, but Chillingworth warned:
With regard to these vampyres, it is believed by those who are inclined to give credence to so dreadful a superstition, that they always endeavor to make their feast of blood, for the revival of their bodily powers, on some evening immediately preceding a full moon, because if any accident befalls them, such as, being shot, or otherwise killed or wounded, they can recover by lying down somewhere where the full moon’s rays will fall upon them.
In the next chapter Rymer vividly describes the effects of the moon:
As the moonbeams, in consequence of the luminary rising higher and higher in the heavens, came to touch the figure that lay extended on the rising ground, a perceptible movement took place in it. The limbs appeared to tremble, and although it did not rise up, the whole body gave signs of vitality.
Immediately afterward Varney arose and escaped from his pursuers. Bram Stoker departed from this fictional convention. In Dracula, the moon was used for atmosphere, but possessed no supernatural qualities. In the first chapter, for example, the moonlight provided added emphasis to Dracula’s command over the wolves. Later, in chapter 4, the three women who resided in Castle Dracula made their appearance in the dust dancing in the moonbeams. Subsequent authors of vampire fiction followed Stoker’s lead; it was the deadly sun, not the moon (except as it was an important element in the larger nocturnal environment), that became a significant element of vampire lore.
The idea of the moon reviving a vampire was not repeated in the movies until 1945, in The Vampire’s Ghost, a movie loosely based on Polidori’s “The Vampyre.” Meanwhile, the moon became closely associated with werewolves, as it is the full moon that often triggers the transformation of the werewolf from his human into his wolf form. In the artwork accompanying contemporary fiction, the use of the moon on the cover very frequently indicates to the potential reader that the book has werewolf characters.
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Moon
(religion, spiritualism, and occult)Many seers, diviners, witches, Wiccans, magicians, shamans, and others believe that it is important to work according to the phases of the moon. Basically, constructive magical work is done during the waxing phase and eliminating work during the waning phase, which is an extension of sympathetic magic. Working to bring about something constructive, to bring increase, the person works as the moon is growing and increasing. Working to get rid of bad habits, for example, they work as the moon is decreasing.
Some divination is done specifically at the full moon. It is seldom that any is done especially at the new moon. For best results, it is said that scrying, for example, should be undertaken at the full moon, though it actually can be done at any time. Some scryers “renew” the energy of their crystal balls by placing them out in the light of the full moon every month.
A guide to the phases of the moon can be found in the times of its rising. The new moon always rises at sunrise. The first quarter always rises at noon. The full moon always rises at sunset. The last quarter always rises at midnight. For each day following the above, the moon will rise about fifty minutes later than the previous day.
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What does it mean when you dream about the moon?
The moon reflects our inner emotional feelings and the ways we express those feelings. The ebb and flow of the oceans of the planet with respect to the lunar phases often reflect cyclic increases and decreases of emotional energy states within human beings. How the moon appears to the dreamer determines the meaning of this symbol.
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(dreams)Moon
the earth’s only natural satellite and the celestial body closest to us. Its astronomical symbol is (.
Motion. The moon moves around the earth at a mean velocity of 1.02 km/sec in an approximately elliptical orbit in the same direction as most other bodies of the solar system, that is, counterclockwise if the moon’s orbit is viewed from the earth’s north pole. The semimajor axis of the moon’s orbit, which is equal to the mean distance between the centers of the earth and the moon, is 384,400 km (approximately 60 earth radii), which corresponds to a horizontal equatorial parallax of 57’2”.6. Because of the orbit’s ellipticity (the orbit’s eccentricity is equal to 0.0549) and perturbations, the distance to the moon varies from 356,400 to 406,800 km. As a result, the visible angular diameter of the moon, which is equal to 31’5” at the mean distance, varies from 33’32” to 29’20” (that is, is greater or less than the solar diameter). The moon’s period of revolution around the earth, known as the sidereal month, is equal to 27.32166 days but is subject to small fluctuations and a very small secular contraction.
The moon’s motion around the earth is very complex, and its study is one of the most difficult tasks of celestial mechanics. Elliptical motion is only a crude approximation; it is subject to many perturbations caused by the attraction of the sun and planets and by the oblateness of the earth. The most important perturbations, or nonuniformities (the equation of the center, evection, variation, annual equation) were discovered from observations long before they were theoretically derived from the law of universal gravitation. The moon’s attraction by the sun is 2.2 times stronger than that by the earth. Thus, strictly speaking, the moon’s motion around the sun and the perturbations of this motion caused by the earth should be examined. However, since the researcher is interested in the motion of the moon as viewed from the earth, the theory of gravitation, which has been developed by many prominent scientists beginning with I. Newton, considers the moon’s motion around the earth.
In the 20th century the American mathematician G. W. Hill developed a theory on the basis of which the American astronomer E. Brown in 1919 calculated mathematical series and compiled tables giving the latitude, longitude, and parallax of the moon as functions of time. The series contain about 1,500 terms derived on the basis of purely gravitational action. A significant empirical term that cannot be explained from the standpoint of the theory of gravitation was added to achieve better agreement with the results of lunar observations. In the 1930’s it was established that the introduction of this term was connected not with the deviation of the moon’s motion from the theory of gravitation, but with the inaccuracy of the time measurement system, which was based on the earth’s rotation about its axis, which proved to be nonuniform. In the current theory of lunar motion this empirical term is not taken into account but an appropriate correction is made in universal time; thus the transition to a uniformly flowing ephemeris time, which is used as the independent variable in Brown’s tables, is accomplished.
The plane of the moon’s orbit is inclined to the plane of the ecliptic at an angle of 5°8’43”, which is subject to small variations. The points of intersection of the orbit and the ecliptic, which are called the ascending and descending nodes, have a nonuniform, retrograde motion—called retrogression of the nodes—and complete a full revolution about the ecliptic in 6,794 days, or about 18.6 years. As a result, the moon returns to a given node after a period of time known as the draconic period, or nodical month, which is shorter than the sidereal month and on the average equals 27.21222 days. The periodicity of solar and lunar eclipses is related to the nodical month. The moon rotates about an axis inclined to the plane of the ecliptic at an angle of 88°28’ with a period precisely equal to the sidereal month, as a result of which it always presents the same side to the earth. The coincidence of the periods of the moon’s rotation and revolution is not accidental and is the result of the tidal friction which the earth produced in the solid or formerly liquid mantle of the moon. However, the combination of uniform axial rotation with nonuniform orbital motion causes small periodic deviations from constant orientation toward the earth, which reach 7°54’ in longitude, and the inclination of the moon’s axis of rotation to the plane of the orbit is responsible for deviations of up to 6°50’ in latitude. As a result of these periodic deviations, known as librations, as much as 59 percent of the lunar surface is visible from the earth at different times (although the areas near the edges of the lunar disk can be seen only in strong foreshortening). The planes of the moon’s equator, the ecliptic, and the lunar orbit always intersect in a single line (Cassini’s law).
Figure. The shape of the moon is very close to a sphere with a radius of 1,737 km, which is equal to 0.2724 times the earth’s equatorial radius. The surface area of the moon is 3.8 X 107 km2 (that is, 0.0743 ≈ 3/40 of the earth’s surface), and its volume is 2.2 X 1025 cm3 (that is, 0.0203 ≈ 1/49 of the earth’s volume). An exact determination of the moon’s shape poses problems because, as a result of the absence of oceans, the moon has no clearly defined fiducial surface with respect to which heights and depths can be determined. Moreover, since the moon keeps the same side toward the earth, it is possible to measure from the earth the radii of points on the surface of the moon’s visible hemisphere (except for points on the very edge of the lunar disk) only on the basis of the weak stereoscopic effects caused by the libration. The study of the libration has made it possible to estimate the difference between the principal semiaxes of the moon’s ellipsoid. The polar axis is 700 m shorter than the equatorial axis facing the earth and 400 m shorter than the equatorial axis that is perpendicular to the direction of the earth. Thus, the moon is slightly elongated in the direction of the earth because of tidal forces.
The moon’s mass can be determined most accurately from observations of artificial lunar satellites. It is 1/81.3 that of the earth and equals 7.35 X 1025 g. The moon’s mean density is 3.34g/cm3 (0.61 that of the earth). The acceleration of gravity at the moon’s surface is one-sixth that for the earth and is equal to 162.3 cm/sec2; 1 km above the surface it decreases to 0.187 cm/sec2. The orbital velocity is 1,680 m/sec, and the surface escape velocity is 2,375 m/sec. Because of its small gravitational field, the moon cannot retain a gaseous envelope or water in the free state.
Phases. The moon is not a self-luminous body, and only that part of the moon that is illuminated by sunlight either directly or after reflection from the earth is visible. This explains the phases of the moon. Each month the moon, moving in orbit, passes approximately between the sun and the earth and faces us with its dark side. This is the new moon. One or two days after this the narrow bright crescent of the new moon appears in the western part of the sky. At this time the rest of the lunar disk is weakly illuminated from the earth, which faces the moon with its sunlit hemisphere. This weak luminosity is the moon’s earth-shine, or earthlight. After seven days the moon has moved away from the sun by 90° and the first quarter sets in. This is when exactly half of the moon’s disk is illuminated and the terminator—the line that separates the bright and dark sides—becomes a straight line and thus a diameter of the lunar disk. In the days that follow, the terminator becomes convex, the shape of the moon approaches a bright circle, and the full moon occurs after 14 or 15 days. Then the western limb of the moon begins to wane. On the 22nd day the last quarter is observed. This is when the moon appears once again as a semicircle, but this time with its convex side to the east. The moon’s angular distance from the sun decreases, the moon again becomes a crescent, and the new moon occurs once again after 29.5 days.
The interval between two successive new moons is called the synodic month and has an average length of 29.53059 days. The synodic month is longer than the sidereal month, since in this time the earth passes through approximately 1/13 of its orbit and the moon, in order to pass again between the earth and the sun, must traverse still another 1/13 of its orbit, which requires a little more than two days. If the new moon occurs near one of the nodes of the lunar orbit, a solar eclipse occurs. When the full moon is near a node, a lunar eclipse occurs. The easily observable succession of lunar phases has served as the basis of many calendar systems.
Surface. The surface of the moon is quite dark and its albedo is equal to 0.073; that is, on the average the moon reflects only 7.3 percent of the sun’s rays. The visual stellar magnitude of the full moon at the mean distance is — 12.7. When the moon is full, it sends 465,000 times less light to the earth than the sun. In its variation with phase, this amount of light decreases much more rapidly than the area of the illuminated part of the moon, so that when the moon is at a quarter and we see half of its disk illuminated, it transmits to us not 50 percent but only 8 percent of the light of the full moon. The color index of moonlight is + 1.2, appreciably redder than sunlight.
The moon rotates with respect to the sun with a period equal to the synodic month, and therefore both the day and the night on the moon each last nearly 15 earth days. Being unprotected by an atmosphere, the surface of the moon heats up to +100° C in the day and cools to — 120°C at night. However, as radio observations have demonstrated, these enormous fluctuations in temperature penetrate only a few decimeters because of the extremely low thermal conductivity of the surface layers. For the same reason, the heated surface cools rapidly during total lunar eclipses, although some places retain their heat longer, probably because of their high specific heat (the “hot spots”).
Irregular extended darkish markings that have been taken as seas can be seen on the moon even by the unaided eye. The term “maria” (seas) has been retained even though it has been established that these formations have nothing in common with seas on the earth. Telescopic observations begun in 1610 by Galileo made it possible to detect the mountainous structure of the moon’s surface. The maria are plains of darker color than other areas, which are sometimes called continental regions, and abound in mountains, most of which are circular (craters).
Detailed maps of the moon have been compiled on the basis of many years of observations. The first such maps were published in Danzig (Gdansk) in 1647 by J. Hevelius. Retaining the term “mare,” Hevelius also named the principal lunar mountain ranges after similar features on the earth: the Apennines, Caucasus, and Alps. In 1651, G. Riccioli of Ferrara gave imaginative names to the extensive dark lowlands, such as the Oceanus Procellarum (Ocean of Storms), Mare Crisium (Sea of Crises), Mare Tranquillitatis (Sea of Tranquillity), and Mare Imbrium (Sea of Showers). He called small dark regions bordering on these maria, bays—for example, Sinus Iridum (Bay of Rainbows)— and small irregular spots, marshes—for example, Palus Putredinis. He named individual mountains, chiefly circular ones, after outstanding scientists, such as Copernicus, Kepler, and Tycho Brahe. These names are still retained on lunar maps, and many new names of outstanding individuals and scientists of later date have been added. The names of K. E. Tsiolkovskii, S. P. Korolev, Iu. A. Gagarin, and others appear on maps of the far side of the moon that have been compiled as a result of observations made from space probes and artificial lunar satellites.
In the 19th century the German astronomers J. von Mädler and J. Schmidt compiled detailed and accurate maps of the moon from telescopic observations. The maps were prepared in orthographic projection for the middle phase of the libration, that is, approximately as the moon is seen from the earth. Photographic observations of the moon were begun at the end of the 19th century. Between 1896 and 1910 a large atlas of the moon was published by the French astronomers M. Loewy and P. Puiseux from photographs taken at the Paris Observatory. Later, a photographic album of the moon was published by the Lick Observatory in the United States, and in the mid-20th century G. Kuiper (United States) compiled several detailed atlases of photographs of the moon that were taken through large telescopes at different astronomical observatories. Modern telescopes make it possible to detect (but not to examine in detail) craters about 0.7 km in size and crevasses a few hundred meters wide.
Surface relief. The relief of the lunar surface has been clarified chiefly as a result of many years of telescopic observations. The maria, which occupy about 40 percent of the visible surface of the moon, are flat plains traversed by crevasses and low winding ridges. There are comparatively few large craters in the maria. Many of the maria are surrounded by concentric circular rims (walls). The remaining, lighter, surface is covered with numerous craters, circular ridges, rills, and other features. Craters smaller than 15-20 km are bowl-shaped. Larger craters (up to 200 km) consist of circular rim with steep interior slopes and have a comparatively flat depressed bottom, often with a central knoll. The heights of mountains above the surrounding terrain are determined photometrically or by the length of the shadows cast on the lunar surface. Hypsometric maps with a scale of 1:1,000,000 have been compiled in this manner for most of the visible side. However, absolute heights and the distances of points on the lunar surface from the moon’s mass, or figure, center can be only very inaccurately determined, and the hypsometric maps based on them provide only a general idea of the lunar relief.
The relief of the limb zone, which limits the lunar disk as a function of the phase of the libration, has been studied in much greater detail and with much better accuracy. The German scientist F. Heyn, the Soviet scientist A. A. Nefed’ev, and the American scientist C. Watts compiled hypsometric maps that are used to calculate irregularities of the lunar limb for the purpose of determining the coordinates of the moon (such observations are made with transit circles and from photographs of the moon taken against the background of surrounding stars and from occultations of the stars by the moon). The selenographic coordinates of several principal reference points, which are used to interlink a large number of other points on the lunar surface, are determined through micrometric measurements with respect to the lunar equator and central meridian. The starting point is the Mosting A crater, a small regular formation that is easily visible in the center of the lunar disk. The structure of the moon has been studied chiefly through photometric and polarimetric observations augmented by radio-astronomical investigations.
The craters on the lunar surface are of different relative ages, ranging from ancient, scarcely discernible, heavily modified formations to young clearly defined craters that are sometimes surrounded by bright streaks called rays. Also, the young craters overlap older ones. In some cases the craters cut into the surface of the maria, and in others the rocks of the maria overlap the craters. Tectonic fractures sometimes traverse the craters and maria and sometimes they themselves are overlapped by younger formations. These and other features make it possible to establish the sequence of the appearance of the various structures on the lunar surface.
In 1949 the Soviet scientist A. V. Khabakov divided lunar formations into several successive age complexes. Further development of this approach toward the end of the 1960’s made it possible to compile intermediate-scale geological maps for much of the lunar surface. The absolute age of lunar formations is known only at a few sites, but by using certain indirect methods it has been determined that the youngest large craters are tens or hundreds of millions of years old and that most of the large craters appeared in the “premaria” era, 3 or 4 billion years ago.
Both internal forces and external influences have had a part in the formation of the lunar relief. Thermal calculations show that soon after the moon’s formation the interior was heated by radioactive sources and was melted to a considerable extent, leading to intensive volcanism on the surface. As a result, huge lava fields, a number of volcanic craters, and numerous clefts, scarps, and other features were formed. At the same time an enormous number of meteorites and asteroids (remnants of the protoplanetary cloud) struck the surface of the moon at different stages, resulting in craters ranging in size from pinholes to ringed structures with a cross section of several tens and perhaps several hundred km. Because of the absence of an atmosphere and hydrosphere, a large number of these craters have been preserved to the present day. Meteorites now strike the moon much less often; volcanism has basically halted, since the moon has expended much of its thermal energy and the radioactive elements have been raised to the outer strata of the moon. Escaping carbon-bearing gases in lunar craters, spectrograms of which were obtained for the first time by the Soviet astronomer A. N. Kozyrev, attest to residual volcanism.
Origin. The origin of the moon has not yet been fully established. Three different hypotheses have been developed in detail. At the end of the 19th century G. Darwin posited that the moon and earth originally constituted a single molten mass whose rate of rotation increased as the mass cooled and contracted. As a result, the mass broke up into two parts: the larger one was the earth, and the smaller, the moon. Darwin’s hypothesis explains the low density of the moon, which was formed from the outer layers of the original mass. However, it encounters serious objections from the standpoint of the mechanics of the process. Moreover, there are significant geochemical differences between the rocks of the earth’s mantle and lunar rocks.
The capture hypothesis proposed by the German scientist C. von Weizsacker, the Swedish scientist H. Alfven, and the American scientist H. Urey assumes that the moon was originally an asteroid that, on passing near the earth, became a satellite as a result of the effects of gravity. The probability of such an event is extremely small, and if it happened, there would be a greater difference between earth rocks and lunar rocks.
According to the third hypothesis, worked out by the Soviet scientist O. Iu. Schmidt and his followers in the mid-20th century, the moon and the earth were formed at the same time through the consolidation and compaction of a large cluster of small particles. However, since the moon as a whole has a lower density than the earth, the matter of the interplanetary cloud must have divided with a concentration of the heavy elements in the earth. This led to the assumption that the earth, surrounded by a thick atmosphere enriched with relatively volatile silicates, began to form first. On subsequent cooling, the matter of this atmosphere condensed into a ring of planetesimals, from which the moon formed. This hypothesis seems to be the most plausible at the current level of knowledge (1970’s).
Recent advances. A new stage of lunar research began with the launching of the first unmanned interplanetary probes to the moon. Studies have been conducted in the USSR with the Luna (21 spacecraft had been launched by September 1973) and the Zond probes, and in the United States the Ranger, Lunar Orbiter, Surveyor, and Apollo programs have been carried out. In early 1959, the Soviet Luna 1 was accelerated to escape velocity for the first time, and thus the first artificial planet was created. On Sept. 14, 1959, Luna 2 spacecraft delivered on the moon a banner bearing the USSR State Seal, and on Oct. 7, 1959, Luna 3 took the first photographs of about two-thirds of the moon’s far side at a distance of about 65,000 km. The transmitted television pictures made it possible to compile the first atlas of the far side. On July 20, 1965, the Zond 3 probe transmitted much clearer pictures of almost all the rest of the far side, which differs from the visible side in the absence of maria, with a few exceptions (for example, the Mare Moscoviense [Sea of Moscow]). Virtually the entire surface is mountainous and covered with craters of different sizes. Chains of craters up to several hundred kilometers long were also observed on the far side. As a result of studies of photographs of the far side taken by Luna 3 and Zond 3, the Atlas of the Far Side of the Moon cataloging about 4,000 newly observed formations was published in the USSR. In 1966-67 the first complete map of the moon and a complete globe of the moon were made in the USSR from the data of this atlas and photographs of the visible side of the moon. An atlas consisting of seven maps of the equatorial zone of the visible lunar hemisphere was published in 1968.
The American Ranger 7 unmanned spacecraft, which was launched on July 28, 1964, transmitted about 200 photographs taken at distances of 1,800 to 0.3 km from the moon. The photographs revealed the presence of craters ranging in size from those visible from the earth to others 1-2 m in diameter even on the apparently smooth surface of the maria. Luna 9, launched on Jan. 31, 1966, made the first soft landing on the moon on Feb. 3, 1966. A panoramic picture of the surrounding terrain was transmitted to the earth. Individual rocks or clods that were probably ejected on the impact of meteorites or during volcanic eruptions could be seen on the microgranular surface. Luna 10, launched on Mar. 31, 1966, became the first artificial satellite of the moon on Apr. 3, 1966. Between June and December 1966, American and Soviet spacecraft conducted experiments on the mechanical properties of the soil, determining the soil’s density and bearing strength. The outermost layer has a density of 1.1-1.2 g/cm3 and can withstand loads of up to 1 kg/cm2; at a depth of a few decimeters the density and bearing strength increase considerably. American artificial lunar satellites of the Lunar Orbiter series transmitted medium-scale photographs of virtually the entire lunar surface and large-scale photographs of a number of particular areas. Measurement of the velocity of motion of these satellites around the moon made it possible to compile gravitational maps of the moon. In the process, it was discovered that masses of high-density matter (mascons) occur in the vicinity of the circular maria.
On July 21, 1969, the American astronauts N. Armstrong and E. Aldrin, who traveled to the moon aboard the Apollo 11 spacecraft, were the first humans to set foot on the moon. Another ten men visited the moon on subsequent flights of Apollo spacecraft. The astronauts brought back to the earth several hundred kilograms of lunar rock samples; they also conducted a number of studies on the moon itself, including measurements of the heat flux, magnetic field, radiation level, and the intensity and composition of the solar wind (the stream of particles emanating from the sun). It was shown that the heat flux from the lunar interior is approximately one-half that from the earth’s interior. Residual magnetization was found in lunar rocks, attesting to the presence, at one time, of a magnetic field on the moon. Instruments that automatically transmit information to the earth, including seismometers that record oscillations in the body of the moon, were left on the moon. The seismometers have recorded shocks from the impact of meteorites and “moonquakes” of internal origin. On the basis of seismic data, it has been determined that to a depth of a few tens of kilometers the moon consists of a relatively light “crust”; below that lies a denser “mantle.” The duration of seismic oscillations on the moon, which is several times longer than on the earth, is apparently connected with the strongly fractured nature of the upper “crust.”
At the same time, studies of the moon were conducted by the Soviet Luna probes. In September 1970 Luna 16 drilled a 35-cm core sample and brought it back to the earth. In November 1970, Luna 17 delivered to the moon’s Mare Imbrium Lunokhod 1, a self-propelled lunar vehicle, which traveled a distance of 10,540 m in 11 lunar days (or 10.5 months) and transmitted a large number of panoramic pictures, individual photographs of the lunar surface, and various scientific data. A French reflector mounted on the Lunokhod made it possible, using a laser beam, to measure the distance to the moon with an accuracy of a fraction of a meter. In February 1972, Luna 20 probe brought back samples of lunar soil that were picked up in an almost inaccessible region of the moon. In January 1973 Luna 21 placed Lunokhod 2 in the Le Monnier crater (Mare Serenitatis) to conduct a comprehensive study of the transition zone between the mare and continental regions. Lunokhod 2 operated for five lunar days (four months) and traversed a distance of about 37 km.
Soil. Everywhere that spacecraft have made landings, the moon is covered with regolith, a layer of loose rock material, ranging in thickness from a few meters to several tens of meters. The regolith formed as a result of the crushing, intermixing, and sintering of lunar rocks under the impact of meteorites and micrometeorites. As a result of the solar wind, it is saturated with neutral gases. Particles of meteoritic matter are found in the regolith. It has been established from radioisotopes that some fragments on the surface of the regolith have been there tens or hundreds of millions of years.
Two types of rocks are found among the samples brought back to the earth: volcanic rocks (lava) and rocks formed by the crushing and melting of lunar formations under the impact of meteorites (glasses and breccias). Most of the volcanic rocks are similar to terrestrial basalts and contain plagioclases, pyroxenes, ilmenite, olivine, spinel, zircon, apatite, metallic iron, copper, and other minerals. All the lunar maria apparently consist of such rocks. Moreover, fragments of other rocks, which are similar to such terrestrial rocks as norites, anorthosites, and dacites, have been observed in the lunar soil, as well as KREEP—a rock that is rich in potassium, rare-earth elements, and phosphorus. Apparently these rocks represent fragments of the matter of the lunar continent. Luna 20 and Apollo 16, which soft-landed in continental regions, brought back anorthosite-type rocks from these sites. All types of rocks (see Table 1) were formed as a result of the prolonged evolution of melts in the lunar interior. Lunar rocks differ from terrestrial rocks in a number of respects: (1) they contain very little water and little potassium, sodium, and other volatile elements, and (2) there is a great deal of titanium and iron in some samples, but on the whole the moon is poor in siderophile elements. The age of these rocks, as determined from the relationships among the radioactive elements, is 3-4.5 billion years, which corresponds to the earliest periods of the earth’s development.
| Table 1. Principal varieties of lunar rocks (composition by percent) | |||||
|---|---|---|---|---|---|
| Mare basalt1 | Gabbro- anorthosite2 | Anorthosite3 | Norite, or nonmare basalt4 | Dacite5 | |
| SiO2 | 40.5 | 42.4 | 44.1 | 50 | 61 |
| Al2O3 | 9.7 | 20.2 | 35.5 | 20 | 12 |
| FeO | 19.0 | 6.4 | 0.2. | 7.7 | 10 |
| Ti02 | 11.4 | 0.4 | – | 1.3 | 1.2 |
| CaO | 9.6 | 18.6 | 19.7 | 11 | 6.3 |
| MgO | 8.0 | 12.2 | 0.1 | 8 | 6 |
| Na2O | 0.53 | 0.40 | 0.34 | 0.63 | 0.69 |
| K2O | 0.16 | 0.52 | – | 0.53 | 2.0 |
| 1 Apollo 11, average of four samples2 Luna 203 Apollo 15, no. 154154 Apollo 14, no. 143105 Apollo 12, no. 12013 | |||||
International legal problems. The principal legal problems of exploring the moon have been resolved by the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies (known as the Outer Space Treaty). But the significant achievements in the study of the moon raised the necessity of concluding a special international treaty that would regulate different aspects of the activities of various countries on the moon. The need for a treaty whose range of force would be limited exclusively to the moon is due to the special status of the moon, since the study of the moon is conducted directly by human beings. In June 1971 the USSR proposed a draft of an international treaty regarding the moon at the 26th session of the UN General Assembly; this draft was submitted for appropriate study to the UN Committee on the Use of Outer Space for Peaceful Purposes. The Soviet draft is intended to ensure that the moon is used exclusively for peaceful purposes.
In conducting scientific research on the moon, no state has the right to infringe on the interests of other states or interfere with similar research conducted by other states. By making specific the Outer Space Treaty, which prohibits the appropriation of celestial bodies, the Soviet draft treaty stipulates that the surface and interior of the moon cannot be owned by any state. The question of the responsibility of states for damages inflicted during exploration of the moon are also covered.
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A. A. MIKHAILOV and A. P. VINOGRADOV