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Anthropologists distinguish the U.S. State of California as a primary zone of prehistoric and tribal North America—it was one of the most linguistically and cultural diverse regions on earth. The original population of Native California and traditional cultures were decimated by the Spanish, the Mexicans, and the Anglos, who successively settled California and transformed it. For that reason, knowledge of the character and function of astronomy in what is now California prior to European contact in the 16th century is incomplete and fragmented. Traditional astronomical lore is preserved in a few ethnohistoric commentaries, in some archaeological remains, and in ethnographic research conducted primarily in the early 20th century, when elements of indigenous knowledge still survived. Throughout Native California, the moon’s conspicuous brightness, movement, and systematically changing appearance prompted its affiliation with seasonal change, the passage of time, and cyclical renewal, and most California tribes monitored and counted lunations in one way or another, but not necessarily throughout the entire year. In some cases, individual lunations were affiliated with and named for seasonal circumstances. There is little evidence, however, for even minimal interest in or recognition of the planets visible to the unaided eye, with the exception of Venus as the “Morning Star” or “Evening Star.” Venus, like the moon and other celestial objects, was personified and regarded as a fundamental and active agent of the cosmos. There is no evidence, however, for detailed monitoring of Venus and quantitative knowledge of its synodic behavior.

Article

Von Del Chamberlain

We can be certain that all cultures wondered about the Sun, Moon, planets, and stars, and that they found ways of incorporating what they observed into comprehension of themselves existing within their perceptible surroundings, both on earth and in the sky. Thanks to the gleanings of anthropologists in the late 1800s and early 1900s, we have a treasure trove revealing that the Native American Skidi Band of the Pawnee Nation possessed a unique creation tradition rich in astronomical symbolism. This includes the belief that the two bright planets encompassing within their orbits the orbit of planet Earth were considered by the Skidi to be the cosmic parents of the very first human child, a girl; the Sun and Moon were considered parents of the first male child. This story of human origin includes the legendary journey of the male Great Red Warrior from the east to court the Beautiful Bright White female star of the west, followed by the birth of their daughter transported to earth. This is a striking allegory of the apparent migrations of Mars and Venus, continually changing in brightness, undergoing retrograde motions and sometimes seeming to unite in close conjunctions. Watching these interrelations, repeated over and over with intriguing variations, likely led to and continually reinforced this tradition. Likewise, the apparent monthly relationships of Sun and Moon, with occasional eclipses, visually reinforced the account of the initial male human birth. Thus, the Skidi Pawnee tradition of human origins is an interesting, indeed beautiful, example of human interpretation of natural phenomena.

Article

Nicolas Mangold, Jessica Flahaut, and Véronique Ansan

Planetary surface compositions are fundamental to an understanding of both the interior activity through differentiation processes and volcanic activity and the external evolution through alteration processes and accumulations of volatiles. While the Moon has been studied since early on using ground-based instruments and returned samples, observing the surface composition of the terrestrial planets did not become practical until after the development of orbital and in situ missions with instruments tracking mineralogical and elemental variations. The poorly evolved, atmosphere-free bodies like the Moon and Mercury enable the study of the formation of the most primitive crusts, through processes such as the crystallization of a magma ocean, and their volcanic evolution. Nevertheless, recent studies have shown more diversity than initially expected, including the presence of ice in high latitude regions. Because of its heavy atmosphere, Venus remains the most difficult planetary body to study and the most poorly known in regards to its composition, triggering some interest for future missions. In contrast, Mars exploration has generated a huge amount of data in the last two decades, revealing a planet with a mineralogical diversity close to that of the Earth. While Mars crust is dominated by basaltic material, recent studies concluded for significant contributions of more felsic and alkali-rich igneous material, especially in the ancient highlands. These ancient terrains also display widespread outcrops of hydrous minerals, especially phyllosilicates, which are key in the understanding of past climate conditions and suggest a volatile-rich early evolution with implications for exobiology. Recent terrains exhibit a cryosphere with ice-rich landforms at, or close to the surface, of mid- and high latitudes, generating a strong interest for recent climatic variability and resources for future manned missions. While Mars is certainly the planetary body the most similar to Earth, the observation of specific processes such as those linked to interactions with solar wind on atmosphere-free bodies, or with a thick acidic atmosphere on Venus, improve our understanding of the differences in evolution of terrestrial bodies. Future exploration is still necessary to increase humankind’s knowledge and further build a global picture of the formation and evolution of planetary surfaces.

Article

Pascal Rosenblatt, Ryuki Hyodo, Francesco Pignatale, Antony Trinh, Sebastien Charnoz, Kevin Dunseath, Mariko Dunseath-Terao, and Hidenori Genda

The origin of the natural satellites or moons of the solar system is as challenging to unravel as the formation of the planets. Before the start of the space probe exploration era, this topic of planetary science was restricted to telescopic observations, which limited the possibility of testing different formation scenarios. This era has considerably boosted this topic of research, particularly after the Apollo missions returned samples from the Moon’s surface to Earth. Observations from subsequent deep space missions such as Viking 1 and 2 Orbiters, Voyager 1 and 2, Phobos-2, Galileo, Cassini-Huygens, and the most recent Mars orbiters such as Mars Express, as well as from the Hubble space telescope, have served to intensify research in this area. Each moon system has its own specificities, with different origins and histories. It is widely accepted that the Earth’s Moon formed after a giant collision between the proto-Earth and a body similar in size to Mars. The Galilean moons of Jupiter, on the other hand, appear to have formed by accretion in a circum-Jovian disk, while smaller, irregularly shaped satellites were probably captured by the giant planet. The small and medium-sized Saturnian moons may have formed from the rings encircling the planet. Among the terrestrial planets, Mercury and Venus have no moons, the Earth has a single large moon, and Mars has two very small satellites. This raises some challenging questions: What processes can lead to moon formation around terrestrial planets and what parameters determine the possible outcomes, such as the number and size of moons? The answer to such fundamental questions necessarily entails a thorough understanding of the formation of the Martian system and may have relevance to the possible existence of (exo)moons orbiting exoplanets. The formation of such exomoons is of great importance as they could influence conditions for habitability or for maintaining life over long periods of time on the surface of Earth-like exoplanets, for example by limiting the variations of the orientation of the planet’s rotation axis and thus preventing frequent changes of its climate. Our current knowledge concerning the origin of Phobos and Deimos has been acquired from observational data as well as theoretical work. Early observations led to the idea that the two satellites were captured asteroids but this created difficulties in reconciling the current orbits of Phobos and Deimos with those of captured bodies, hence suggesting the need for an alternative theory. A giant-impact scenario provides a description of how moons similar to Phobos and Deimos can be formed in orbits similar to those observed today. This scenario also restricts the range of possible composition of the two moons, providing a motivation for future missions that aim for the first time to bring material from the Martian system back to Earth.

Article

While the moon naturally featured in Mediterranean cultures from time immemorial, principally noted in the earliest literature as a marker of time, time-dependent constructs such as the calendar, and time-related activities, awareness and recognition of the five visible planets came relatively late to the Greeks and thence to the Romans. The moon underlies the local calendars of the Greeks, with documentary and literary evidence from the Late Bronze Age through the Imperial Roman period, and there are signs that the earliest Roman calendar also paid homage to the moon in its divisions of the month. However, although Homer in the 8th century BCE knows of a Morning and an Evening Star, he shows no indication of realizing that these are one and the same, the planet Venus. That particular identification may have come in the 6th century BCE, and it appears to have been not until the 4th century BCE that the Greeks recognized the other four planets visible to the naked eye—Saturn, Jupiter, Mars, and Mercury. This awareness probably came via contact with Babylonian astronomy and astrology, where identification and observations of the planets had figured from the 2nd millennium BCE and served as a basis for astrological prognostications. But it is time, not astrology, that lies at the heart of Greek and Roman concerns with the moon and the planets. Indeed, the need to tell time accurately has been regarded as the fundamental motivation of Greek astronomy. A major cultural issue that long engaged the Greeks was how to synchronize the incommensurate cycles of the moon and the sun for calendrical purposes. Given the apparent irregularities of their cycles, the planets might seem to offer no obvious help with regard to time measurement. Nonetheless they were included by Plato in the 4th century BCE in his cosmology, along with the sun and moon, as heavenly bodies created specifically to compute time. Astrology then provided a useful framework in which the sun, moon, planets, and stars all combined to enable the interpretation and forecasting of life events. It became necessary for the Greeks, and their successors the Romans, to be able to calculate as accurately as possible the positions of the heavenly bodies in order to determine readings of the past, present, and future. Greek astronomy had always had a speculative aspect, as philosophers strove to make sense of the visible cosmos. A deep-seated assumption held by Greek astronomers, that the heavenly bodies moved in uniform, circular orbits, lead to a desire over the centuries to account for or explain away the observed irregularities of planetary motions with their stations and retrogradations. This intention “to save the phenomena,”— that is, to preserve the fundamental circularity—was said to have originated with Plato. While arithmetical schemes had sufficed in Babylonia for such calculation, it was a Greek innovation to devise increasingly complex geometric theories of circular motions (eccentrics and epicycles) in an effort to understand how the sun, moon, and planets moved, so as to place them more precisely in time and space.

Article

Susan Milbrath

What is known about the Moon among the ancient Maya of southern Mexico and Guatemala and the Nahuatl-speaking people of central Mexico, especially the Aztecs who lived in the Valley of Mexico and their neighbors in Puebla-Tlaxcala Valley, has been obtained from records related to astronomy and lunar cycles inscribed on Classic Maya monuments dating between ad 250 and 850/900. Modern scholarship focusing on the mathematical units and glyphic writing has helped in deciphering the records. Postclassic Maya codices dating from 1300 to 1500, sent to Europe shortly after the Spanish conquest, also have lunar tables that have been decoded by study of the lunar cycles and glyphs. Painted books dating prior to the conquest in 1521 are also known from central Mexico, but these can only be understood with the help of books that were painted by native artists later in the 16th century and annotated with texts written in Spanish and Nahuatl. These glosses provide information about lunar deities and beliefs about the Moon. Furthermore, knowledge of the Moon in Meso-America is greatly enhanced by ethnographic studies and study of iconographic representations of deities representing different lunar roles and phases.

Article

Jay M. Pasachoff and Roberta J.M. Olson

Since the landmark lunar landing of Apollo 11 on July 20, 1969, NASA’s Lunar Reconnaissance Orbiter (launched in 2009), and the Japanese Aerospace Exploration Agency’s Kaguya spacecraft (2007–2009), among other efforts, have now mapped the Moon’s surface. Before those technological advances and since the beginning of recorded time, people and civilizations have been entranced by Earth’s only natural satellite, which is the second-brightest celestial object visible in the sky from the surface of the planet. Selected examples, among thousands, show how the history of the Moon has been regarded, illustrated, and mapped in visual culture in the Western world. Early examples include representations of a formulaic crescent Moon in Babylonian times; later this dominant stylized depiction of the Moon gave way to more naturalistic images based on observation, culminating in Leonardo da Vinci’s manuscript drawings, which study the lunar structure and cratered surface, and Galileo Galilei’s first telescopic images of the Moon recorded in wash drawings and woodcuts for his book Sidereus Nuncius. Both the artistic and scientific visual acuity that made this evolution possible belonged to the burgeoning empiricism of the 14th through the 17th centuries, which eventually yielded modern observational astronomy and impacted lunar iconography. The subsequent dramatic mapping of the Moon’s surface and the naming of its features became a preoccupation of many astronomers and some artists, who assisted scientists in illustrating their work. With the seeming physical mapping of the Earth-facing side of the Moon well underway in the late 18th and early 19th centuries, the function of Earth’s satellite as a Romantic symbol gained force in the all the arts but most dramatically in the works of landscape painters in Germany (e.g., Caspar David Friedrich and Carl Gustav Carus) and in England (e.g., Samuel Palmer). At the same time, William Blake, who was obsessed with astronomical imagery, used the Moon for expressive purposes, which reached a fever pitch later in the century in the work of Vincent Van Gogh. Along with the increasing accuracy of the Moon’s portrayal through both artists’ and scientists’ representations, the dramatic history of its mapping from Earth crescendoed with the development of photography and William Cranch Bond’s first successful daguerreotype of the Moon in 1851. Further exploration of the Moon, including its far side, has gravitated to aerospace engineers in cooperation with physicists, astronomers, mathematicians, and Apollo astronauts. Nevertheless, the Moon has remained an enduring object of fascination for artists—among the many, Surrealist Joan Miró, Veja Celmins, and Andy Warhol.

Article

Long Xiao and James W. Head

The geological characteristics of the Moon provide the fundamental data that permit the study of the geological processes that have formed and modified the crust, that record the state and evolution of the lunar interior, and that identify the external processes that have been important in lunar evolution. Careful documentation of the stratigraphic relationships among these features can then be used to reconstruct the sequence of events and the geological history of the Moon. These results can then be placed in the context of the geological evolution of the terrestrial planets, including Earth. The Moon’s global topography and internal structures include landforms and features that comprise the geological characteristics of its surface. The Moon is dominated by the ancient cratered highlands and the relatively younger flat and smooth volcanic maria. Unlike the current geological characteristics of Earth, the major geological features of the Moon (impact craters and basins, lava flows and related features, and tectonic scarps and ridges) all formed predominantly in the first half of the solar system’s history. In contrast to the plate-tectonic dominated Earth, the Moon is composed of a single global lithospheric plate (a one-plate planet) that has preserved the record of planetary geological features from the earliest phases of planetary evolution. Exciting fundamental outstanding questions form the basis for the future international robotic and human exploration of the Moon.

Article

The great rise and diversification of the use of outer space raises the question of the limitations to space activities. The ultimate restriction posed by space law is the use of outer space “for peaceful purposes.” Regardless of the semantic approach one adopts with respect to the definition of the term “peaceful purposes” in the text of the Outer Space Treaty, it is the underlying substantive legal normativity which constitutes the determining factor. The applicable international legal rules confirm that the ultimate limit is the prohibition of the use of force laid down in Article 2 (4) of the UN Charter, which applies to outer space along with the exceptions stipulated in the UN Charter and general international law. In addition, the Outer Space Treaty establishes a particular legal regime on celestial bodies, declaring them a demilitarized zone, and bans the stationing of weapons of mass destruction in outer space. Space law, as any other branch of public international law, is of evolutive nature, so future adjustments and developments of its legal normativity in light of the abrupt growth and multiplication of the exploration and uses in the space arena remain open.