Bisakah Cold Moon Dilihat Secara Langsung?
Ternyata Cold Moon dapat disaksikan secara langsung tanpa perlu menggunakan perangkat optik seperti teleskop. Dijelaskan dalam laman The Economic Times bahwa waktu terbaik untuk menyaksikan Cold Moon adalah pada tanggal 15 Desember 2024.
Pada tanggal tersebut seseorang bisa menyaksikan Cold Moon di daerah masing-masing dengan pemandangan tanpa halangan. Bahkan cahaya yang dihasilkan oleh Bulan Purnama diperkirakan bakal terlihat terang dan jelas.
Illumination and phases
Half of the Moon's surface is always illuminated by the Sun (except during a lunar eclipse). Earth also reflects light onto the Moon, observable at times as Earthlight when it is reflected back to Earth from areas of the near side of the Moon that are not illuminated by the Sun.
Since the Moon's axial tilt with respect to the ecliptic is 1.5427°, in every draconic year (346.62 days) the Sun moves from being 1.5427° north of the lunar equator to being 1.5427° south of it and then back, just as on Earth the Sun moves from the Tropic of Cancer to the Tropic of Capricorn and back once every tropical year. The poles of the Moon are therefore in the dark for half a draconic year (or with only part of the Sun visible) and then lit for half a draconic year. The amount of sunlight falling on horizontal areas near the poles depends on the altitude angle of the Sun. But these "seasons" have little effect in more equatorial areas.
With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different lunar phases during the synodic month. The phase is equal to the area of the visible lunar sphere that is illuminated by the Sun. This area or degree of illumination is given by ( 1 − cos e ) / 2 = sin 2 ( e / 2 ) {\displaystyle (1-\cos e)/2=\sin ^{2}(e/2)} , where e {\displaystyle e} is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).
Brightness and apparent size of the Moon changes also due to its elliptic orbit around Earth. At perigee (closest), since the Moon is up to 14% closer to Earth than at apogee (most distant), it subtends a solid angle which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee.[207] A full (or new) moon at such a position is called a supermoon.[201][202][208]
There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago.[209][210]
The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.[72][j] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon.[211] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light.[212]
At times, the Moon can appear red or blue. It may appear red during a lunar eclipse, because of the red spectrum of the Sun's light being refracted onto the Moon by Earth's atmosphere. Because of this red color, lunar eclipses are also sometimes called blood moons. The Moon can also seem red when it appears at low angles and through a thick atmosphere.
The Moon may appear blue depending on the presence of certain particles in the air,[212] such as volcanic particles,[213] in which case it can be called a blue moon.
Because the words "red moon" and "blue moon" can also be used to refer to specific full moons of the year, they do not always refer to the presence of red or blue moonlight.
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses.[214] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
Because the distance between the Moon and Earth is very slowly increasing over time,[185] the angular diameter of the Moon is decreasing. As it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing.[k] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.[215]
As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.[216] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years.[217]
Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[l][218] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.[219]
Moon Treaty and explorational absence (1976–1990)
Following the last Soviet mission to the Moon of 1976, there was little further lunar exploration for fourteen years. Astronautics had shifted its focus towards the exploration of the inner (e.g. Venera program) and outer (e.g. Pioneer 10, 1972) Solar System planets, but also towards Earth orbit, developing and continuously operating, beside communication satellites, Earth observation satellites (e.g. Landsat program, 1972), space telescopes and particularly space stations (e.g. Salyut program, 1971).
Negotiation in 1979 of Moon treaty, and its subsequent ratification in 1984 was the only major activity regarding the Moon until 1990.
Renewed exploration (1990–present)
In 1990 Hiten-Hagoromo,[254] the first dedicated lunar mission since 1976, reached the Moon. Sent by Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.
In 1994, the U.S. dedicated a mission to fly a spacecraft (Clementine) to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface.[255] In 1998, this was followed by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.[256]
The next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon. Between 2004 and 2006 the first spacecraft by the European Space Agency (ESA) (SMART-1) reached the Moon, recording the first detailed survey of chemical elements on the lunar surface.[257] The Chinese Lunar Exploration Program reached the Moon for the first time with the orbiter Chang'e 1 (2007–2009),[258] obtaining a full image map of the Moon. India reached, orbited and impacted the Moon in 2008 for the first time with its Chandrayaan-1 and Moon Impact Probe, becoming the fifth and sixth state to do so, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.[259]
The U.S. launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor on June 18, 2009. LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on October 9, 2009,[260] whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery.
China continued its lunar program in 2010 with Chang'e 2, mapping the surface at a higher resolution over an eight-month period, and in 2013 with Chang'e 3, a lunar lander along with a lunar rover named Yutu (Chinese: 玉兔; lit. 'Jade Rabbit'). This was the first lunar rover mission since Lunokhod 2 in 1973 and the first lunar soft landing since Luna 24 in 1976, making China the third country to achieve this.
In 2014 the first privately funded probe, the Manfred Memorial Moon Mission, reached the Moon.
Another Chinese rover mission, Chang'e 4, achieved the first landing on the Moon's far side in early 2019.[261]
Also in 2019, India successfully sent its second probe, Chandrayaan-2 to the Moon.
In 2020, China carried out its first robotic sample return mission (Chang'e 5), bringing back 1,731 grams of lunar material to Earth.[262]
The U.S. developed plans for returning to the Moon beginning in 2004,[263] and with the signing of the U.S.-led Artemis Accords in 2020, the Artemis program aims to return the astronauts to the Moon in the 2020s.[264] The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on the Moon Treaty and the ESA-led Moon Village concept.[265][266][267]
2023 and 2024 India and Japan became the fourth and fifth country to soft land a spacecraft on the Moon, following the Soviet Union and United States in the 1960s, and China in the 2010s.[268] Notably, Japan's spacecraft, the Smart Lander for Investigating Moon, survived 3 lunar nights.[269] The IM-1 lander became the first commercially built lander to land on the Moon in 2024.[270]
China launched the Chang'e 6 on May 3, 2024, which conducted another lunar sample return from the far side of the Moon.[271] It also carried a Chinese rover to conduct infrared spectroscopy of lunar surface.[272] Pakistan sent a lunar orbiter called ICUBE-Q along with Chang'e 6.[273]
Nova-C 2, iSpace Lander and Blue Ghost are all planned to launch to the Moon in 2024.
Beside the progressing Artemis program and supporting Commercial Lunar Payload Services, leading an international and commercial crewed opening up of the Moon and sending the first woman, person of color and non-US citizen to the Moon in the 2020s,[274] China is continuing its ambitious Chang'e program, having announced with Russia's struggling Luna-Glob program joint missions.[275][276] Both the Chinese and US lunar programs have the goal to establish in the 2030s a lunar base with their international partners, though the US and its partners will first establish an orbital Lunar Gateway station in the 2020s, from which Artemis missions will land the Human Landing System to set up temporary surface camps.
While the Apollo missions were explorational in nature, the Artemis program plans to establish a more permanent presence. To this end, NASA is partnering with industry leaders to establish key elements such as modern communication infrastructure. A 4G connectivity demonstration is to be launched aboard an Intuitive Machines Nova-C lander in 2024.[277] Another focus is on in situ resource utilization, which is a key part of the DARPA lunar programs. DARPA has requested that industry partners develop a 10–year lunar architecture plan to enable the beginning of a lunar economy.[278]
In 1959 the first extraterrestrial probes reached the Moon (Luna program), just a year into the space age, after the first ever orbital flight. Since then humans have sent a range of probes and people to the Moon. The first stay of people on the Moon was conducted in 1969, in a series of crewed exploration missions (the Apollo Program), the last having taken place in 1972.
Uninterrupted presence has been the case through the remains of impactors, landings and lunar orbiters. Some landings and orbiters have maintained a small lunar infrastructure, providing continuous observation and communication at the Moon.
Increasing human activity in cislunar space as well as on the Moon's surface, particularly missions at the far side of the Moon or the lunar north and south polar regions, are in need for a lunar infrastructure. For that purpose orbiters in orbits around the Moon or the Earth–Moon Lagrange points, have since 2006 been operated. With highly eccentric orbits providing continuous communication, as with the orbit of Queqiao and Queqiao-2 relay satellite or the planned first extraterrestrial space station, the Lunar Gateway.[279][280]
While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed.[282] If there is astronomy performed from the Moon, it will need to be free from any physical and radio pollution. While the Moon has no significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content.[283] Scholar Alice Gorman asserts that, although the Moon is inhospitable, it is not dead, and that sustainable human activity would require treating the Moon's ecology as a co-participant.[284]
The so-called "Tardigrade affair" of the 2019 crashed Beresheet lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection.[285]
Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions.[286][287] As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle.[288][289]
Human remains have been transported to the Moon, including by private companies such as Celestis and Elysium Space. Because the Moon has been sacred or significant to many cultures, the practice of space burials have attracted criticism from indigenous peoples leaders. For example, then–Navajo Nation president Albert Hale criticized NASA for sending the cremated ashes of scientist Eugene Shoemaker to the Moon in 1998.[290][291]
Beside the remains of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six lunar plaques, the Fallen Astronaut memorial, and other artifacts.[281]
Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational.[292] Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical librations through laser ranging to the Moon.
There are several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.
Lunar geologic timescale
Millions of years before present
The lunar geological periods are named after their characteristic features, from most impact craters outside the dark mare, to the mare and later craters, and finally the young, still bright and therefore readily visible craters with ray systems like Copernicus or Tycho.
Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System.[36][37] Historically, several formation mechanisms have been proposed,[38] but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force[39] would require too great an initial rotation rate of Earth.[40] Gravitational capture of a pre-formed Moon[41] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon.[40] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[40] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.[42]
The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The oblique impact blasted material into orbit about the Earth and the material accreted and formed the Moon[43][44] just beyond the Earth's Roche limit of ~2.56 R🜨.[45]
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth.[46] However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth.[47][48][49][50] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,[51] although this is debated.[52]
The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system.[53][54] The newly formed Moon would have had its own magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[53]
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[55] Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements.[56] A study published in 2022, using high-resolution simulations (up to 108 particles), found that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.[57]
On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside the Earth.[58][59]
The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other, eclipses were more frequent, and tidal effects were stronger.[60] Due to tidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.[61]
Following formation, the Moon has cooled and most of its atmosphere has been stripped.[62] The lunar surface has since been shaped by large impact events and many small ones, forming a landscape featuring craters of all ages.
The Moon was volcanically active until 1.2 billion years ago, which laid down the prominent lunar maria. Most of the mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years[63] and some as old as 4.2 billion years.[64] There are differing explanations for the eruption of mare basalts, particularly their uneven occurrence which mainly appear on the near-side. Causes of the distribution of the lunar highlands on the far side are also not well understood. Topological measurements show the near side crust is thinner than the far side. One possible scenario then is that large impacts on the near side may have made it easier for lava to flow onto the surface.[65]
Pre-telescopic observation (before 1609)
It is believed by some that the oldest cave paintings from up to 40,000 BP of bulls and geometric shapes,[220] or 20–30,000 year old tally sticks were used to observe the phases of the Moon, keeping time using the waxing and waning of the Moon's phases.[221] One of the earliest-discovered possible depictions of the Moon is a 3,000 BCE rock carving Orthostat 47 at Knowth, Ireland.[222][223] Lunar deities like Nanna/Sin featuring crescents are found since the 3rd millennium BCE.[224] Though the oldest found and identified astronomical depiction of the Moon is the Nebra sky disc from c. 1800–1600 BCE.[225][226]
The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.[230][231]: 227 Elsewhere in the 5th century BC to 4th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses,[232] and Indian astronomers had described the Moon's monthly elongation.[233] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses.[231]: 411
In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries.[234] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System.[235] In the 2nd century BC, Seleucus of Seleucia correctly thought that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun.[236] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance.
The Chinese of the Han dynasty believed the Moon to be energy equated to qi and their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun; Jing Fang (78–37 BC) noted the sphericity of the Moon.[231]: 413–414 Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters, close to the correct values of about 60 and 0.273 respectively.[237] In the 2nd century AD, Lucian wrote the novel A True Story, in which the heroes travel to the Moon and meet its inhabitants. In 510 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon.[238][239] The astronomer and physicist Ibn al-Haytham (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.[240] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.[231]: 415–416 During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was "perfectly smooth".[241]
Coordination and regulation
Increasing human activity at the Moon has raised the need for coordination to safeguard international and commercial lunar activity. Issues from cooperation to mere coordination, through for example the development of a shared Lunar time, have been raised.
In particular the establishment of an international or United Nations regulatory regime for lunar human activity has been called for by the Moon Treaty and suggested through an Implementation Agreement,[265][267] but remains contentious. Current lunar programs are multilateral, with the US-led Artemis program and the China-led International Lunar Research Station. For broader international cooperation and coordination the International Lunar Exploration Working Group (ILEWG), the Moon Village Association (MVA) and more generally the International Space Exploration Coordination Group (ISECG) has been established.
Since pre-historic times people have taken note of the Moon's phases and its waxing and waning cycle, and used it to keep record of time. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon.[221][321][322] The counting of the days between the Moon's phases gave eventually rise to generalized time periods of lunar cycles as months, and possibly of its phases as weeks.[323]
The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English month as well as moon, and its cognates in other Indo-European languages (e.g. the Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month")[324][325][326][327] stem from the Proto-Indo-European (PIE) root of moon, *méh1nōt, derived from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual).[328][329][330] To give another example from a different language family, the Chinese language uses the same word (月) for moon as well as for month, which furthermore can be found in the symbols for the word week (星期).
This lunar timekeeping gave rise to the historically dominant, but varied, lunisolar calendars. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.[331]
Of particular significance has been the occasion of full moon, highlighted and celebrated in a range of calendars and cultures, an example being the Buddhist Vesak. The full moon around the southern or northern autumnal equinox is often called the harvest moon and is celebrated with festivities such as the Harvest Moon Festival of the Chinese lunar calendar, its second most important celebration after the Chinese lunisolar Lunar New Year.[332]
Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu.
History of exploration and human presence
Fase Bulan Bulan Desember 2024
Seperti yang telah diungkapkan sebelumnya bahwa Cold Moon ada di titik tertingginya di bulan Desember 2024 ini. Hal tersebut tak terlepas dari fase bulan yang berlangsung di bulan ini. Setidaknya ada 5 fase bulan yang terjadi sepanjang bulan Desember 2024. Masih mengacu dari laman Almanac, berikut rincian lengkapnya:
Itulah tadi gambaran mengenai Cold Moon sebagai fenomena Bulan Purnama yang diperkirakan bakal muncul pada 15 Desember 2024. Semoga informasi ini dapat menambah wawasan baru bagi detikers, ya.
Natural satellite orbiting Earth
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period (lunar day) at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.
In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the United States from coast to coast). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth largest and most massive moon overall, and larger and more massive than all known dwarf planets.[17] Its surface gravity is about one sixth of Earth's, about half of that of Mars, and the second highest among all Solar System moons, after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia.
The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks, rilles and, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled lava. These maria were formed when molten lava flowed into ancient impact basins. The Moon is, except when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases.[18] The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun completely during a total solar eclipse. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible.
The Moon has been an important source of inspiration and knowledge for humans, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. The first human-made objects to fly to an extraterrestrial body were sent to the Moon, starting in 1959 with the flyby of the Soviet Union's Luna 1 and the intentional impact of Luna 2. In 1966, the first soft landing (by Luna 9) and orbital insertion (by Luna 10) followed. On July 20, 1969, humans for the first time stepped on an extraterrestrial body, landing on the Moon at Mare Tranquillitatis with the lander Eagle of the United States' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Since then, exploration of the Moon has continued robotically, and crewed missions are being planned to return beginning in the late 2020s.
The usual English proper name for Earth's natural satellite is simply Moon, with a capital M.[19][20] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn,[21] which in turn comes from Proto-Indo-European *mēnsis 'month'[22] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb 'measure' (of time).[23]
Occasionally, the name Luna is used in scientific writing[24] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon.[25] Cynthia is another poetic name, though rare, for the Moon personified as a goddess,[26] while Selene (literally 'Moon') is the Greek goddess of the Moon.
The English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, lūna. Selenian [27] is an adjective used to describe the Moon as a world, rather than as a celestial object,[28] but its use is rare. It is derived from σελήνη selēnē, the Greek word for the Moon, and its cognate selenic was originally a rare synonym[29] but now nearly always refers to the chemical element selenium.[30] The element name selenium and the prefix seleno- (as in selenography, the study of the physical features of the Moon) come from this Greek word.[31][32]
Artemis, the Greek goddess of the wilderness and the hunt, came to also be identified as the goddess of the Moon (Selene) and was sometimes called Cynthia, after her birthplace on Mount Cynthus.[33] Her Roman equivalent is Diana. The names Luna, Cynthia, and Selene are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.
The astronomical symbol for the Moon is a crescent\decrescent, \, for example in M☾ 'lunar mass' (also ML).
Telescopic exploration (1609–1959)
In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book Sidereus Nuncius, and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier.
Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 Mappa Selenographica of Wilhelm Beer and Johann Heinrich von Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography.[242] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions.[72] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s,[243] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.[72]
