The celestial body known as the Moon orbits our planet Earth, serving as its only natural satellite. This wondrous object holds the distinction of being the fifth largest Moon in our Solar System, a true giant compared to its parent planet. With a diameter equivalent to the width of Australia, the Moon stands out as a planetary-mass object boasting a rugged, differentiated exterior.
This unique feature places it among the ranks of satellite planets and elevates it above all known dwarf planets in our system. However, despite its impressive size and composition, the Moon lacks any substantial atmosphere, hydrosphere, or magnetic field. The surface gravity of this celestial marvel is an awe-inspiring one-sixth of Earth’s, a feat only matched by Jupiter’s own moon Io in terms of density and strength.
Orbiting our planet Earth at an average distance of 384,400 km, the Moon’s presence exerts a powerful gravitational influence that drives the tides and ever-so-slowly extends the length of each Earthly day. With a sidereal period of 27.3 days, the Moon’s orbital journey around our planet is a marvel to behold.
Over the course of 29.5 days, the amount of visible lunar surface illuminated by the Sun fluctuates, resulting in the phases of the Moon and forming the basis of a lunar calendar. The Moon is tidally locked to Earth, meaning its rotation is synchronized with its orbital period, always presenting the same side to us, the near side. Despite this, a full 59% of the lunar surface can still be seen from Earth as a result of the cyclical shifts known as libration.
The prevailing theory of the Moon’s origin is that it formed approximately 4.51 billion years ago, shortly after the formation of Earth, from the debris left behind following a catastrophic collision between our planet and a Mars-sized body named Theia. Over time, the Moon receded to its current orbit due to the influence of tidal interactions with Earth.
The near side of the Moon is characterized by large, dark volcanic regions known as “maria” that are interspersed among ancient, bright highlands and prominent impact craters. The majority of these impact basins and mare surfaces were formed by the end of the Imbrian period, some three billion years ago.
The surface of the Moon has relatively low reflectivity, with soil reflectance comparable to that of asphalt. However, due to its large angular diameter, the full Moon is the brightest object in the night sky, appearing nearly as large as the Sun and able to completely obscure it during a total solar eclipse.
Throughout history, the Moon’s prominence in the sky and its cyclical phases have been an enduring source of inspiration and influence for human cultures, reflected in language, calendars, art, and mythology. The Soviet Union’s Luna 2 was the first artificial object to reach the Moon in 1959, followed by the successful landing of Luna 9 in 1966. The United States
Apollo program was the first and only human mission to the Moon, with twelve astronauts walking on its surface between 1969 and 1972. These and subsequent uncrewed missions have collected lunar rocks, which have been instrumental in our understanding of the Moon’s formation, internal structure, and evolution. As of 2023, the Moon remains the only celestial body, apart from Earth, that has been visited by humans.
Names and etymology
The celestial satellite of Earth, commonly referred to as the Moon, holds a unique and important place in human culture and history. Its constant presence in the night sky, as well as its predictable cycles of phases, have captivated and inspired people for centuries, leaving a lasting impact on language, art, mythology, and even timekeeping.
The term “Moon” is derived from Old English “mōna,” which can be traced back to the Proto-Indo-European word “mēnsis,” meaning “month.” While the scientific community sometimes refers to the Moon as “Luna,” this name is more commonly found in science fiction, while “Cynthia” and “Selene” are used poetically to personify the celestial body.
To date, the only other celestial body that humans have visited besides Earth is the Moon, with the United States Apollo program being the first to land 12 men on its surface between 1969 and 1972.
The adjective lunar refers to the Moon in English. The adjective selenian, derived from the Greek word for the Moon, “selēnē,” is rarer and refers to the Moon as a world. The prefix seleno- is derived from the Greek word for the Moon and is used in the study of the physical features of the Moon, such as selenography, as well as in the name of the chemical element selenium.
The Greek goddess of the wilderness, Artemis, who was associated with the Moon, was also called Cynthia, reflecting the names of lunar orbits such as apolune, pericynthion, and selenocentric.
The astronomical symbol for the Moon is a crescent ☾, which is often used to represent the Moon in astronomical or scientific contexts, such as “M☾” to denote ‘lunar mass’ or “ML” for the same.
The Natural history of the Moon
The isotopic dating of lunar specimens has sparked a new revelation that suggests the Moon was formed around 50 million years post the formation of the Solar System. Despite numerous proposals for its formation mechanism throughout history, none have been able to fully explain the unique characteristics of the Earth-Moon system.
The idea of the Moon separating from Earth’s crust via centrifugal force was debunked due to the excessive initial rotation rate it would require, while the notion of capturing a pre-formed Moon through Earth’s atmosphere was deemed implausible due to the energy required to dissipate.
The co-formation theory of Earth and Moon in the primordial accretion disk was likewise dismissed as it fails to account for the metal depletion in the Moon.
However, the current leading theory is that the Earth-Moon system was created as a result of a massive impact of a Mars-sized object, referred to as Theia, on the proto-Earth. This impact sent debris into orbit around the Earth, which eventually accreted and formed the Moon, situated just outside the Earth’s Roche limit of ~2.56 R🜨. This explanation provides the best explanation for the high angular momentum of the Earth-Moon system.
Giant impacts, similar to the one that formed the Earth-Moon system, were believed to have been a common occurrence during the early stages of the Solar System’s formation. Computer simulations of such impacts have generated results that are in line with the observations of the Moon’s core mass and the Earth-Moon system’s angular momentum.
Initially, it was believed that most of the Moon was derived from the impactor and not the proto-Earth, but more recent simulations indicate that a larger fraction of the Moon could have originated from the proto-Earth.
However, other inner Solar System bodies, such as Mars and Vesta, have vastly different oxygen and tungsten isotopic compositions compared to Earth, as indicated by meteorites from those bodies.
But, the Earth and the Moon have almost identical isotopic compositions, which could be explained by the post-impact mixing of the vaporized material that formed the two, although this remains a matter of debate.
The massive energy released by the impact would have caused the Earth’s crust and the ejecta to liquefy and form a magma ocean. The liquid ejecta could then have re-accreted into the Earth-Moon system.
The newly formed Moon, similarly, would have had its own lunar magma ocean, estimated to be anywhere between 500 km to 1,737 km deep.
Despite providing explanations for various lines of evidence, the giant-impact theory still leaves some questions unanswered, mostly regarding the Moon’s composition.
A study from 2022 published at a high-resolution threshold for simulations found that giant impacts have the ability to place a satellite with a similar mass and iron content as the Moon into orbit far beyond Earth’s Roche limit. Even satellites that initially pass within the Roche limit have the ability to survive and are predicted to be partially stripped and then pushed onto wider and more stable orbits.
The Moon’s natural development
Despite the many questions that still remain unresolved, the giant-impact theory continues to be the most widely accepted explanation for the formation of the Earth-Moon system. The impact, thought to have taken place early in the Solar System’s history, would have released enough energy to liquefy both the Moon’s surface and Earth’s crust, forming magma oceans on both bodies.
Over time, the Moon settled into its current orbit around Earth, and its surface was shaped by a combination of impact events and volcanic activity. The Moon’s volcanic activity has long since subsided, but its surface still bears the scars of its tumultuous past, with a heavily cratered landscape and the prominent lunar maria. The causes of the Moon’s volcanic eruptions, particularly their uneven distribution, remain an enigma, but they continue to be a subject of ongoing investigation and study.
The Moon’s distinct shape is not just a result of tidal forces but also holds within it a history of its past. As an ellipsoid that is slightly off-balance, with its longest axis tilted at an angle of 30°, it bears testimony to the gravitational anomalies resulting from impact basins and the stretching brought about by tidal forces.
But what truly sets it apart is the elongated shape that extends beyond what current tidal forces can account for – a phenomenon referred to as the “fossil bulge.” This subtle yet remarkable feature points towards a time when the Moon orbited much closer to Earth and was warm enough for its shape to adjust to its then-orbit. Now, it stands frozen in time, its shape a permanent reminder of its past and its place in the cosmic history of our solar system.
What are the Moon’s size and mass?
The Moon is the largest natural satellite relative to its primary planet, Earth, and is the fifth largest Moon in the Solar System. It has a diameter of 3,500 km and a surface area of 38 million square km. The Moon has a mass of 1/81 of Earth’s and surface gravity of 0.1654 g, making it the second densest among planetary moons and with the second highest escape velocity of 2.38 km/s.
And what is its structure?
The Moon is a differentiated body with a distinct crust, mantle, and core. It is believed to have formed from the crystallization of a global magma ocean after its formation 4.5 billion years ago. The mantle is composed of mafic minerals like olivine, clinopyroxene, and orthopyroxene, and the crust is mostly made of anorthosite and is about 50 km thick.
The Moon’s inner core is small and has a radius of about 350 km or less, and is believed to be made of metallic iron alloyed with a small amount of sulfur and nickel. The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).
Despite its rocky surface and airless environment, the Moon does have a trace atmosphere with a total mass of less than 10 tonnes. This atmosphere is characterized by its low surface pressure, varying with the lunar day, and is largely composed of elements such as sodium and potassium, produced by sputtering; helium-4, neon, and argon-40, outgassed after creation by radioactive decay; and water vapor.
These gases, including those found on Mercury and Io, are largely absent of neutral species such as oxygen, nitrogen, carbon, hydrogen and magnesium, the absence of which is still not fully understood. The trace atmosphere is subject to constant change, with its constituents either returning to the regolith due to the Moon’s gravity or being lost to space through solar radiation pressure or ionization from the solar wind’s magnetic field.
Magnetic and gravitational fields of the Moon
The magnetic field surrounding the Moon, though it once had a magnetic dynamo operating 4 billion years ago, has since lost its strength, currently measuring a mere 0.2 nanoteslas. In comparison, Earth’s magnetic field is 100,000 times stronger. Despite this, remnants of magnetization still persist on the lunar crust, possibly originating from transient magnetic fields generated during large impacts.
These impacts lead to the expansion of plasma clouds, and the largest crustal magnetizations are observed near the antipodes of giant impact basins, supporting this theory. This intriguing and mysterious shift from a strong magnetic field to its current state, less than one-hundred thousandth of Earth’s, continues to be a topic of scientific curiosity and investigation.
The Moon’s surface gravity, averaging at 1.62 m/s2, is a mere fraction of Earth’s, being roughly half of Mars and a sixth of Earth’s. However, this gravity is not evenly distributed, with mascons being the main features influencing its gravitational field.
These positive anomalies, linked to giant impact basins and the dense mare basaltic lava flows that fill them, have been measured through tracking the Doppler shift of radio signals from orbiting spacecraft.
The existence of mascons without any correlation to mare volcanism presents a conundrum for scientists, as lava flows alone cannot fully explain the observed gravitational signature. The uneven distribution of the Moon’s gravity continues to be a subject of scientific investigation and fascination.
The Moon’s Atmosphere
Despite its rocky surface and airless environment, the Moon does have a trace atmosphere with a total mass of less than 10 tonnes. This atmosphere is characterized by its low surface pressure, varying with the lunar day, and is largely composed of elements such as sodium and potassium, produced by sputtering; helium-4, neon, and argon-40, outgassed after creation by radioactive decay; and water vapor.
These gases, including those found on Mercury and Io, are largely absent of neutral species such as oxygen, nitrogen, carbon, hydrogen, and magnesium, the absence of which is still not fully understood. The trace atmosphere is subject to constant change, with its constituents either returning to the regolith due to the Moon’s gravity or being lost to space through solar radiation pressure or ionization from the solar wind’s magnetic field.
The Moon, once upon a time, was graced with a thick and abundant atmosphere, according to studies of magma samples retrieved by the Apollo missions. This ancient atmosphere, which lasted for around 70 million years between 3 and 4 billion years ago, was sourced from volcanic eruptions on the Moon and was two times thicker than the present-day Martian atmosphere.
However, solar winds eventually stripped away the lunar atmosphere and dissipated it into space. Additionally, the Moon is surrounded by a permanent dust cloud caused by the impact of small particles from comets. It is estimated that 5 tons of comet particles hit the Moon’s surface every 24 hours, releasing dust particles into the air. The dust hovers above the Moon for approximately 10 minutes, rising and falling in equal 5-minute intervals.
The average amount of dust present above the Moon is 120 kilograms, rising up to a height of 100 kilometers above the surface. The Lunar Dust Experiment (LDEX) by LADEE found that particle counts peak during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers when the Earth and Moon pass through comet debris. The lunar dust cloud is not uniform and is denser near the boundary between the Moon’s day and night side.
The Moon’s surface conditions
The cosmic rays, solar radiation, and resulting neutron radiation produce average ionizing radiation levels on Earth’s surface of 1,369 microsieverts per day, a figure that is nearly 200 times greater than on the surface of the planet. By comparison, astronauts on the International Space Station, located 400 km above Earth, experience 2-3 times lower radiation levels, while those on a trans-Atlantic flight are exposed to 5-10 times less radiation. The radiation exposure during a flight to Mars is roughly 1.84 millisieverts per day, and on the red planet itself, it stands at 0.64 millisieverts per day.
In contrast to Earth’s axial tilt of 23.44°, the Moon’s axial tilt is a mere 1.5427°, leading to much more stable solar illumination throughout the year and the presence of peaks of eternal light at the Moon’s north pole.
The surface of the Moon is subject to extreme temperature fluctuations, ranging from 140°C to -171°C, due to the lack of atmosphere and the strong influence of solar irradiance on local surface temperatures.
The Lunar Reconnaissance Orbiter has measured the coldest temperatures ever recorded in the Solar System, with temperatures reaching a frigid 35 K (−238 °C) in southern polar craters during the summer and an even more startling 26 K (−247 °C) close to the winter solstice in the north polar crater Hermite.
The Moon’s crust is covered by a layer of regolith, a highly fragmented and impact-gardened surface layer that is mostly gray in color. This layer of lunar soil, comprised mostly of silicon dioxide glass, has a texture that resembles snow and a scent that is reminiscent of spent gunpowder.
Older surfaces have thicker regolith layers, varying in thickness from 10-15 m in the highlands to 4-5 m in the maria. Beneath this finely fragmented layer is the megaregolith, a layer of highly fractured bedrock that is many kilometers thick.
Given these extreme conditions on the Moon, it is highly unlikely that bacterial fragments could survive on spacecraft for more than one lunar orbit.
Surface features of the Moon
The Moon’s surface is an enigmatic canvas of extreme conditions and stark contrasts, characterized by its intricate topography, ferocious radiation, and volatile temperature changes. It is an extraterrestrial frontier, both captivating and challenging in equal measure.
The Moon’s axial tilt of just 1.5427° in relation to the ecliptic pales in comparison to Earth’s 23.44° tilt, resulting in far less seasonal variation in solar illumination. This lack of variance enables the existence of “peaks of eternal light” at the north pole and has made the Moon a prime target for exploration.
The extreme temperatures that the Moon’s surface is exposed to, ranging from 140 °C to −171 °C, are dictated by the intensity of solar irradiance. With no atmosphere to regulate temperatures, areas are either basked in searing heat or plunged into freezing darkness, depending on whether they are in sunlight or shadow.
Parts of many craters, particularly at the poles, are permanently shadowed, resulting in what have been dubbed “craters of eternal darkness.” The Lunar Reconnaissance Orbiter measured temperatures in these areas to be as low as 26 K (-247°C), the coldest temperature in the Solar System ever measured by a spacecraft.
The Moon’s surface is blanketed by a layer of regolith, a highly comminuted and impact-gardened layer formed by impact processes. The finest regolith, or lunar soil, is made up of silicon dioxide glass and resembles snow, with a scent that brings to mind spent gunpowder. The thickness of the regolith layer varies, with older surfaces being generally thicker than younger ones.
Beneath the regolith layer lies the megaregolith, a layer of highly fractured bedrock many kilometers thick. These extreme conditions make it unlikely for spacecraft to harbor bacterial fragments on the Moon for more than one lunar orbit.
The Moon’s topography has been meticulously mapped through laser altimetry and stereo image analysis, revealing its most extensive topographic feature, the giant far-side South Pole–Aitken basin. This basin, with a diameter of 2,240 km (1,390 mi), is the largest confirmed impact crater in the Solar System, and its floor is the lowest point on the surface of the Moon.
Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, are characterized by regionally low elevations and elevated rims. The discovery of fault scarp cliffs suggests that the Moon has shrunk by about 90 meters within the past billion years, a similar phenomenon observed on Mercury. The north pole’s Mare Frigoris, once thought to be geologically dead, has shifted and cracked, evidence of the slow tectonic activity of a world without tectonic plates.
Volcanic features on the Moon
The naked eye from Earth perceives the moon’s dark, nearly featureless lunar plains known as “maria,” meaning “seas” in Latin. These ancient pools of basaltic lava, rich in iron and devoid of water-altered minerals, formed from the majority of lunar basaltic eruptions or flows into impact basins.
Shield volcanoes and volcanic domes are present in several geologic provinces within the maria on the near side of the moon, covering 31% of the near side and just 2% of the far side. The concentration of heat-producing elements beneath the near side’s crust, which heated and partially melted the mantle to rise and erupt, could explain this disparity.
Most of the mare basalts on the moon erupted between 3.3 to 4.2 billion years ago, with evidence of recent lunar volcanism at 70 irregular mare patches, some as young as 50 million years.
A 2006 study at a depression called Ina in Lacus Felicitatis uncovered dust-free features that appear to be only 2 million years old, hinting at the moon’s ongoing activity through moonquakes and gas releases. This raises the possibility of a warmer lunar mantle, especially on the near side, with a deeper, substantially warmer crust rich in radioactive elements.
There is also evidence of 2 to 10 million-year-old basaltic volcanism within the Lowell crater and Orientale basin on the far side. The lighter-colored highlands, or terrae, on the moon, believed to have formed 4.4 billion years ago from plagioclase cumulates of the lunar magma ocean, are distinct from the maria as they are higher and lack major mountain formations caused by tectonic events.
The concentration of maria on the near side may reflect the thicker crust of the far side highlands formed from a slow-velocity impact of a second moon or asymmetrical tidal heating during the moon’s proximity to Earth.
Impact craters on the Moon
The Moon’s surface has been shaped by a series of celestial events, one of the most prominent being impact cratering. With roughly 300,000 craters wider than 1 km on the near side, the lunar landscape is a testament to its violent past. These craters form when asteroids and comets collide with the lunar surface, leaving behind their mark in the form of deep, multi-ring basins that provide a glimpse into the Moon’s geologic timeline.
The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, and is characterized by uplifted material and a broad apron of ejecta deposits. With no atmosphere, weather, or recent geological processes, many of these craters remain well-preserved, offering valuable information about the Moon’s past.
By counting the number of craters per unit area, scientists can estimate the age of the surface. Radiometric ages of impact-melted rocks collected during the Apollo missions indicate a Late Heavy Bombardment period that lasted between 3.8 and 4.1 billion years ago.
Recent high-resolution images from the Lunar Reconnaissance Orbiter have revealed a higher contemporary crater-production rate than previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of the regolith on a timescale of 81,000 years, 100 times faster than the rate computed from models based solely on direct micrometeorite impacts. This ongoing process further highlights the Moon’s dynamic surface and ongoing geological evolution.
The Moon’s surface is dotted with enigmatic lunar swirls – captivating features that are widely recognized for their striking high albedo and sinuous shapes. These swirls are often accentuated by contrasting low albedo regions, winding through the bright swirls, and are often found in areas with elevated magnetic fields. These features are thought to possess an optically immature nature, giving the appearance of a relatively young regolith.
The location of many of these swirls is particularly intriguing, as they are often situated at the antipodal points of major impacts. Their unique placement has led to hypotheses about the cause of these features. It is believed that the swirling patterns are formed by areas that have been partially shielded from the solar wind, leading to slower space weathering and preserving their striking appearance.
Well-known examples of lunar swirls include the Reiner Gamma feature and Mare Ingenii, which continue to captivate scientists and space enthusiasts alike. These swirls present a puzzle, with questions yet to be answered about their formation, evolution, and significance in the broader context of lunar history.
In conclusion, lunar swirls are mysterious and fascinating features, providing a glimpse into the complexities of the Moon’s surface and its interaction with the solar wind.
Is there a presence of water on the Moon?
The existence of water on the lunar surface has been a topic of interest among scientists for several decades. Initially, it was believed that liquid water could not persist on the Moon’s surface as it quickly decomposes upon exposure to solar radiation.
However, scientists proposed the idea of water ice deposits on the Moon, produced by impacting comets or through the reaction of lunar rocks with hydrogen from the solar wind. This water ice could potentially persist in cold, shadowed craters on the Moon’s poles.
Over the years, evidence of water on the lunar surface has been found. In 1994, the Clementine spacecraft discovered small pockets of frozen water, but these findings were later disputed by radar observations. In 1998, the Lunar Prospector spacecraft detected high concentrations of hydrogen near the Moon’s polar regions.
The 2008 Chandrayaan-1 spacecraft confirmed the existence of surface water ice through the observation of absorption lines commonly associated with hydroxyl in reflected sunlight. The LCROSS mission in 2009 also detected at least 100 kg of water in a plume of ejected material from a polar crater.
In 2011, the discovery of 615-1410 ppm of water in melt inclusions in a lunar sample was reported, which is comparable to the concentration of magma in Earth’s upper mantle. The Moon Mineralogy Mapper (M3) also provided definitive evidence for the presence of water-ice on the lunar surface in 2018, with deposits found on both the North and South poles, with more abundance in the South.
In 2020, molecular water was detected on the sunlit surface of the Moon by several spacecraft, including the Stratospheric Observatory for Infrared Astronomy.
The discovery of water on the Moon is of great significance, as it makes lunar habitation a cost-effective plan and eliminates the prohibitive cost of transporting water from Earth. These findings hold significant selenological and scientific importance, and further exploration is needed to understand the full extent of water on the Moon and its implications.
Earth-Moon orbit system
The Earth-Moon system is a celestial duo, bound together by a shared center of mass or barycenter, that always stays stationed 1,700 km beneath the Earth’s surface, causing the Moon to seem as though it orbits the Earth.
This barycenter is positioned at a quarter of Earth’s radius, a testament to the unique relationship between these celestial bodies.
Despite its seemingly circular orbit, the Earth-Moon system is actually slightly elliptical, as evidenced by its orbital eccentricity of 0.055.
The Lunar distance, the semi-major axis of the geocentric lunar orbit, is a staggering 400,000 km, equivalent to 1.28 light-seconds or a quarter of a million miles. This should not be confused with the instantaneous Earth-Moon distance, the momentary measurement from the center of Earth to the center of the Moon.
As the Moon revolves around Earth with respect to the fixed stars, it completes a sidereal period, a full orbit, in about 27.3 days. However, due to the simultaneous movement of the Earth-Moon system in its orbit around the Sun, it takes slightly longer, 29.5 days, to return to the same lunar phase, as seen from Earth, and complete a full cycle, commonly known as a synodic period or synodic month. This synodic month, equal in length to the solar day on the Moon, is better known as the lunar month.
Tidal locking has resulted in the Moon having a 1:1 spin-orbit resonance, where its rotation and orbital periods around Earth are equal. This phenomenon is the reason why only one side of the Moon, the so-called near side, is visible from Earth.
However, despite being in resonance, the Moon’s movements are not without intricacies, such as libration, which causes slight variations in its perspectives, resulting in approximately 59% of its surface being visible from Earth at different times and locations.
Another interesting aspect of the Moon’s orbit is that it is closer to the ecliptic plane, which governs the Earth’s path around the Sun, rather than the planet’s equatorial plane.
The Moon’s orbit is also affected by the Sun and Earth in many small, complex, and interrelated ways, such as the gradual rotation of its orbit plane once every 18.61 years, as described by Cassini’s laws. These follow-on effects add to the subtle intricacies of the Moon’s motion.
The tidal effects of our Moon
The gravitational pull that the Earth, Moon, and Sun exert on each other creates tidal forces that can be observed in the form of ocean tides, changes in the lunar solid crust, and even moonquakes. The Earth and Moon are in synchronous rotation, causing a fixed component in the lunar tides, while the variable component arises from changing distance and libration as a result of the Moon’s orbital eccentricity and inclination. The tides also contribute to maintaining Earth’s magnetic field, as recently discovered by scientists.
Ocean tides are the most widely recognized effect of tidal forces, with two bulges in the Earth’s oceans, one facing the Moon and the other opposite, resulting in two high tides and two low tides every 24 hours. However, the tides are greatly modified by other factors such as frictional coupling, water inertia, shallow ocean basins near land, and sloshing between different ocean basins. The timing of tides at most points on Earth is a product of both observations and theory.
The delays in the tidal peaks of both ocean and solid-body tides produce a torque that slows down Earth’s rotation, transferring angular momentum and rotational kinetic energy from Earth to the Moon. This process, known as tidal acceleration, results in the Moon’s increasing distance from Earth and a slowing of Earth’s rotation, with the Moon’s distance increasing by 38mm per year and Earth’s day lengthening by 17 microseconds every year.
Eventually, after 50 billion years, the tidal drag will cause the rotation of Earth and the orbital period of the Moon to match, resulting in the lighter body of the system being tidally locked, as is already the case with the Moon. However, this will not occur as the Sun will become a red giant and engulf the Earth-Moon system long before this event takes place.
The Lunar solid crust is not immune to the tidal forces exerted upon it, with its tides of around 10 cm (4 in) over 27 days, made up of three distinct components – a fixed one due to Earth’s synchronous rotation, a variable one resulting from the orbital eccentricity and inclination, and a minor varying component from the Sun.
The Earth-induced variable component arises from changes in distance and libration caused by the Moon’s orbital eccentricity and inclination. Were the Moon’s orbit to be perfectly circular and un-inclined, it would only result in solar tides. Scientific investigations suggest that the Moon’s effect on the Earth may even play a role in maintaining the latter’s magnetic field.
The tidal forces generated by the Earth and Moon’s gravitational attraction cause stress and pressure to accumulate within the lunar solid crust, resulting in the phenomenon known as moonquakes. Despite being less frequent and intense compared to earthquakes, moonquakes can endure for a remarkably long duration of up to one hour due to the scattering of seismic vibrations in the dry and fragmented upper lunar crust. The presence of moonquakes was an unexpected finding, revealed by seismometers deployed on the Moon by the Apollo astronauts from 1969 to 1972.
Ocean tides are a result of the gravitational interaction between the Earth, Moon, and Sun. The Moon’s gravitational pull creates a bulge in the Earth’s oceans, causing ocean tides to rise and fall twice a day. The Sun’s gravitational pull also contributes to the tides, accounting for about 40% of the total tidal force. The combined gravitational influence of the Moon and Sun results in the formation of spring and neap tides, which are patterns of ocean tides with varying amplitudes.
The time between successive high tides, called the tidal period, is about 12 hours and 25 minutes due to the Moon’s orbital period around the Earth. Additionally, the gravitational attraction between the Earth, Moon, and Sun creates a complex interplay of forces, causing the tidal patterns to change over time, resulting in spring and neap tides.
The effect of delays in tidal peaks, both in the oceans and in solid-body tides, is a potent force that exerts torque opposing the rotation of the Earth. This process “drains” the planet’s rotational angular momentum and kinetic energy, gradually slowing down the Earth’s rotation. As a result, the Earth’s lost angular momentum is transferred to the Moon through a process known as tidal acceleration. This shift in energy lifts the Moon into a higher orbit while lowering its orbital speed around the Earth.
Measurements taken by laser reflectors left during the Apollo missions revealed that the distance between the Earth and the Moon is increasing by 38 mm (1.5 in) per year. This increase is roughly equivalent to the rate at which human fingernails grow. Additionally, atomic clocks show that Earth’s day lengthens by around 17 microseconds every year, causing the rate at which UTC is adjusted through the addition of leap seconds to slowly increase.
This process of tidal drag ultimately results in the gradual synchronization of the rotation of the Earth and the orbital period of the Moon. This gradual matching causes the lighter body of the orbital system to become tidally locked, as is the case with the Moon. However, it is projected that after 50 billion years, even the Earth will become tidally locked with the Moon, presenting the same face always to its companion. However, long before this occurs, the Sun will have expanded into a red giant, engulfing the Earth-Moon system.
The Moon’s Position and appearance
The Moon’s highest point in the sky during its journey across the celestial sphere, known as culmination, is constantly shifting based on its phase in its orbit and the Earth’s position in relation to the Sun. This can be observed through the changing altitude of the Moon, with the full Moon reaching its peak in the sky during the winter months (for each hemisphere), only to sink to its lowest point during the summer.
At the North and South Poles, the Moon shines brilliantly for 24 hours straight, twice a month, during a period equivalent to the polar day. This prolonged exposure to moonlight is relied upon by Arctic zooplankton as they thrive in an environment where the Sun is hidden below the horizon for extended periods.
The appearance of the Moon, viewed from the northern hemisphere, can appear upside down in comparison to how it is seen in the southern hemisphere. This variation is a result of the Moon’s position in the sky and the location of the observer on Earth. The “horns” of a crescent Moon may appear to point upwards more frequently in the tropics, a phenomenon known as a “wet moon.”
The distance between the Moon and Earth fluctuates between 356,400 km and 406,700 km, causing the Moon’s size to appear to change. On average, the Moon’s angular diameter measures about 0.52°, roughly the same as the Sun, but its apparent size can seem larger when close to the horizon due to the “Moon illusion.”
Despite being locked into its rotation with the Earth, the effect of libration allows for 59% of the Moon’s surface to be visible from Earth over the course of one month.
The Moons rotation
The Moon’s mesmerizing journey around Earth is marked by a stunning display of celestial mechanics, as its synchronous rotation causes it to always keep one face turned towards the planet.
The near side of our Moon, which is constantly facing Earth, is a familiar sight, but the far side remains a mystery to many, despite being illuminated just as frequently. Misnamed as the “dark side,” the far side actually experiences a lunar cycle, alternating between dark and new moon, just like the near side.
But the Moon’s current state of tidally locked rotation wasn’t always so. In the early days of its existence, the Moon spun at a much brisker pace. However, the relentless gravitational pull of Earth gradually slowed its rotation, causing friction and tidal deformations that led to its current synchronous state. Over time, the Moon’s rotational energy dissipated into heat, until it was no longer rotating relative to Earth.
Recent discoveries have shed new light on the Moon’s ancient history. In 2016, planetary scientists pored over data collected during the NASA Lunar Prospector mission, uncovering two hydrogen-rich areas on opposite sides of the Moon, which are believed to be former water ice deposits. These fascinating findings had led to speculation that the Moon may have had polar regions billions of years ago before it became tidally locked to Earth.
The Moon’s illumination and phases
The Moon, a celestial object that never ceases to amaze, is constantly bathed in the sunlight except during rare moments of lunar eclipses. The Sun, with its immense power, illuminates half of the Moon’s surface at all times, casting its radiant glow on various lunar features.
But did you know Earth also contributes to the illumination of the Moon? Yes, it’s true! Earth’s reflection of light onto the lunar surface, known as Earthlight, can be observed at certain times, illuminating areas on the near side of the Moon that are not directly basked in the Sun’s rays.
As the Moon orbits the Earth, its position changes, causing different areas of its surface to be illuminated by the Sun. These variations in lunar illumination, as seen from the perspective of Earth, resulting in the fascinating phases of the Moon we all know and love. A phase is simply defined as the portion of the visible lunar sphere that is illuminated by the Sun. This portion is determined by the equation (1-cos e)/2=sin^2(e/2), where e represents the elongation, or the angle between the Moon, an observer on Earth, and the Sun.
So, the next time you look up at the Moon, remember the intricate dance between the Sun, Earth, and the Moon that results in its captivating appearance.
On November 14th, 2016, the Moon reached a remarkable milestone. It was at its fullest phase and at its closest distance to Earth since 1948. This cosmic event meant that the Moon was 14% closer and larger than its furthest position from Earth, known as apogee.
This close encounter with Earth also coincided with a full moon, making it a spectacular sight in the night sky. The increased apparent size of the Moon, combined with its proximity to Earth, made it a dazzling “supermoon,” appearing 30% brighter than at its farthest distance.
But how does the human eye perceive changes in the brightness of the Moon? The formula 100 × sqrt(actual reduction % / 100) provides the perceived reduction in brightness as a percentage. For instance, when the actual reduction is 1.00 / 1.30, or approximately 0.770, the perceived reduction is calculated to be 0.877, or 1.00 / 1.14. This means that between apogee and perigee moons of the same phase, the perceived increase in brightness can be a maximum of 14%.
So, the next time you catch a glimpse of a “supermoon,” remember the science behind this celestial phenomenon and how it affects our perception of the Moon’s brightness.
The Moons albedo and color
The Moon has a remarkably dull albedo, resulting in a luminance that is barely brighter than the reflectance of old asphalt. This makes it the most radiant object in the sky, aside from the Sun, due to a combination of factors.
The brightness surge during opposition amplifies its radiance, making a full moon ten times brighter than a quarter moon, which would only be half as bright. Furthermore, the human visual system adjusts the relationship between the color of an object and its environment, allowing the brightly lit Moon to appear even more brilliant against the relatively dark sky.
The edges of a full moon shine as brightly as the center, without any signs of shadow, thanks to the reflective properties of lunar soil, which redirects light back toward the Sun more effectively than in other directions.
The hue of the Moon depends on the type of light it reflects, which is shaped by the features and surface of the Moon, including larger, darker regions. Overall, the Moon reflects light with a grayish-brown tint.
“Blood moon” or “Blue moon”
From Earth, the atmosphere filters the reflected light, sometimes causing it to appear red, based on the Moon’s position in the sky and the thickness of the air, or with a blue cast due to the presence of particles in the air, such as those produced by volcanic activity.
However, terms like “blood moon” or “blue moon” do not always indicate red or blue hues of the Moon but rather refer to specific full moons within a cultural context.
There has been an ongoing debate regarding whether the features on the Moon’s surface change over time. Modern scientific consensus holds that many of these claims were mistaken, caused by varying lighting conditions, subpar astronomical observations, or flawed illustrations.
However, instances of outgassing have been documented and may contribute to a small portion of reported lunar variations. More recently, evidence suggests that a region of the Moon’s surface with a diameter of about 3 km (1.9 mi) underwent alteration due to gas release about a million years ago.
Eclipses are celestial phenomena that occur when the Sun, Earth, and Moon align in a straight line, known as “syzygy.” They can be either solar or lunar, depending on which celestial body is obscured by the other.
Solar eclipses take place during the new Moon, when the Moon passes between the Sun and Earth, casting a shadow on the latter. On the other hand, lunar eclipses occur at full Moon, when Earth obstructs the Sun’s light from reaching the Moon.
The Moon and the Sun appear to have the same size from Earth, with both appearing to be about half a degree wide. Despite being much smaller, the Moon appears to be the same size as the Sun because it is much closer to Earth. The non-circular orbits of the Moon and the Sun lead to variations in their apparent sizes, resulting in both total and annular solar eclipses. In a total solar eclipse, the Moon fully covers the Sun, making its corona visible to the naked eye.
However, this celestial dance is not static. The Moon is slowly moving away from Earth, causing its apparent size to decrease over time. At the same time, the Sun is evolving into a red giant, leading to an increase in its size and apparent diameter in the sky. These changes mean that millions of years ago, total solar eclipses were the norm, and annular eclipses were not possible. Conversely, in the future, the Moon will no longer fully cover the Sun during eclipses, leading to the end of total solar eclipses.
Eclipses are not a frequent occurrence as the Moon’s orbit around Earth is tilted by 5.145° with respect to Earth’s orbit around the Sun. This means that for an eclipse to occur, the Moon must be close to the point where these two orbital planes intersect. The frequency and recurrence of solar and lunar eclipses are determined by the saros cycle, which has a period of approximately 18 years.
What is the phenomenon of occultation?
The Moon is constantly blocking a half-degree-wide circular area of the sky, leading to the phenomenon of occultation. This occurs when a bright star or planet is hidden from view as it passes behind the Moon. In this sense, a solar eclipse is an occultation of the Sun. However, occultations of individual stars are not visible everywhere on Earth and not at the same time, as the lunar orbit is in a state of precession, meaning different stars are occulted each year.
History of exploration and human presence on the Moon
The exploration of the Moon has been a significant part of human history, with ancient cultures observing and worshipping the celestial body. In the 20th century, the Cold War space race between the United States and the Soviet Union led to the first successful human landing on the Moon in 1969 by NASA’s Apollo 11 mission. This marked a major milestone in human space exploration and furthered our understanding of the Moon’s composition, geology, and environment.
After the Apollo missions, there have been no human landings on the Moon, although there have been numerous robotic missions to explore its surface and gather data. In recent years, there has been renewed interest in returning to the Moon, with multiple nations and private companies developing plans for lunar exploration and exploitation. The goal of these efforts is to establish a sustainable human presence on the Moon, potentially for scientific research, commercial purposes, and as a stepping stone for further exploration of the solar system.
The history of human presence on the Moon has been brief but impactful, with the Apollo missions leaving behind equipment and waste, as well as planting the seeds for future exploration and development. The Moon remains a subject of fascination and wonder, and its exploration will likely continue to play a significant role in human history in the years to come.
Pre-telescopic observation of the Moon (before 1609)
For millennia, humans have been captivated by the phases of the Moon, with some evidence suggesting that as early as 20-30,000 years ago, tally sticks were used to track its waxing and waning. One of the earliest known depictions of the Moon, a 5000-year-old rock carving, can be found at Knowth, Ireland.
The ancient Greek philosopher Anaxagoras, who lived in the 5th century BC, believed that the Moon was a giant spherical rock that reflected the light of the Sun. Meanwhile, Babylonian astronomers were studying the 18-year Saros cycle of lunar eclipses, and Indian astronomers were describing the Moon’s monthly elongation. The Chinese astronomer Shi Shen, who lived in the 4th century BC, even wrote instructions for predicting solar and lunar eclipses.
Aristotle, who lived in the 4th century BC, saw the Moon as the boundary between the spheres of the mutable elements on Earth and the imperishable stars of the aether. This idea dominated for centuries. The mathematician Archimedes designed a planetarium that could calculate the movements of the Moon and other celestial bodies.
In the 2nd century BC, Seleucus of Seleucia discovered that the tides were caused by the attraction of the Moon and that their height changed based on the Moon’s position relative to the Sun. Around the same time, Aristarchus estimated the size and distance of the Moon from Earth, concluding that it was about 20 times the radius of Earth away.
In the Han Dynasty of China, the Moon was considered to be a form of energy, represented by qi, but the idea that its light was merely a reflection of the Sun was also recognized. The mathematician Jing Fang, who lived in the 1st century BC, even noted the Moon’s spherical shape.
The astronomer Ptolemy, who lived in the 2nd century AD, improved on Aristarchus’s calculations, determining that the Moon was 59 times the radius of Earth away, with a diameter close to 0.292 Earth diameters.
In the 2nd century AD, the author Lucian wrote a novel about a journey to the Moon, where travelers encounter its inhabitants.
In the 5th century, the Indian astronomer Aryabhata stated that the Moon’s shine was caused by reflected sunlight. The physicist Alhazen, who lived in the 11th century, found that sunlight was not reflected by the Moon like a mirror, but that light was emitted from every part of its sunlit surface. The Song dynasty astronomer Shen Kuo created an allegory that compared the waxing and waning of the Moon to a round ball of reflective silver, appearing as a crescent when dusted with white powder and viewed from the side.
By the Middle Ages, before the invention of the telescope, the Moon was widely recognized as a sphere, though many believed it was “perfectly smooth.”
Telescopic exploration of the Moon (1609-1959)
In the year 1609, Galileo Galilei made a groundbreaking discovery using an early telescope, creating drawings of the Moon’s mountainous and crater-filled surface for his book “Sidereus Nuncius.” Although Thomas Harriot had made similar observations months earlier, he never published his findings.
The mapping of the Moon through telescopic technology continued to evolve, with Giovanni Battista Riccioli and Francesco Maria Grimaldi later contributing to the naming system of lunar features still in use today.
From 1834 to 1836, the trigonometrically precise Mappa Selenographica created by Wilhelm Beer and Johann Heinrich Mädler, along with their book “Der Mond,” marked a new era in the study of the Moon. The map revealed the heights of over a thousand mountains and opened the door to lunar geography studies with unprecedented accuracy.
Initially, Galileo’s observations of lunar craters were believed to be the result of volcanic activity. However, this theory was challenged in the 1870s by Richard Proctor, who proposed that craters were formed by collisions.
This hypothesis gained support from geologist Grove Karl Gilbert’s experiments in 1892 and from comparative studies conducted from 1920 to 1940, leading to the development of lunar stratigraphy. By the 1950s, this new field of astrogeology was growing rapidly.
Space Race to the Moon (1959–1990)
Amidst the chaos of World War II, humanity’s pursuit of the final frontier reached new heights. The development of launch systems, initially seen as a mere afterthought in the aftermath of war, transformed into a closely-followed competition between the Soviet Union and the United States. Fueled by the intensity of the Cold War, this Space Race and its later phase, the Moon Race, saw a rapid acceleration of efforts and interest in the exploration of our celestial neighbor.
As the world looked on, the Soviet Union’s Luna program made history with its 1957 spaceflight of Sputnik 1 during International Geophysical Year. In quick succession, the Luna program achieved a series of firsts, from the escape of Luna 1 from Earth’s gravitational pull to the intentional impact of Luna 2 on the Moon’s surface to the reveal of the previously unseen far side of the Moon in the first photographs taken by Luna 3.
As the years went by, the Soviet Union’s mastery of lunar exploration only grew. In 1966, the Luna 9 made a successful soft landing, marking the first time a spacecraft accomplished this feat. The same year, the Luna 10 became the first vehicle to orbit the Moon.
With each accomplishment, the Space Race heated up and sparked a renewed interest in the mysteries of our celestial neighbor. But the true legacy of the Luna program was the impact it had on humanity’s understanding of our place in the universe.
In 1961, President John F. Kennedy issued a bold challenge to the nation: to land a man on the Moon before the end of the decade. With this commitment, the United States set out on a quest to unlock the secrets of the lunar surface and pave the way for human missions.
Under the leadership of NASA, the country launched a series of uncrewed probes through programs such as the Jet Propulsion Laboratory’s Ranger, the Lunar Orbiter, and the Surveyor. Meanwhile, the crewed Apollo program was being developed in parallel with a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit.
Driven by the potential for a Soviet lunar human landing, the Apollo 8 mission marked the first human journey to the lunar orbit in 1968. Finally, in 1969, the historic landing of the first humans on the Moon was a crowning achievement in the Space Race and a defining moment for humanity.
The achievements of the Apollo program represent a shining example of humanity’s boundless potential for exploration and discovery. To this day, the first lunar landing continues to inspire new generations of scientists, engineers, and explorers to reach for the stars.
On July 21, 1969, history was made as astronaut Neil Armstrong became the first person to step foot on the Moon. As the commander of the groundbreaking Apollo 11 mission, Armstrong took that famous “giant leap for mankind.”
An estimated 500 million people around the world tuned in to watch the live transmission of Armstrong’s moonwalk, marking the largest television audience for a live broadcast at the time. The Apollo missions from 11 to 17, excluding the aborted landing of Apollo 13, brought back a staggering 380.05 kilograms of lunar rock and soil in 2,196 separate samples.
This pivotal moment in human history serves as a testament to the power of human determination and innovation. Armstrong’s historic step on the Moon inspires us to continue pushing the boundaries of exploration and discovery, both on Earth and beyond.
Throughout the Apollo missions, scientific instrument packages were placed on the lunar surface, providing valuable data and insights into the Moon’s composition and behavior. Five of the Apollo landing sites, including Apollo 12, 14, 15, 16, and 17, were equipped with long-lived stations that housed instruments like heat flow probes, seismometers, and magnetometers.
Although direct data transmission to Earth ended in 1977, these instruments are still providing valuable information to scientists today, as they continue to be used for lunar laser ranging.
After the Apollo 17 mission in 1972, which marked the last crewed trip to the Moon, the U.S. did not send any dedicated probes to the Moon until the 1990s. Meanwhile, the Soviet Union continued to explore the Moon through its robotic missions, deploying the first remote-controlled rover, Lunokhod 1, in 1970 and collecting rock and soil samples through three sample return missions.
These missions have helped us to better understand the Moon and have provided a foundation for future exploration efforts. By studying the Moon and other celestial bodies, we can gain a greater understanding of the universe and our place in it.
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Moon treaty and explorational absence (1976–1990)
After the Soviet Union’s final mission to the moon in 1976, a profound hush settled over the lunar landscape for the next 14 years. The focus of astronautics shifted to a variety of new frontiers, from delving into the inner Solar System with programs like Venera to venturing into the outer reaches with missions like Pioneer 10 in 1972.
Earth’s orbit became a prime area of focus too, with the development and continuous operation of communication satellites, Earth observation satellites such as the Landsat program (1972), space telescopes, and particularly space stations like the Salyut program (1971).
Despite the until 1979 negotiated Moon Treaty, with its ratification by a mere handful of signatories in 1984, there was little activity surrounding the moon until 1990. This treaty remained, by and large, the sole significant undertaking related to the moon.
Renewed exploration of the Moon (1990-present)
Hiten-Hagoromo, the first mission to the moon dedicated solely to lunar exploration since 1976, was launched by Japan in 1990, becoming the first mission to the moon not sent by the Soviet Union or the United States.
In 1994, the US reignited their interest in lunar exploration and sent a spacecraft, Clementine, back to the moon for the first time since 1973. This mission achieved numerous milestones, including obtaining the first near-global topographic map of the moon and the first global multispectral images of the lunar surface. Four years later, in 1998, the Lunar Prospector mission followed, and its instruments detected an excess of hydrogen at the lunar poles, which was believed to have been caused by the presence of water ice in the upper layers of the regolith within permanently shadowed craters.
In the following years, a new group of states actively began exploring the moon and made several first missions to the celestial body. Between 2004 and 2006, the European Space Agency’s (ESA) first spacecraft, SMART-1, reached the moon, capturing the first detailed survey of chemical elements on the lunar surface. The Chinese Lunar Exploration Program started with Chang’e 1 between 2007 and 2009, resulting in a full image map of the moon. In 2008, India made its first successful trip to the moon with Chandrayaan-1, creating a high-resolution chemical, mineralogical, and photo-geological map of the lunar surface and confirming the existence of water molecules in lunar soil.
In 2009, the US launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor, with LCROSS completing its mission by making a planned impact in the Cabeus crater on October 9th, 2009. LRO continues to operate, providing precise lunar altimetry and high-resolution imagery.
China continued to advance its lunar program with the 2010 launch of Chang’e 2, which mapped the surface at a higher resolution over an eight-month period, followed by the 2013 launch of Chang’e 3, a lunar lander along with a lunar rover named Yutu (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.
In 2014, history was made with the arrival of the privately funded Manfred Memorial Moon Mission at the moon’s surface, marking a pivotal moment in the realm of space exploration. This feat was followed by the groundbreaking landing of China’s Chang’e 4 on the moon’s far side in early 2019, solidifying China’s position as a major player in the space race. India too joined the lunar club in 2019 with the successful launch of its second lunar probe, Chandrayaan-2.
As the decade progressed, China continued to make strides in lunar exploration with the launch of its first robotic sample return mission, Chang’e 5, in 2020. This mission brought back an impressive 1,731 grams of lunar material to Earth, fueling new discoveries and scientific breakthroughs.
With the signing of the U.S.-led Artemis Accords in 2020, a new era of lunar exploration is set to begin. The Artemis program aims to return astronauts to the moon in the coming years and has received support from a growing number of countries. The Artemis Accords have sparked a renewed discussion on international cooperation and the framework for lunar activity, building upon existing treaties such as the Moon Treaty and the ESA-led Moon Village concept.
The U.S. has had plans for returning to the moon since 2004, leading to the development of several programs. The Artemis program, however, has advanced the farthest and includes plans to send the first woman to the moon, as well as establish an international lunar space station called Lunar Gateway.
The lunar exploration continues to reach new heights, with several upcoming missions on the horizon. The Artemis program missions, spearheaded by the United States, are set to build on the achievements of past lunar missions and carry out cutting-edge research on the moon.
Russia, too, is making its mark on the lunar frontier with its first lunar mission, Luna-Glob. This ambitious mission includes an uncrewed lander equipped with seismometers, as well as an orbiter drawing from the lessons learned from Russia’s previous Martian mission, Fobos-Grunt.
As the excitement for lunar exploration continues to grow, China and Russia announced their collaboration to develop and construct an International Lunar Research Station in the 2030s. This groundbreaking partnership between two major space-faring nations promises to bring new discoveries and advancements in our understanding of the moon and beyond.
The Human presence and impact
The last time humans set foot on the moon was during the Apollo Program, a series of manned missions carried out between 1969 and 1972. Since then, the lunar orbit has seen a continuous presence of orbiters, mainly tasked with lunar observation and providing communication support for robotic missions on the lunar surface.
To facilitate further human activities in cislunar space and on the moon’s surface, lunar orbits and orbits around Earth-Moon Lagrange points are being utilized to establish a near-lunar infrastructure. More specialized missions, such as those at the far side of the moon or the lunar north and south polar regions, require spacecraft with unique orbits, such as the Queqiao relay satellite or the upcoming Lunar Gateway, which will be the first extraterrestrial space station.
In summary, the advancements in lunar exploration have paved the way for a sustained human presence in space and on the moon, opening up new avenues for scientific discovery and technological progress.
The Moon, despite being assigned the lowest planetary protection classification, has garnered much attention in recent years due to its degradation as a pure and scientific environment.
The issue of preserving the Moon’s natural state has become increasingly pressing, especially in light of the potential for astronomy to be performed on its surface. It is imperative that any human activity on the Moon be devoid of physical and radio pollution, as even the smallest amount of traffic and impact can cause clouds of dust to spread and potentially alter the Moon’s original state and scientific content.
According to scholar Alice Gorman, the Moon, despite its inhospitable nature, is not devoid of life, and that sustainable human activity on the Moon would require treating its ecology as an equal partner.
The “Tardigrade Affair” of 2019, involving the crash of the Beresheet lander and its carrying of tardigrades, serves as a cautionary tale, highlighting the lack of measures and international regulation for planetary protection.
With the increasing number of missions to the Moon, space debris beyond Earth and around the Moon has been identified as a future challenge, posing a potential danger to such missions. This has led to the call for the establishment of lunar waste management, which future lunar missions, particularly on the surface, will need to address.
The Moon holds not only remnants of human endeavors but also purposeful installations such as the Moon Museum, Apollo 11 goodwill messages, six lunar plaques, the Fallen Astronaut memorial, and other historically significant artifacts. However, even now, some long-term missions are still in progress, such as the Lunar Reconnaissance Orbiter, launched in 2009, which continues to survey the lunar surface for future missions.
The Chang’e 3 lander, launched in 2013, is still operational, equipped with its Lunar Ultraviolet Telescope. Additionally, five retroreflectors have been placed on the Moon since the 1970s and are used to determine the physical librations via laser ranging. There are ambitious plans by various space agencies and private companies to establish a sustainable human presence on the Moon, with the Lunar Gateway project being the most advanced one as part of the Artemis program.
Astronomy from the Moon
For many decades, the Moon has been considered a prime location for telescopes due to its proximity, lack of astronomical seeing issues, and presence of permanently dark and cold craters near the poles that are ideal for infrared telescopes. Radio telescopes placed on the far side of the Moon would also be protected from the radio noise of Earth.
The lunar soil can be combined with carbon nanotubes and epoxies to construct mirrors up to 50 meters in diameter, overcoming the challenges posed by its dusty surface. Even a lunar zenith telescope can be created at a low cost using an ionic liquid.
The Apollo 16 mission, which took place in April 1972, captured astronomical photos and spectra in ultraviolet using the Far Ultraviolet Camera/Spectrograph. Moreover, the Moon serves as a platform for observing Earth, including the iconic Earthrise imagery.
Living on the lunar surface
Human habitation on the Moon has only been possible through brief stays in the Apollo Lunar Module, such as during the Apollo 17 mission. One of the difficulties faced by astronauts during their time on the lunar surface is the accumulation of lunar dust on their suits, which they can taste and smell, and is known as the “Apollo aroma.” This fine dust can pose health risks to astronauts.
In 2019, the Chang’e 4 lander conducted an experiment in which at least one plant seed was able to sprout. The seed was part of the Lunar Micro Ecosystem, which also included other living organisms transported from Earth.
Space law and the Moon
Despite the fact that Soviet Union’s Luna landers left pennants on the Moon and U.S. flags were planted at Apollo landing sites, no country has claimed ownership of any portion of the lunar surface. Similarly, no private ownership claims of parts of the Moon or as a whole are considered credible.
The 1967 Outer Space Treaty designates the Moon and all outer space as the “province of all mankind.” The treaty imposes restrictions on the use of the Moon for peaceful purposes only, and explicitly prohibits military installations and weapons of mass destruction.
A majority of the world’s nations are parties to this treaty. The 1979 Moon Agreement was established to further regulate the exploitation of the Moon’s resources, but by an international regulatory regime that has not yet been determined. As of January 2020, it has been signed and ratified by 18 countries, none of which have human spaceflight capabilities.
However, starting in 2020, a number of countries have joined the U.S. in the Artemis Accords, which challenge the treaty. The U.S. has stated in a presidential executive order, “Encouraging International Support for the Recovery and Use of Space Resources,” that it does not view outer space as a “global commons” and considers the Moon Agreement to be a failed attempt to limit free enterprise.
As commercial and national interests in the Moon grow, with the prospect of territories being prospected, there has been a call for harmonizing the Moon Treaty and the Artemis Accords. This has led to the advocacy of an Implementation Agreement for the Moon Treaty as a way to address its shortcomings and bring it in line with other laws, increasing its acceptability.
In response to this growing interest, U.S. lawmakers have introduced legislation for the conservation of historic landing sites on the Moon. Meanwhile, interest groups have advocated for the designation of these sites as World Heritage Sites, protected zones of scientific value, further solidifying the legal and territorial claims on the Moon.
In 2021, a group of lawyers, space archaeologists, and concerned citizens created the Declaration of the Rights of the Moon, drawing inspiration from the Rights of Nature movement and the idea of granting legal personality to non-human entities in space.
These organizations aim to facilitate cooperation and coordination among various space agencies, governments, and private companies with interests in lunar exploration.
The International Lunar Exploration Working Group (ILEWG) focuses on scientific, technical, and programmatic aspects of lunar exploration, while the Moon Village Association (MVA) promotes a collaborative approach to the sustainable use of lunar resources.
The International Space Exploration Coordination Group (ISECG) works to harmonize the exploration plans and objectives of its member space agencies. By fostering collaboration and coordination among these organizations, these groups aim to ensure that future lunar development is carried out in a sustainable and responsible manner, maximizing benefits for all of humanity.
The Moon in culture and life
For thousands of years, humans have been observing the phases of the Moon – its waxing and waning – and utilizing it to track the passage of time. Archeologists have uncovered evidence of tally sticks, notched bones dating back as far as 20,000-30,000 years, which are believed by some to symbolize the phases of the Moon. Over time, the counting of days between the phases of the Moon gave rise to the creation of generalized time periods for the full lunar cycle as months, and perhaps even the phases of the Moon as weeks.
The etymology of the words used to describe the month in various languages reflect this relationship between the period of the month and the Moon. The English word “month” and its cognates in other Indo-European languages, such as the Latin “mensis” and Ancient Greek “meis” or “mēn,” derive from the Proto-Indo-European (PIE) root of “moon,” *méh1nōt. This root is derived from the PIE verbal root *meh1-, meaning “to measure,” indicating a functional conception of the Moon as a marker of the month. In a different language family, the Chinese language uses the same word (月) for both “moon” and “month,” which can also be found in the symbols for “week” (星期).
This method of lunar timekeeping gave rise to the historically prevalent, but diverse, lunisolar calendars. One example is the 7th-century Islamic calendar, which is a purely lunar calendar, where months are traditionally determined by the sighting of the hilal, or the earliest crescent Moon, over the horizon.
The full Moon has been particularly significant and has been celebrated in various calendars and cultures. Around the autumn equinox, the full Moon is known as the Harvest Moon and is marked with festivities such as the Harvest Moon Festival in the Chinese Lunar Calendar, which is the second most important celebration after the Chinese New Year.
Additionally, the relationship between time and the Moon can also be found in religion. For example, in ancient Egypt, the deity Khonsu was both a temporal and lunar deity. These connections between the Moon, time, and religion serve to highlight the timeless importance and influence of the Moon in human history.
Throughout history, humans have been captivated by the Moon, interpreting its changing phases and employing it for religious and astrological purposes.
The symbol of the crescent 🌙, particularly, has been utilized by cultures across the world to represent the Moon and its phases. For example, the Chinese writing system has transformed the crescent into the symbol 月, representing the Moon. Ancient Egyptians also utilized the crescent, spelling it 𓇹, as a representation of the lunar deity Iah, meaning Moon.
In Mesopotamia, the crescent was employed as the primary symbol of Nanna/Sîn, the ancient Sumerian lunar deity, and was associated with magic and sorcery. This symbol was frequently depicted as part of headgear or crowns, giving the appearance of horns, a feature seen in the ancient Greek Selene and Egyptian Khonsu.
These lunar deities, such as Selene and Luna, were often paralleled with gods and goddesses of other pantheons, forming triple deities and being associated with other celestial bodies. For instance, in Roman mythology, the Moon was associated with Juno and Diana, and was considered a byname of Luna, who was part of a triplet (diva triformis) with Diana and Proserpina, with Hecate being their binding manifestation as trimorphos.
The star and crescent (☪️) arrangement has roots dating back to the Bronze Age, symbolizing either the Sun and Moon or the Moon and planet Venus. Over time, this symbol came to represent the goddesses Artemis or Hecate and was eventually used as a symbol of Byzantium. The use of this symbol became widespread, and it became popular in Islam as the hilal of the Islamic calendar, as well as a symbol for many nations.
In Roman Catholic tradition, the Virgin Mary is depicted with a crescent and adorned with stars, while in Islam, the Prophet Muhammad is associated with the Moon through the miraculous event known as the “splitting of the Moon.”
Cultures have interpreted the contrast between the brighter highlands and darker maria of the Moon to form abstract shapes, such as the Man in the Moon or the Moon Rabbit, as seen in Chinese Tu’er Ye or in Indigenous American mythologies, such as the Mayan Moon goddess.
In Western alchemy, the Moon is associated with silver and the Sun with gold.
The Moon in modern culture representation
The way in which we view the Moon in contemporary society has undergone a profound transformation, as our understanding of this celestial body has been shaped by the advancements of modern astronomy, as well as by the tangible effects of human activity on the Moon’s surface.
From the first human landing on the Moon to the slew of cultural references it has inspired, including romantic musings and science-fiction tales, the Moon has come to occupy a unique place in our collective imagination.
In recent years, the Moon has also been seen as a place of economic opportunity, as missions seek to extract its resources and stake a claim in the new frontier of space.
This has sparked renewed discussions about the ethical and legal implications of humanity’s relationship with the Moon, including the specter of colonialism.
From calls to preserve the Moon’s natural beauty to arguments for its status as a common heritage of humankind, the Moon continues to captivate us and spark important debates about our place in the cosmos.
The idea of the “lunar effect” suggests that there is a connection between specific phases of the 29.5-day lunar cycle and changes in behavior and physiological responses in living beings on Earth, including humans.
This idea has been around for centuries, with the words “lunacy” and “lunatic” tracing their origins back to the Latin word for the Moon, “Luna.”
In the past, philosophers like Aristotle and Pliny the Elder believed that the full Moon could cause insanity in certain individuals, but this idea has been debunked by modern science.
Despite this, some people still believe that during a full moon, there are increases in admissions to psychiatric hospitals, traffic accidents, homicides, and suicides. However, numerous studies have found no evidence to support these claims.
In conclusion, the Moon holds a significant place in our Solar System as the only natural satellite of the Earth. It holds great cultural and scientific importance with its tidal influence, phases, and history of human exploration.
Thank you for taking the time to read this article about our fascinating natural satellite, the Moon. We hope it has provided you with a deeper understanding and appreciation of this important celestial body and its role in our Solar System.