1.4 The Moon and Its Phases
4 min read•august 14, 2024
The Moon, Earth's only natural satellite, plays a crucial role in our planet's tides and night sky. Its rocky composition, cratered surface, and lack of atmosphere make it a unique celestial body. Understanding the Moon's properties helps us grasp Earth's place in the solar system.
The Moon's phases, caused by its orbit around Earth, create a mesmerizing monthly cycle visible from our planet. These phases, from to and back, showcase the interplay between the Sun, Earth, and Moon, highlighting the dynamic nature of our cosmic neighborhood.
Physical Properties of the Moon
Composition and Structure
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- The Moon has a rocky composition similar to Earth's mantle, with a crust, mantle, and partially molten core
- Its average density is 3.34 g/cm^3, making it the second densest moon in the solar system (after Io)
- The Moon lacks an atmosphere, hydrosphere, and magnetic field, which contributes to its unique surface features and environment
Size and Mass
- The Moon is Earth's only natural satellite, with a diameter of about 3,474 km (roughly 1/4 the size of Earth)
- It has a mass of 7.34 x 10^22 kg (approximately 1/81 of Earth's mass), which influences its gravitational interaction with Earth
Surface Features
- The Moon's surface is divided into two distinct regions: the bright, heavily cratered highlands and the darker, smoother maria (ancient solidified lava plains)
- It has a heavily cratered surface with numerous impact basins (), mountains (), and volcanic features ()
- The Moon's is responsible for Earth's tides, with the tidal force being about 1/300,000 that of Earth's surface gravity
Phases of the Moon
Causes of Lunar Phases
- The Moon's phases are caused by its orbit around Earth and the changing relative positions of the Moon, Earth, and Sun
- As the Moon orbits Earth, the amount of its illuminated surface visible from Earth changes, resulting in the apparent phases
- The Moon's orbital period (27.3 days) and its synodic period (29.5 days) are different due to Earth's motion around the Sun, which determines the time between two identical phases (New Moon to New Moon)
Eight Main Lunar Phases
- New Moon: The Moon is positioned between Earth and the Sun, with its unilluminated side facing Earth
- : As the Moon moves in its orbit, a thin crescent of its illuminated surface becomes visible from Earth
- : Half of the Moon's illuminated surface is visible from Earth, appearing as a "half moon"
- : More than half of the Moon's illuminated surface is visible, but not yet a full moon
- Full Moon: Earth is located between the Sun and the Moon, with the Moon's fully illuminated side facing Earth
- : Less than a full moon is visible, but more than half of the illuminated surface can be seen
- (or Last Quarter): Half of the Moon's illuminated surface is visible, appearing as a "half moon" again
- : Only a thin crescent of the Moon's illuminated surface is visible from Earth before the cycle begins anew
Synchronous Rotation of the Moon
Tidal Locking and Its Effects
- The Moon is in , meaning it orbits Earth once and rotates on its axis once in the same period (27.3 days)
- Synchronous rotation is a result of tidal locking, which occurs when a celestial body's orbital period matches its rotational period due to exerted by its parent body (Earth)
- As a result of tidal locking, the same side of the Moon always faces Earth, known as the near side
Near Side and Far Side Differences
- The far side of the Moon, often incorrectly referred to as the "dark side," is permanently hidden from Earth's view and was first photographed by the Soviet Luna 3 spacecraft in 1959
- The near side and far side of the Moon have distinct geological features: the near side is dominated by maria (), while the far side is more heavily cratered and lacks large maria
- Synchronous rotation has implications for lunar exploration and potential human settlements, as the far side is shielded from Earth's radio interference, making it suitable for radio astronomy and other sensitive scientific observations
Lunar Surface Features
Impact Craters and Basins
- are the most common lunar surface features, formed by the collision of asteroids, comets, and meteoroids with the Moon's surface
- Examples of prominent impact include Tycho, Copernicus, and Aristarchus
- Large impact basins, such as the Orientale Basin, were created by massive collisions and are often surrounded by concentric rings and filled with solidified lava (maria)
Volcanic Features
- Lunar maria are vast, dark, basaltic plains formed by ancient volcanic eruptions that filled large impact basins
- Major maria include Mare Tranquillitatis, , and
- Domes are small, rounded, volcanic features on the lunar surface, often found in clusters near the edges of maria and thought to be formed by viscous lava extrusions
- Rilles, such as Vallis Schröteri and Rima Hadley, are long, narrow, and sinuous depressions on the lunar surface, believed to be either ancient lava channels or tectonic faults
Other Notable Features
- Mountain ranges on the Moon, such as the Montes Apenninus and Montes Caucasus, are thought to have formed as a result of the impact events that created the large basins
- Lunar swirls, such as Reiner Gamma, are enigmatic, bright, curvilinear features found across the lunar surface. Their origin is still debated, but they may be related to magnetic anomalies or surface alteration by cometary impacts
Key Terms to Review (31)
Apollo Missions: The Apollo missions were a series of spaceflights conducted by NASA from 1961 to 1972, aimed primarily at landing humans on the Moon and bringing them safely back to Earth. These missions significantly advanced our understanding of lunar geology and the Moon's phases, providing invaluable data about its surface and composition, as well as human capabilities in space exploration.
Aristarchus Plateau: The Aristarchus Plateau is a prominent geological feature on the Moon, known for its high albedo and unique basaltic formations. This area stands out due to its bright, reflective surface, which is primarily composed of volcanic rock. Its distinctive characteristics make it an important site for studying lunar geology and understanding the Moon's volcanic history.
Craters: Craters are large, bowl-shaped depressions on the surface of celestial bodies, formed by the impact of meteoroids, asteroids, or comets. These structures are key features of the Moon's surface, showcasing the history of impacts and geological processes that have shaped the lunar landscape over billions of years. The characteristics and distribution of craters provide insights into the Moon's age and the dynamics of our solar system.
First quarter: The first quarter is one of the lunar phases that occurs about a week after the new moon, during which half of the Moon's visible surface is illuminated from Earth’s perspective. This phase signifies a key point in the lunar cycle, marking the transition from the new moon to the full moon and highlighting the increasing illumination of the lunar surface as it orbits the Earth.
Full Moon: A full moon occurs when the Moon is on the opposite side of the Earth from the Sun, fully illuminating its surface and making it appear as a bright, round disk in the night sky. This phase marks the culmination of the lunar cycle, during which the Moon transitions from new moon to full moon and back again, showcasing various phases that affect natural phenomena like tides and eclipses.
Galileo Galilei: Galileo Galilei was an Italian astronomer, physicist, and mathematician, often referred to as the 'father of modern observational astronomy.' His pioneering work in telescope technology and his observations of celestial bodies, including the Moon, provided crucial evidence that challenged the geocentric model of the universe and supported the heliocentric theory.
Gravitational Pull: Gravitational pull is the attractive force exerted by a celestial body, such as the Earth or the Moon, on another body due to gravity. This force is responsible for various phenomena, including the orbiting of moons around planets and the movement of tides in oceans. The strength of gravitational pull depends on the mass of the bodies involved and the distance between them, leading to significant effects on other celestial mechanics.
Impact Craters: Impact craters are depressions formed on a planetary surface when an object, such as a meteorite or asteroid, collides with it at high speed. These features are key indicators of the geological history of celestial bodies, revealing information about their age and the frequency of impacts over time. The study of impact craters provides insights into both the Moon’s surface evolution and the early history of Earth, as both bodies experienced significant bombardment during their formative years.
Johannes Kepler: Johannes Kepler was a German astronomer and mathematician known for his groundbreaking laws of planetary motion, which describe the orbits of planets around the Sun. His work laid the foundation for modern astronomy, connecting the motions of celestial bodies with their observed positions in the sky. Kepler's insights into the elliptical nature of orbits and his emphasis on mathematical precision transformed our understanding of the solar system, influencing future scientists like Isaac Newton.
Lunar cycle: The lunar cycle refers to the sequence of phases the Moon goes through as it orbits Earth over a period of approximately 29.5 days. This cycle includes several distinct phases, including the New Moon, First Quarter, Full Moon, and Last Quarter, which result from the changing positions of the Moon relative to Earth and the Sun. The lunar cycle plays a crucial role in understanding various astronomical phenomena and influences natural events on Earth.
Lunar highlands: Lunar highlands refer to the mountainous regions on the Moon's surface, characterized by their lighter color and elevated terrain compared to the darker maria (seas). These highlands are composed primarily of anorthosite, a rock rich in calcium and aluminum, and cover about 83% of the Moon's surface. The highlands are also heavily cratered due to their age, making them an essential part of understanding the Moon's geological history and evolution.
Lunar Lander: A lunar lander is a spacecraft designed specifically to transport astronauts or equipment from lunar orbit to the surface of the Moon and back again. This type of vehicle is crucial for missions aimed at exploring the Moon, as it allows for the safe descent to the surface and return to orbit, making it integral to human space exploration efforts focused on our natural satellite.
Lunar mare: Lunar mare refers to the large, dark, basaltic plains on the Moon's surface, formed by ancient volcanic eruptions. These areas are less cratered than the highlands, indicating that they are younger and were created after the more heavily cratered regions of the Moon. The presence of lunar mare is key to understanding the Moon's geological history and its volcanic activity.
Mare Imbrium: Mare Imbrium, also known as the Sea of Rains, is a vast lunar mare located on the Moon's surface, formed by ancient volcanic activity. This expansive area is one of the largest and most prominent basins on the Moon, covering about 1.145 million square kilometers and characterized by its dark basaltic plains. Its significance is tied to its role in lunar geology, the study of impact craters, and the Moon's history.
Mare serenitatis: Mare Serenitatis, or the Sea of Serenity, is one of the large, dark basaltic plains on the Moon formed by ancient volcanic eruptions. This feature is significant in understanding lunar geology and the history of volcanic activity on the Moon, as well as how it relates to the phases of the Moon observed from Earth.
Mare tranquillitatis: Mare Tranquillitatis, or the Sea of Tranquility, is one of the largest and most well-known lunar maria, located on the Moon's surface. This vast, flat area was formed by ancient volcanic activity and is characterized by its smooth basalt plains, making it a significant site for lunar exploration. It gained fame as the landing site of Apollo 11 in 1969, where humans first set foot on the Moon, further cementing its importance in the study of lunar geology and the history of space exploration.
Montes Apenninus: Montes Apenninus is a prominent mountain range located on the Moon, extending about 600 kilometers across the lunar surface. This range is significant not only for its height, with some peaks reaching approximately 2,500 meters, but also for its geological history and the insights it provides into the Moon's formation and evolution. The Montes Apenninus is often associated with the Apennine Mountains on Earth due to its name, which reflects the connection between lunar features and terrestrial geography.
New moon: A new moon occurs when the Moon is positioned between the Earth and the Sun, making it invisible from Earth due to the illuminated side facing away from us. This phase marks the beginning of the lunar cycle, influencing not only our perception of the Moon but also playing a critical role in phenomena like solar eclipses and tidal patterns.
Orientale Basin: The Orientale Basin is a large impact crater located on the far side of the Moon, known for its distinctive circular shape and complex structure. This basin is one of the best-preserved lunar impact features and provides insight into the Moon's geological history, particularly the processes that formed its surface and the effects of large impacts. The study of Orientale Basin helps us understand not only the Moon but also similar features on other celestial bodies.
Partial solar eclipse: A partial solar eclipse occurs when the Moon covers only a portion of the Sun's disk as seen from Earth, allowing some sunlight to reach the observer. This phenomenon happens during a new moon phase when the Moon is positioned between the Earth and the Sun but is not perfectly aligned, leading to varying degrees of sunlight blockage. Partial solar eclipses provide valuable insight into the alignment of celestial bodies and their movements.
Photometry: Photometry is the science of measuring visible light in terms of its brightness to the human eye. It plays a crucial role in understanding celestial bodies and their illumination, particularly when examining how the Moon reflects sunlight during its phases and how light interacts during eclipses and affects tidal patterns.
Regolith: Regolith is a layer of loose, fragmented material covering solid bedrock, which includes soil, dust, and broken rock. This term is important for understanding the surface composition of celestial bodies, as it forms the basis for soil development and provides insight into geological processes. Regolith can influence erosion, weathering, and even the potential for supporting life on other planets or moons.
Spectroscopy: Spectroscopy is the study of how light interacts with matter, specifically focusing on the analysis of the spectrum of light emitted, absorbed, or scattered by materials. This technique is crucial in understanding the composition and physical properties of celestial bodies, including the Moon, by providing insights into their surface materials, chemical makeup, and even temperature variations based on spectral lines.
Synchronous rotation: Synchronous rotation is a phenomenon where an astronomical body rotates on its axis in the same amount of time it takes to orbit another body, resulting in the same side always facing the latter. This is most commonly observed with the Moon, which takes about 27.3 days to complete both its rotation and orbit around the Earth. As a result, we only see one hemisphere of the Moon from Earth, which has significant implications for understanding lunar features and phases.
Third quarter: The third quarter is one of the eight primary phases of the Moon, occurring approximately one week after the full moon. During this phase, half of the Moon's surface is illuminated, making it appear as a half-circle in the sky. This phase signifies a transition where the illuminated portion starts to decrease, leading toward the new moon phase.
Tidal Forces: Tidal forces are the gravitational effects exerted by celestial bodies, primarily the Moon and the Sun, that lead to the rise and fall of sea levels on Earth, known as tides. These forces are a result of the differences in gravitational attraction experienced at different points on Earth due to the positions of these bodies, leading to varying water levels in the oceans. Tidal forces not only influence ocean tides but also affect the behavior of other natural systems on Earth.
Total Lunar Eclipse: A total lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, causing the Earth's shadow to completely cover the Moon. This astronomical event results in the Moon appearing a reddish color, often referred to as a 'blood moon,' due to the scattering of sunlight through the Earth's atmosphere. The visibility and duration of a total lunar eclipse are influenced by various factors such as the Moon's orbit and the Earth's position in relation to the Sun.
Waning crescent: The waning crescent is a phase of the Moon that occurs after the third quarter and before the new moon, characterized by a small sliver of illumination on the left side of the Moon as observed from the Northern Hemisphere. This phase signifies that the Moon's illuminated portion is decreasing, leading up to the new moon, where it will be completely dark. The waning crescent is an important part of the lunar cycle, influencing various natural phenomena and cultural practices.
Waning Gibbous: The waning gibbous is a phase of the Moon that occurs after the full moon and before the last quarter. During this phase, more than half of the Moon's surface is illuminated, but the amount of visible light is decreasing each night as it moves toward the new moon. This phase plays a crucial role in lunar cycles and influences various natural phenomena on Earth.
Waxing Crescent: The waxing crescent is a phase of the Moon that occurs after the new moon and before the first quarter. During this phase, a small, illuminated portion of the Moon becomes visible, shaped like a crescent, as it transitions from complete darkness to being more illuminated. This phase reflects the gradual increase in sunlight hitting the Moon's surface as it orbits Earth.
Waxing gibbous: A waxing gibbous is a phase of the Moon that occurs between the first quarter and full Moon stages, characterized by more than half of the Moon's visible surface being illuminated by sunlight. During this phase, the illuminated portion continues to increase, creating a bulging appearance that is prominent in the night sky. The waxing gibbous is an important phase as it marks the progression towards the full Moon, which is when the entire face of the Moon is visible from Earth.