4.5 Phases and Motions of the Moon
4 min read•june 12, 2024
The Moon's and motions captivate us with their cyclical nature. As it orbits Earth, the Moon's illuminated portion changes shape, creating a mesmerizing dance of light and shadow in our night sky.
Understanding helps us grasp the Moon's relationship with Earth and the Sun. From the invisible to the fully illuminated , each phase reveals the intricate interplay of celestial bodies in our cosmic neighborhood.
Lunar Phases and Motions
Cycle of lunar phases
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- Lunar phases represent the different shapes of the illuminated portion of the Moon visible from Earth throughout its orbit
- New Moon occurs when the Moon is positioned between Earth and the Sun, with the unilluminated side facing Earth ()
- phase shows a thin crescent on the right side of the Moon as it moves eastward in its orbit (right side illuminated after sunset)
- phase displays half of the Moon's Earth-facing side illuminated, appearing as a "half moon" (90° angle between Earth, Moon, and Sun)
- phase reveals more than half of the Moon's Earth-facing side illuminated (between First Quarter and Full Moon)
- Full Moon happens when the Moon is on the opposite side of Earth from the Sun, with the fully illuminated side facing Earth ()
- phase shows less than half of the Moon's Earth-facing side illuminated (between Full Moon and )
- Last Quarter phase illuminates the opposite half of the Moon's Earth-facing side compared to the First Quarter (270° angle between Earth, Moon, and Sun)
- phase displays a thin crescent on the left side of the Moon as it moves westward in its orbit (left side illuminated before sunrise)
- Changing relative positions of the Moon, Earth, and Sun cause the lunar phases
- Moon orbits Earth approximately once every 27.3 days ()
- Moon's orbital plane tilted about 5° relative to Earth's orbital plane around the Sun ()
- Amount of the Moon's illuminated surface visible from Earth changes as it orbits, creating the lunar phases (due to the Sun's light reflecting off the Moon)
- The time between successive identical Moon phases is called the
Synchronization of Moon's movements
- Moon's rotation period (time to spin once on its axis) equals its orbital period (time to orbit Earth once)
- or maintains this synchronization
- Gravitational interaction between Earth and the Moon causes tidal locking (tidal forces)
- Moon always shows the same face to Earth as a consequence of
- of the Moon constantly faces Earth (visible from Earth)
- of the Moon perpetually faces away from Earth (not visible from Earth)
- Tidal forces from Earth have slowed down the Moon's rotation over time
- Earth's gravity creates tidal bulges on the Moon, generating a that slows the Moon's rotation
- Moon's rotation period eventually matched its orbital period, resulting in tidal locking (stable configuration)
Moon's position and appearance
- Moon's phase and position in the sky relative to the Sun determine its visibility and appearance
- New Moon not visible from Earth, as the unilluminated side faces Earth; rises and sets with the Sun (daytime Moon)
- Waxing Crescent visible in the western sky after sunset; sets shortly after the Sun (evening Moon)
- First Quarter visible in the southern sky during evening; rises at noon and sets at midnight (half-illuminated Moon)
- Waxing Gibbous visible most of the night; rises in the afternoon and sets in the early morning (more than half-illuminated Moon)
- Full Moon visible all night; rises at sunset and sets at sunrise (fully illuminated Moon)
- Waning Gibbous visible most of the night; rises in the late evening and sets in the late morning (more than half-illuminated Moon)
- Last Quarter visible in the southern sky during morning; rises at midnight and sets at noon (half-illuminated Moon)
- Waning Crescent visible in the eastern sky before sunrise; rises shortly before the Sun (morning Moon)
- Tilt of the Moon's orbital plane relative to Earth's orbital plane around the Sun () causes variations in the Moon's altitude in the sky throughout the year
- Full moon appears higher or lower in the sky during certain times of the year (summer vs. winter full moons)
- Variation in altitude affects the Moon's visibility and the duration of its presence above the horizon (longer visibility during higher altitudes)
Lunar Orbit and Surface Features
- The Moon's elliptical orbit around Earth results in varying distances
- occurs when the Moon is closest to Earth in its orbit
- is the point where the Moon is farthest from Earth
- The lunar surface is characterized by two main types of terrain
- are the dark, flat plains visible on the Moon's surface
- are the lighter, more heavily cratered regions of the Moon
Key Terms to Review (35)
Apogee: Apogee is the point in an orbit around Earth where the orbiting object is farthest from the planet. It is one of two apsides, the other being perigee, which is the closest point.
Apogee: Apogee refers to the point in an object's orbit around Earth when it is farthest from the planet's surface. This term is particularly relevant in the context of the motions of satellites and spacecraft, the phases and motions of the Moon, ocean tides, and eclipses of the Sun and Moon.
Conjunction: A conjunction is an astronomical event where two or more celestial bodies, such as planets, stars, or the Moon, appear to pass close to one another in the sky from the perspective of an observer on Earth. This phenomenon is particularly relevant in the context of the Moon's phases and motions.
Earthshine: Earthshine refers to the faint glow that can be seen on the unilluminated portion of the Moon's surface. This phenomenon occurs when sunlight reflects off the Earth's surface and then bounces back to the Moon, illuminating the otherwise dark side of the lunar disk.
Ecliptic: The ecliptic is the apparent path that the Sun traces out in the sky over the course of a year as observed from Earth. It also represents the plane of Earth's orbit around the Sun.
Ecliptic: The ecliptic is the apparent path that the Sun appears to take across the sky over the course of a year, as viewed from Earth. It is the projection of Earth's orbit around the Sun onto the celestial sphere, and it serves as a reference plane for many important astronomical phenomena.
Far Side: The far side, also known as the dark side, refers to the hemisphere of the Moon that is permanently facing away from the Earth. This side of the Moon is never directly visible from the Earth's surface due to the Moon's synchronous rotation, where the same side of the Moon always faces the Earth.
First Quarter: The first quarter is the phase of the Moon when the Moon's illuminated half is facing the right side from the perspective of an observer on Earth, appearing as a half-moon. This phase occurs approximately one week after a new moon, when the Moon has completed a quarter of its orbit around the Earth.
Full Moon: A full moon is the lunar phase that occurs when the Moon's Earth-facing side is fully illuminated by the Sun's rays. This happens when the Moon is on the opposite side of the Earth from the Sun, resulting in the Moon's disk appearing completely lit from the perspective of an observer on Earth.
Highlands: Highlands are elevated regions on the lunar surface, characterized by rugged terrain and a high density of impact craters. They are typically older than the lunar maria and have a bright appearance due to their composition of anorthosite rocks.
Highlands: Highlands refer to the elevated, mountainous regions on the surface of the Moon. These areas are characterized by their rugged, cratered terrain and are distinct from the relatively flat and smooth lowland regions known as maria.
Last Quarter: The Last Quarter, also known as the Third Quarter, is a specific phase of the Moon's cycle where the Moon's illuminated surface appears as a half-moon, with the right half of the lunar disk visible from Earth. This phase occurs when the Moon is positioned between the Earth and the Sun, with the Moon's unilluminated side facing the Earth.
Libration: Libration is the slight, oscillating motion of the Moon relative to the Earth, which causes different portions of the lunar surface to be visible from the Earth at different times. This phenomenon allows us to see slightly more than 50% of the Moon's surface over the course of the Moon's orbit around the Earth.
Lunar Orbit: The lunar orbit is the path that the Moon takes as it revolves around the Earth. This cyclical motion is a fundamental aspect of the Earth-Moon system and is closely tied to the phases of the Moon and the tides experienced on Earth.
Lunar Phases: Lunar phases refer to the different appearances of the Moon as it orbits the Earth and its position relative to the Sun. These phases are caused by the Moon's revolution around the Earth and the illumination of its surface by the Sun's light.
Maria: Maria are the large, dark, basaltic plains on the surface of the Moon. They are the result of ancient volcanic eruptions that filled in the large impact basins on the lunar surface, creating the distinctive dark regions we see on the Moon.
Near Side: The near side of the Moon refers to the hemisphere of the Moon that constantly faces the Earth, making it the most visible and well-studied portion of the lunar surface. This side of the Moon is the one we observe from Earth, as the Moon's rotation and revolution are synchronized, causing the same face to always be oriented towards our planet.
New Moon: The New Moon is the first phase of the lunar cycle, when the Moon is positioned between the Earth and Sun, and the side of the Moon facing the Earth is not illuminated. This marks the beginning of a new lunar cycle, as the Moon's orbit around the Earth brings it to the point where the Moon's position relative to the Earth and Sun results in the lunar disk being completely dark from our perspective on Earth.
Opposition: Opposition refers to the alignment of two celestial bodies, such as the Earth and a planet, in which they are positioned on opposite sides of the sky relative to the observer. This is a significant event in the context of the Moon's phases and motions, as it marks a specific point in the Moon's orbit around the Earth.
Perigee: Perigee is the point in an orbit of a celestial body where it is closest to Earth. It applies to any object, such as the Moon or a satellite, that is orbiting our planet.
Perigee: Perigee refers to the point in the orbit of a satellite, the Moon, or any other celestial body when it is closest to the Earth. This term is particularly important in the context of understanding the motions of satellites and spacecraft, the phases and motions of the Moon, ocean tides, and eclipses of the Sun and Moon.
Phases: Phases are the different appearances of the Moon as seen from Earth, which change in a predictable cycle. These phases result from the relative positions of the Earth, Moon, and Sun.
Sidereal month: A sidereal month is the time it takes for the Moon to complete one full orbit around Earth relative to the background stars. It lasts approximately 27.3 days.
Sidereal Period: The sidereal period is the time it takes for an object, such as the Moon, to complete one full orbit around a larger body, like the Earth, relative to the fixed stars in the night sky. This term is particularly relevant in the context of understanding the phases and motions of the Moon.
Solar month: A solar month is the period it takes for the Sun to return to the same position in the sky as observed from Earth, typically averaging about 30.44 days. It represents one-twelfth of a tropical year, aligning closely with calendar months.
Synchronous rotation: Synchronous rotation is when an astronomical body's rotational period matches its orbital period around a partner, causing the same side to always face that partner. The Moon exhibits synchronous rotation with Earth, which is why we only see one hemisphere of the Moon from our planet.
Synchronous Rotation: Synchronous rotation, also known as tidal locking, is a phenomenon where the rotational period of a celestial body, such as a moon or planet, is equal to its orbital period around another body. This results in the same side of the rotating body always facing the body it orbits.
Synodic Period: The synodic period refers to the time it takes for an object, such as the Moon, to complete one full cycle of its phases as observed from Earth. It is the period between two consecutive occurrences of the same phase of the object, such as two new moons or two full moons.
Terminator: The terminator is the boundary between the illuminated and unilluminated portions of a planetary body, such as the Moon, as seen from an observer. It marks the division between the day and night sides of the celestial object and is an important concept in understanding the phases and motions of the Moon.
Tidal Locking: Tidal locking is a phenomenon where a celestial body, such as a moon or a planet, rotates at the same rate as it orbits around another body, causing one side of the body to always face the object it orbits. This synchronization of rotation and orbital period is a result of the gravitational forces between the two bodies.
Tidal Torque: Tidal torque refers to the gravitational force exerted by one celestial body on another, causing a rotational effect. This concept is particularly relevant in the context of the Moon's phases and motions, as the gravitational pull of the Earth and Moon interact to influence the Moon's orientation and orbital dynamics.
Waning Crescent: A waning crescent is the phase of the Moon when it appears as a thin, crescent-shaped sliver in the sky, with the illuminated portion decreasing each night. This phase occurs as the Moon moves from the full Moon to the new Moon, with the visible portion of the lunar surface gradually diminishing.
Waning Gibbous: The Waning Gibbous is a lunar phase that occurs when the Moon's illuminated area is decreasing, and more than half of the Moon's disk appears illuminated. This phase is observed between the Full Moon and the Last Quarter Moon.
Waxing Crescent: A waxing crescent is the first phase of the Moon's cycle after the new moon, where the Moon appears as a thin, crescent-shaped sliver of light in the sky. This phase occurs as the Moon's illuminated portion gradually increases, leading up to the first quarter moon.
Waxing Gibbous: A waxing gibbous is a phase of the Moon where more than half of the Moon's visible surface is illuminated, and the illuminated area is increasing. This phase occurs between the first quarter and full Moon.