Phases of the Moon
Understand why the Moon's appearance changes over 29.5 days as it orbits Earth. Learn the eight lunar phases, how the Sun-Moon-Earth angle creates them, and why Earth's shadow is not the cause.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Phases of the Moon, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Have you ever noticed the Moon looks different on different nights? Sometimes it appears as a thin crescent, sometimes a full circle, and sometimes you can't see it at all. These changes aren't random — they follow a predictable pattern that repeats roughly every month. By understanding how the Moon orbits Earth and where sunlight comes from, you'll discover why the Moon's shape seems to transform over about 29.5 days. This knowledge helps astronomers predict when the Moon will look a certain way, and it explains an important difference between lunar phases and lunar eclipses.
What Are Lunar Phases?
A lunar phase is the shape of the illuminated portion of the Moon that we see from Earth at any given time. As the Moon orbits Earth, the angle between the Sun, Earth, and Moon constantly changes. Since the Sun always illuminates the same half of the Moon (the side facing the Sun), the amount and shape of that illuminated half visible from Earth varies depending on where the Moon is in its orbit.
The Moon takes about 29.5 days to complete one full orbit around Earth, which is called a synodic month or lunar month. During this time, the illuminated shape we observe goes through a complete cycle of changes and returns to the starting point. Understanding phases requires thinking about the geometry of three objects in space: the Sun far away, Earth in the middle, and the Moon orbiting nearby. The key insight is that the Moon doesn't produce its own light — it only reflects sunlight. What we see depends entirely on how much of the Sun-facing hemisphere of the Moon is visible from our location on Earth.
The Moon takes about 29.5 days to complete one full orbit around Earth, which is called a synodic month or lunar month. During this time, the illuminated shape we observe goes through a complete cycle of changes and returns to the starting point. Understanding phases requires thinking about the geometry of three objects in space: the Sun far away, Earth in the middle, and the Moon orbiting nearby. The key insight is that the Moon doesn't produce its own light — it only reflects sunlight. What we see depends entirely on how much of the Sun-facing hemisphere of the Moon is visible from our location on Earth.
The Eight Phases in Order
The lunar cycle is traditionally divided into eight named phases. The sequence begins with the New Moon, when the Moon is positioned between Earth and the Sun. From Earth, the Sun-facing side of the Moon points away from us, so we see a dark disk (the Moon is actually there, but invisible against the bright daytime sky).
As the Moon orbits, a thin crescent of light appears on the western edge — this is the Waxing Crescent phase. The word "waxing" means growing or brightening. About one week into the cycle, the Moon reaches the First Quarter, where exactly half of the visible disk is illuminated. A week later comes the Waxing Gibbous phase ("gibbous" means humped or swollen), with more than half but not all of the disk lit.
Half a month after New Moon, we see the Full Moon, when Earth is positioned between the Sun and Moon, so the entire Sun-facing side points toward us. The cycle then reverses: Waning Gibbous (growing dimmer), Last Quarter (half lit, opposite side from First Quarter), and Waning Crescent (a thin sliver returns), before returning to New Moon. Waning means shrinking or darkening. Remembering the sequence helps you predict what the Moon will look like on future nights.
As the Moon orbits, a thin crescent of light appears on the western edge — this is the Waxing Crescent phase. The word "waxing" means growing or brightening. About one week into the cycle, the Moon reaches the First Quarter, where exactly half of the visible disk is illuminated. A week later comes the Waxing Gibbous phase ("gibbous" means humped or swollen), with more than half but not all of the disk lit.
Half a month after New Moon, we see the Full Moon, when Earth is positioned between the Sun and Moon, so the entire Sun-facing side points toward us. The cycle then reverses: Waning Gibbous (growing dimmer), Last Quarter (half lit, opposite side from First Quarter), and Waning Crescent (a thin sliver returns), before returning to New Moon. Waning means shrinking or darkening. Remembering the sequence helps you predict what the Moon will look like on future nights.
The Sun-Moon-Earth Angle Explains the Phases
The shape of each phase is determined by the angle between the Sun, Earth, and Moon — not by Earth's shadow. A common misconception is that Earth's shadow falling on the Moon creates the phases, but that's actually what happens during a lunar eclipse, which is a rare event. Phases happen every night and are caused by something much simpler: the direction from which sunlight hits the Moon relative to where you stand on Earth.
Imagine the Moon as a ball painted half white (the sunny side) and half black (the dark side). As this ball orbits around you, the amount of white you see changes based on your viewpoint, even though the ball itself never changes. At New Moon, you're looking at the black side. At Full Moon, you're looking at the white side. At First Quarter, you're positioned so that you see half white and half black, because the Sun is off to the side.
You can test this with a simple model: stand in a dark room, have a friend hold a white ball (the Moon) at arm's length, and shine a flashlight (the Sun) from behind or beside you. As your friend moves the ball around you in a circle, notice how the lit portion changes shape from your perspective. No shadow blocks the light — the geometry of the three positions alone creates all eight phases.
Imagine the Moon as a ball painted half white (the sunny side) and half black (the dark side). As this ball orbits around you, the amount of white you see changes based on your viewpoint, even though the ball itself never changes. At New Moon, you're looking at the black side. At Full Moon, you're looking at the white side. At First Quarter, you're positioned so that you see half white and half black, because the Sun is off to the side.
You can test this with a simple model: stand in a dark room, have a friend hold a white ball (the Moon) at arm's length, and shine a flashlight (the Sun) from behind or beside you. As your friend moves the ball around you in a circle, notice how the lit portion changes shape from your perspective. No shadow blocks the light — the geometry of the three positions alone creates all eight phases.
Why Phases Happen Every Night, Not Monthly
A student sometimes wonders: if the Moon takes 29.5 days to orbit Earth, why don't we see the same phase every night for two weeks? The answer is that the Moon's position in its orbit changes noticeably from night to night. The Moon moves about 13 degrees along its orbital path each day, which is roughly the width of your fist at arm's length in the sky. This means the Sun-Moon-Earth angle shifts every single day, so the visible shape changes gradually and continuously.
Over the course of one night, the phase barely changes — the Moon might look almost identical to how it looked the night before. But comparing the Moon from one week to the next, the difference is dramatic. A crescent becomes a half-moon, then a gibbous moon, then full. This smooth, continuous change is what we observe in the real night sky, and it's why you can use the current phase to estimate how many days until or since New or Full Moon. The predictability of the cycle is one reason the lunar month was historically important for timekeeping and agriculture.
Over the course of one night, the phase barely changes — the Moon might look almost identical to how it looked the night before. But comparing the Moon from one week to the next, the difference is dramatic. A crescent becomes a half-moon, then a gibbous moon, then full. This smooth, continuous change is what we observe in the real night sky, and it's why you can use the current phase to estimate how many days until or since New or Full Moon. The predictability of the cycle is one reason the lunar month was historically important for timekeeping and agriculture.
Common Confusion: Phases vs. Eclipses
The phases of the Moon and lunar eclipses both involve changes in how the Moon appears from Earth, but they are caused by completely different geometry. During the phases, we see different amounts of the Sun-lit side of the Moon because of the Sun-Moon-Earth angle — this happens roughly every two weeks. During a lunar eclipse, Earth's shadow falls directly on the Moon, temporarily darkening it. This occurs only when the Sun, Earth, and Moon line up perfectly, which happens rarely (only a few times per year, if at all).
Another key difference: phases are predictable from the Moon's orbital position alone. Eclipses depend on the specific geometry of Earth's shadow and the Moon's orbital plane relative to Earth's orbit around the Sun. You can predict a phase by looking at where the Moon is in the sky relative to the Sun, but predicting an eclipse requires more detailed calculations. Understanding this distinction helps you explain why the Moon looks different every night (phases) versus why it occasionally turns dark red (eclipse), and why one is common while the other is a special event.
Another key difference: phases are predictable from the Moon's orbital position alone. Eclipses depend on the specific geometry of Earth's shadow and the Moon's orbital plane relative to Earth's orbit around the Sun. You can predict a phase by looking at where the Moon is in the sky relative to the Sun, but predicting an eclipse requires more detailed calculations. Understanding this distinction helps you explain why the Moon looks different every night (phases) versus why it occasionally turns dark red (eclipse), and why one is common while the other is a special event.
Key terms
- Lunar phase.
- The shape of the illuminated portion of the Moon visible from Earth at any given time, determined by the angle between the Sun, Earth, and Moon.
- Synodic month.
- The period of time it takes the Moon to complete one full cycle of phases as seen from Earth, approximately 29.5 days.
- Waxing.
- The period during a lunar cycle when the illuminated portion of the Moon visible from Earth is growing larger.
- Waning.
- The period during a lunar cycle when the illuminated portion of the Moon visible from Earth is shrinking smaller.
- New Moon.
- The lunar phase when the Moon is positioned between Earth and the Sun, with the illuminated side facing away from Earth, making it invisible or barely visible.
- Full Moon.
- The lunar phase when Earth is positioned between the Sun and Moon, with the entire illuminated side of the Moon facing Earth.
- Gibbous.
- A lunar phase that is more than half but not fully illuminated, with a humped or swollen appearance.
- Sun-Moon-Earth angle.
- The relative positions of the Sun, Moon, and Earth that determine how much of the Moon's illuminated side is visible from Earth, thus creating the phases.
Worked example
A student observes the Moon in the night sky on a particular evening and sees a half-moon shape with the lit portion on the right side. Three days later, the student observes the Moon again and sees that the lit portion has grown noticeably larger and bulges outward. Identify which phase the student observed on the first night, explain what phase is approaching, and describe how the Sun-Moon-Earth angle is changing.
On the first night, the student sees a half-moon with light on the right side. This is the First Quarter phase, when exactly half of the Moon's disk is illuminated. At this phase, the Sun is off to the side (to the right from our perspective), so the right half of the Moon is lit and the left half is dark.
Three days later, the lit portion has grown larger and bulges outward in a rounded shape. This describes the Waxing Gibbous phase, which follows First Quarter in the lunar cycle. The word "waxing" tells us the illuminated portion is growing, and "gibbous" describes the rounded or humped appearance when more than half but not all of the Moon is lit.
To explain the change in the Sun-Moon-Earth angle: On the first night at First Quarter, the Sun, Earth, and Moon form a right angle (90 degrees), with the Sun off to the side. As the Moon continues its orbit around Earth over the next three days, it moves further along its path, and the angle changes. The Sun-Moon-Earth angle decreases as the Moon approaches the full Moon position. This means the angle closes: the Moon is moving toward a position where Earth will be between the Sun and Moon (which is the Full Moon geometry). As this angle decreases, more of the Moon's illuminated side becomes visible from Earth, which is why the lit portion grows and becomes more rounded. Eventually, in a few more days, the entire face will be lit (Full Moon).
Three days later, the lit portion has grown larger and bulges outward in a rounded shape. This describes the Waxing Gibbous phase, which follows First Quarter in the lunar cycle. The word "waxing" tells us the illuminated portion is growing, and "gibbous" describes the rounded or humped appearance when more than half but not all of the Moon is lit.
To explain the change in the Sun-Moon-Earth angle: On the first night at First Quarter, the Sun, Earth, and Moon form a right angle (90 degrees), with the Sun off to the side. As the Moon continues its orbit around Earth over the next three days, it moves further along its path, and the angle changes. The Sun-Moon-Earth angle decreases as the Moon approaches the full Moon position. This means the angle closes: the Moon is moving toward a position where Earth will be between the Sun and Moon (which is the Full Moon geometry). As this angle decreases, more of the Moon's illuminated side becomes visible from Earth, which is why the lit portion grows and becomes more rounded. Eventually, in a few more days, the entire face will be lit (Full Moon).
Practice questions
Which of the following statements best explains why we see different phases of the Moon throughout the month?
- Earth's shadow blocks different amounts of sunlight from hitting the Moon each night.
- The Sun-Moon-Earth angle changes as the Moon orbits, changing which portion of the illuminated side of the Moon is visible from Earth.
- The Moon rotates on its axis more slowly during certain parts of its orbit.
- The Moon's brightness changes because it moves closer to and farther from the Sun.
Answer: The Sun-Moon-Earth angle changes as the Moon orbits, changing which portion of the illuminated side of the Moon is visible from Earth.
The phases are caused by the changing geometry of the Sun, Earth, and Moon in space, not by Earth's shadow (which only occurs during rare lunar eclipses). As the Moon orbits Earth, its position relative to the Sun constantly changes. This alters the angle and thus how much of the Sun-lit hemisphere is visible from Earth. The Moon's rotation and distance from the Sun do not explain the regular, predictable cycle of phases we observe. Understanding this distinction is key to separating phases (common, geometric) from eclipses (rare, involving actual shadows).
Draw or describe the positions of the Sun, Earth, and Moon during the Full Moon phase. Then explain why the Moon appears completely illuminated at this phase and not at other times.
Answer: At Full Moon, Earth is positioned between the Sun and the Moon. The Sun is on one side of Earth, and the Moon is on the opposite side. The illuminated (Sun-facing) side of the Moon points directly toward Earth, so we see the entire bright hemisphere.
Full Moon occurs at a specific Sun-Moon-Earth geometry where Earth is in the middle, between the light source (Sun) and the Moon. This positioning allows the entire hemisphere of the Moon that faces the Sun to also face Earth simultaneously. At other phases, the Moon is positioned at different angles relative to the Sun and Earth — for instance, at New Moon the Moon is between Earth and the Sun, so the illuminated side faces away from us. At First and Last Quarter, the Moon is off to the side at a 90-degree angle, so only half the illuminated hemisphere is visible. The Full Moon geometry is the only arrangement where Earth's nighttime location allows us to see the complete lit face.
A student claims that the Moon will look the same every night because it takes 29.5 days to orbit Earth. Explain why this reasoning is incorrect.
Answer: Although the Moon does take 29.5 days to complete one full orbit, the phase changes noticeably from night to night because the Moon moves about 13 degrees around its orbit each day. This continuous change in the Moon's position relative to the Sun and Earth means the Sun-Moon-Earth angle is different each night, so the visible illuminated portion changes gradually and continuously.
This is a common misunderstanding. Students sometimes think that if the orbital period is 29.5 days, the phase should repeat daily. However, the phase depends on the Moon's current position in its orbit, not on the time elapsed. Since the Moon moves roughly one-thirteenth of its orbit per day, the Sun-Moon-Earth angle shifts daily, producing a slightly different phase each night. Over weeks, these small daily changes add up to the dramatic transformations we see — from crescent to half-moon to gibbous to full and back. Comparing the Moon from one week to the next makes these changes obvious. This predictability is what allows us to estimate the lunar phase on any given night based on the visible shape.
FAQ
- Why can't I see the New Moon in the night sky?
- At New Moon, the Moon is positioned between Earth and the Sun, so the illuminated side faces away from us. The dark side of the Moon points toward Earth, making it invisible. Additionally, the New Moon rises and sets with the Sun, so it's lost in the glare of daylight. You cannot see the New Moon because there's nothing visible to see — the entire side facing Earth is dark. A few days before or after New Moon, a thin crescent becomes visible in the twilight sky as the Moon moves away from the Sun's glare.
- Does the Moon really disappear, or is it always there?
- The Moon is always there — it doesn't disappear during the New Moon phase. The Moon is a solid object orbiting Earth continuously. What changes is how much of its illuminated side we can see from Earth. At New Moon, the dark side faces us, so we perceive it as invisible even though it's still in orbit. You can prove this during a solar eclipse, when the New Moon passes directly in front of the Sun and blocks sunlight, revealing that the Moon was there all along. Between the phases, the Moon's position and the amount of reflected sunlight we receive is constantly changing, but the Moon itself never stops orbiting.
- Are lunar phases and lunar eclipses the same thing?
- No, they are very different events caused by different geometry. Lunar phases occur about every two weeks as the Moon orbits and the Sun-Moon-Earth angle changes — they are predictable and happen every month. A lunar eclipse occurs when Earth's shadow falls on the Moon, which happens only during Full Moon when the Sun, Earth, and Moon are perfectly aligned, and only when the Moon's orbital plane crosses Earth's orbital plane. Eclipses are rare and special events. You see a phase every night, but you might only see one or two lunar eclipses per year, and some years have none. Phases are a normal result of orbital geometry; eclipses require a precise alignment.
- If the Moon orbits Earth every 29.5 days, why isn't there a Full Moon exactly 14.75 days after New Moon?
- There is usually a Full Moon roughly 14-15 days after New Moon, but the exact timing can vary slightly because the Moon's orbit is elliptical (slightly oval), not perfectly circular. Additionally, the dates of New and Full Moon depend on when the geometric alignment occurs, and that moment can happen at different times of the day in different parts of the world. The synodic month (the time for one complete phase cycle) averages 29.5 days, but individual phases may occur a day earlier or later than expected depending on when in the day the geometric alignment happens. This small variation is why moon phase calendars sometimes show Full Moon on slightly different dates in different time zones.
Learn this with a teacher, not a page
The Crimsora tutor teaches Phases of the Moon live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.