Key Takeaways
- Moon phases are caused by the changing angle between the Sun, Earth, and Moon - not Earth's shadow
- The lunar cycle repeats every 29.5 days from New Moon to New Moon
- The best time to observe lunar details is during Quarter phases, not Full Moon
📑 Table of Contents
The Moon is our closest celestial neighbor, and understanding its phases is one of the fundamental skills for any amateur astronomer. Whether you're planning an observing session or simply curious about why the Moon changes shape, this guide has you covered.
Moon phases affect everything from stargazing conditions to ocean tides to traditional farming calendars. Let's explore what causes these phases, how to predict them, and when to observe the Moon for the best views.
What Causes Moon Phases?
The Short Answer: Moon phases are caused by the changing angle between the Sun, Earth, and Moon as the Moon orbits our planet.
The Detailed Explanation:
The Moon doesn't produce its own light - it shines by reflecting sunlight. Just like Earth, half of the Moon is always illuminated by the Sun, and half is always in shadow. What changes is how much of the illuminated half we can see from Earth.
As the Moon orbits Earth (taking about 27.3 days), our viewing angle changes:
- When the Moon is between Earth and the Sun (New Moon), the illuminated side faces away from us, and we see the dark side
- As the Moon moves in its orbit, we begin to see more of the illuminated side (Waxing phases)
- When the Moon is opposite the Sun (Full Moon), we see the entire illuminated half
- As the Moon continues, we see less of the illuminated side again (Waning phases)
Key Point: It's NOT Earth's Shadow
A common misconception is that moon phases are caused by Earth's shadow falling on the Moon. This is incorrect. Earth's shadow only falls on the Moon during a lunar eclipse, which is a rare event. Moon phases happen every month due to the changing viewing angle, not shadows.
The Eight Phases of the Moon
Astronomers divide the lunar cycle into eight distinct phases. Here's what each looks like and when to see it:
1. New Moon 🌑
What you see: Nothing - the Moon is invisible
Why: The illuminated side faces completely away from Earth
When it rises/sets: Rises at sunrise, sets at sunset
Best for: Deep-sky stargazing (no moonlight interference)
Duration: Technically an instant, but the Moon appears very thin for 2-3 days
2. Waxing Crescent 🌒
What you see: A thin sliver of light on the right side (Northern Hemisphere)
Why: We begin to see a small portion of the illuminated half
When it rises/sets: Rises mid-morning, sets after sunset
Best for: Observing "Earthshine" - the dark part of the Moon faintly lit by sunlight reflecting off Earth
Visibility: Look west just after sunset
Duration: Approximately 3-4 days
3. First Quarter 🌓
What you see: Exactly half of the Moon illuminated (right half in Northern Hemisphere)
Why: The Moon is 90 degrees from the Sun in the sky - we see half lit, half dark
When it rises/sets: Rises around noon, sets around midnight
Best for: Telescope observation (dramatic shadows along the terminator line)
Duration: Technically an instant; appears as "half moon" for about a day
Note: Despite showing a "half moon," it's called "Quarter" because it's one-quarter of the way through the lunar cycle
4. Waxing Gibbous 🌔
What you see: More than half but not fully illuminated
Why: We see more than half of the illuminated side as the Moon approaches Full
When it rises/sets: Rises mid-afternoon, sets after midnight
Visibility: Visible in the eastern sky during the day, overhead in evening
Duration: Approximately 3-4 days
Gibbous meaning: The word "gibbous" comes from Latin meaning "hump" or "bulging"
5. Full Moon 🌕
What you see: The entire face of the Moon illuminated
Why: The Moon is opposite the Sun - we see the complete illuminated half
When it rises/sets: Rises at sunset, sets at sunrise
Best for: Naked-eye viewing (very bright), not ideal for telescope detail (too bright, few shadows)
Duration: Technically an instant; appears "full" for about 1-2 days
Impact: Washes out faint stars and deep-sky objects
6. Waning Gibbous 🌖
What you see: More than half illuminated, but decreasing
Why: We see more than half of the illuminated side, but now the lit portion is shrinking
When it rises/sets: Rises mid-evening, sets mid-morning
Visibility: Visible late at night and in the morning sky
Duration: Approximately 3-4 days
Direction: Light appears on the left side (Northern Hemisphere) and decreases from right to left
7. Last Quarter (Third Quarter) 🌗
What you see: Exactly half illuminated (left half in Northern Hemisphere)
Why: The Moon is again 90 degrees from the Sun, but on the opposite side from First Quarter
When it rises/sets: Rises around midnight, sets around noon
Best for: Morning telescope observations with dramatic shadows
Duration: Technically an instant; appears as "half moon" for about a day
Observation tip: Perfect for early morning astronomers
8. Waning Crescent 🌘
What you see: A thin sliver of light on the left side (Northern Hemisphere)
Why: Only a small part of the illuminated side is visible as we approach New Moon
When it rises/sets: Rises after midnight, sets mid-afternoon
Best for: Observing Earthshine again; challenging naked-eye spotting just before sunrise
Visibility: Look east just before sunrise
Duration: Approximately 3-4 days
Memory Aid: Waxing vs. Waning
- Waxing = Growing (getting larger) - from New to Full
- Waning = Shrinking (getting smaller) - from Full to New
Northern Hemisphere trick:
- If the right side is illuminated → Waxing (getting brighter)
- If the left side is illuminated → Waning (getting dimmer)
Or remember: "Light on Right, Getting Bright" / "Light on Left, Light is Left"
Understanding the Lunar Cycle
The Synodic Month: 29.5 Days
The time from one New Moon to the next New Moon is called a synodic month and averages 29.5 days (29 days, 12 hours, 44 minutes).
This is longer than the Moon's orbital period (27.3 days) because Earth is also moving around the Sun. The Moon must travel a bit farther in its orbit to return to the same phase relative to the Sun and Earth.
Phase Duration Breakdown
- New Moon to First Quarter: ~7.4 days
- First Quarter to Full Moon: ~7.4 days
- Full Moon to Last Quarter: ~7.4 days
- Last Quarter to New Moon: ~7.4 days
Each "quarter" of the cycle takes about a week, making it easy to estimate phases.
Predicting Moon Phases
You can predict moon phases in several ways:
-
Astronomy Apps (easiest)
- SkySafari, Star Walk, Stellarium
- Show current phase and upcoming phases
-
Online Calendars
- timeanddate.com/moon/phases
- NASA's moon phase calendar
-
Simple Estimation
- If you know when the last New Moon was, add 7 days for First Quarter, 14 days for Full, 22 days for Last Quarter
Observing the Moon
The Moon is one of the easiest and most rewarding objects to observe. Here's how to get the best views:
Best Equipment for Each Skill Level
Naked Eye 👁️
- Great for learning phases and tracking the cycle
- Observe Earthshine during crescent phases
- Best during: All phases
- Tip: Use a Moon phase app to identify what you're seeing
Binoculars 🔭
- Excellent entry-level lunar observation
- 7x50 or 10x50 recommended
- Shows craters, maria (dark plains), and mountain ranges
- Best during: Crescent to Gibbous phases (not Full - too bright!)
- Tip: Use a tripod or stabilize against a wall
Telescope 🔬
- Reveals incredible detail
- Start with low power (40-60x), work up to high power (100-200x)
- Moon filter recommended for Full Moon (reduces glare)
- Best during: Quarter phases (best shadows and contrast)
- Tip: Focus on the terminator line (day/night boundary) for maximum detail
Best Times to Observe
For Maximum Detail: First and Last Quarter
Counterintuitively, the Full Moon is NOT the best time for detailed lunar observation. Here's why:
-
Full Moon: The Sun shines directly onto the Moon from our perspective, creating few shadows. The surface appears flat and washed out.
-
Quarter Phases: The Sun illuminates the Moon at an angle, creating long, dramatic shadows that reveal craters, mountains, and valleys in stunning detail.
The Terminator Line:
The terminator is the line dividing day and night on the Moon. Features along this line show maximum relief due to extreme shadows. This line moves across the Moon's face as phases change, revealing new details each night.
Lunar Features to Observe
Easy to Find:
- Maria (dark plains) - ancient lava flows forming the "Man in the Moon" pattern
- Tycho Crater - brilliant crater with bright rays extending across the Moon
- Copernicus Crater - spectacular crater with terraced walls
With Binoculars/Telescope:
- Alpine Valley - a prominent cleft in the lunar Alps
- Straight Wall - a 110km long fault escarpment
- Clavius Crater - massive crater with smaller craters inside
Photographing Moon Phases
The Moon is one of the most accessible subjects for amateur astrophotography.
Smartphone Photography
Settings:
- Use your phone's telephoto lens if available
- Tap to focus on the Moon
- Lower exposure (moon is bright)
- Use night mode (if available)
- Tripod strongly recommended
Advanced: Mount your smartphone to binoculars or a telescope using a smartphone adapter (£15-30)
DSLR/Mirrorless Camera
Recommended settings:
- ISO: 100-400 (moon is bright - you don't need high ISO)
- Shutter Speed: 1/125 to 1/500 second
- Aperture: f/8 to f/11 (sweet spot for sharpness)
- Lens: 200mm minimum; 400-600mm ideal
Tips:
- Shoot in RAW format for maximum editing flexibility
- Use a tripod
- Use mirror lock-up and timer/remote to avoid vibration
- Focus manually using Live View (zoom in 10x on the screen)
Through a Telescope
Afocal Method (for smartphones/compact cameras):
- Hold camera/phone up to telescope eyepiece
- No special adapter needed, but adapters make it easier
- Works well for bright objects like the Moon
Prime Focus (for DSLR/mirrorless):
- Remove telescope eyepiece and camera lens
- Attach camera directly to telescope using T-adapter
- Telescope becomes a giant telephoto lens
- Sharper results than afocal method
Common Misconceptions
Misconception 1: "Moon Phases Are Caused by Earth's Shadow"
Reality: Moon phases are caused by the changing angle between the Sun, Earth, and Moon. Earth's shadow only causes lunar eclipses, which are rare events.
Misconception 2: "The Far Side of the Moon Is Always Dark"
Reality: The far side receives just as much sunlight as the near side. We call it the "far side," not the "dark side." We never see it because the Moon is tidally locked - it rotates once per orbit, keeping the same face toward Earth.
Misconception 3: "Full Moon Happens at Midnight"
Reality: A Full Moon can occur at any time of day or night. It's simply the moment when the Moon is exactly opposite the Sun from Earth's perspective. That moment happens according to orbital geometry, not our clocks.
Misconception 4: "The Moon Only Comes Out at Night"
Reality: The Moon is often visible during the day, especially around First and Last Quarter phases. It's most visible during daylight when it's about 90 degrees from the Sun.
Misconception 5: "Full Moons Make People Crazy"
Reality: Extensive scientific studies have found no correlation between Full Moons and human behavior, hospital admissions, crime rates, or any other measurable phenomenon. This is a persistent myth with no scientific support.
Conclusion
Understanding moon phases connects you to one of the most ancient astronomical observations humans have made. The same phases that helped ancient cultures create calendars are visible in your sky tonight.
Key Takeaways:
- Phases are caused by changing viewing angles, not Earth's shadow
- The cycle repeats every 29.5 days from New Moon to New Moon
- Quarter phases show the best detail for telescopic observation
- The Moon is visible during the day for roughly half of each month
- Anyone can observe the Moon - no special equipment required
Now that you understand moon phases, you can:
- Predict when the Moon will rise and set
- Plan stargazing sessions around Moon phases
- Impress friends with your lunar knowledge
- Begin systematic lunar observation and photography
Clear skies, and enjoy exploring our closest cosmic companion!
Moon Phase Observation Log Template
Track your lunar observations with this template:
Date: _______________ Time: _______________ Phase: _______________ (New, Waxing Crescent, First Quarter, etc.) Equipment: _______________ (Naked eye, binoculars, telescope) Seeing Conditions: _______________ (Excellent, Good, Fair, Poor) Features Observed:
Notes/Sketches:
Ready to explore more celestial phenomena? Check out our Winter Stargazing Guide or learn about Observing Mercury.
