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Lunar Phase
Sequence

Understand why the Moon changes shape in our sky and learn to predict the eight phases in their correct order using the Sun-Earth-Moon model.

Earth & Space Science

What Are Lunar Phases?

A lunar phase is the shape of the sunlit portion of the Moon as seen from Earth. The Moon does not produce its own light: it reflects sunlight. As the Moon orbits Earth, the angle between the Sun, Earth, and Moon changes, so we see different amounts of the lit side.

The Sun-Earth-Moon System

Sun Earth Full Moon New Moon First Quarter Third Quarter You

Why Do Phases Matter?

★ Tides
New and Full Moons produce spring tides (extra-high and extra-low), while Quarter Moons produce neap tides.
★ Eclipses
Solar eclipses can only occur at New Moon; lunar eclipses only at Full Moon.
★ Navigation
For thousands of years, cultures used Moon phases for calendars, agriculture, and timekeeping.

The Eight Phases in Order

New Moon Waxing Crescent First Quarter Waxing Gibbous Full Moon Waning Gibbous Third Quarter Waning Crescent 1-4 5-8

Waxing vs Waning: The Core Rule

The entire 29.5-day lunar cycle divides into two halves:

Waxing Half
New Moon → Full Moon. The lit area increases each night. The bright side is on the right (Northern Hemisphere).
Waning Half
Full Moon → New Moon. The lit area decreases each night. The bright side is on the left (Northern Hemisphere).

Phase vs Eclipse vs Tide

Phase Eclipse Tide Pattern
What causes it? Moon's orbital position Alignment of Sun, Earth, Moon Gravitational pull
How often? Continuous cycle (29.5 days) 2-4 times per year Daily, varies with phase
Visible to whom? Everyone on Earth Narrow path or hemisphere Coastal areas
Key Moon position All positions New (solar) or Full (lunar) New & Full = spring tides

Crescent vs Gibbous

Crescent
< \( 50\% \) lit
 Gibbous
> \( 50\% \) lit
 vs

How to Identify a Phase

1
Check illumination: Is the Moon more than half lit, exactly half lit, less than half, or not visible?
2
Determine waxing or waning: Is the lit side on the right (waxing) or left (waning) in the Northern Hemisphere?
3
Combine the two answers: e.g., "more than half lit + right side bright" = Waxing Gibbous.
4
Special cases: Fully lit = Full Moon. Fully dark = New Moon. Exactly half = First or Third Quarter.

Example 1: Name That Phase

Which phase is this? Bright side

Example 2: Predicting the Next Phase

Now: Waxing Gibbous
\( \approx 3.7 \) days
 Next: Full Moon
\( \approx 3.7 \) days
 Then: Waning Gibbous

Example 3: Time Between Phases

Problem: A Full Moon was observed on March 7. Approximately when will the next New Moon occur?

1
Full Moon to New Moon spans exactly half the cycle: four phases.
2
Calculate: \( 4 \times 3.7 \approx 14.8 \) days, or about 15 days.
3
Answer: March 7 + 15 days = approximately March 22.

The Orbital Angle Model

Earth Sunlight
\( 0^\circ \)
 New
\( 90^\circ \)
 1st Qtr
\( 180^\circ \)
 Full
\( 270^\circ \)
 3rd Qtr
\( 90^\circ \)

Illumination at Each Phase

Phase Illumination Elongation Rises At
New Moon\( 0\% \)\( 0^\circ \)Sunrise
Waxing Crescent\( 1-49\% \)\( 1^\circ-89^\circ \)Mid-morning
First Quarter\( 50\% \)\( 90^\circ \)Noon
Waxing Gibbous\( 51-99\% \)\( 91^\circ-179^\circ \)Afternoon
Full Moon\( 100\% \)\( 180^\circ \)Sunset
Waning Gibbous\( 99-51\% \)\( 181^\circ-269^\circ \)Evening
Third Quarter\( 50\% \)\( 270^\circ \)Midnight
Waning Crescent\( 49-1\% \)\( 271^\circ-359^\circ \)Pre-dawn

Common Mistakes

✕ Earth's Shadow
Phases are not caused by Earth's shadow on the Moon. That only happens during a lunar eclipse. Phases are caused by the viewing angle.
✕ Mixing Up Quarters
First Quarter is lit on the right; Third Quarter is lit on the left (Northern Hemisphere). Swapping them is the most common test error.
✕ Wrong Cycle Length
The synodic period is 29.5 days, not exactly 28 or 30. The sidereal month (27.3 days) measures orbit time, not phase cycle.

Question 1 of 5

What causes the changing appearance of the Moon's phases?

Question 2 of 5

If you see the Moon with the right half illuminated and exactly \( 50\% \) lit, what phase is it?

Question 3 of 5

Approximately how many days does it take to go from a New Moon to a Full Moon?

Question 4 of 5

Which phase comes immediately after the Full Moon in the lunar cycle?

Question 5 of 5

A student says: "We see a crescent Moon because Earth's shadow covers most of the Moon." What is wrong with this explanation?