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Grade 9 / Middle 3 (age 14-15)

Phases of the Moon

Phases of the Moon

The Moon does not shine on its own; it reflects sunlight, and as it orbits Earth its phase (shape) changes. From new moon through crescent, first quarter, full, last quarter, and old crescent, one full cycle takes about 29.5 days (synodic month). Full moon appears around day 15, and the bright side grows from the right and shrinks from the left. On this page, change the day and the phase to check the Moon's look and its place on the orbit.

Why Does the Moon Change Shape?
🌙 Intuition — The Moon Does Not Shine on its Own
①The Moon reflects the Sun's light to look bright
②As it orbits Earth, Sun–Moon–Earth positions change
③So the lit side we see changes
④That is the lunar phase change!
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8 Moon Phases
🔄 A Month of Change
①New Moon: invisible
②Waxing Crescent: thin sliver on the right
③First Quarter: right half lit
④Full Moon: fully lit
⑤Last Quarter: left half lit
⑥Waning Crescent: thin sliver on the left
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Lunar Cycle
📅 The 29.5-day Secret
①New Moon to next New Moon: about 29.5 days
②Called the synodic month
③Basis of the lunar calendar
④First Q: ~day 7, Full: ~day 15, Last Q: ~day 22
Summary
Why the Phases Change
Relative positions of Sun, Moon, Earth
Because the Moon orbits Earth
Synodic Month
Synodic month ≈ 29.5 days
New Moon to next New Moon
Phase Order
New → Cres → 1st Q → Full → Last Q → Old Cres → New
Lights up from the right, fades from the left
🎯 Exam Points
①The Moon reflects sunlight — does not glow on its own
②Phases come from the Moon orbiting Earth
③New Moon: Sun–Moon–Earth in line (Moon between)
④Full Moon: Sun–Earth–Moon in line (Earth between)
⑤First Q lights right half, Last Q lights left half
Examples and Unit Test
Example 1
Why does the shape of the Moon change a little each day, from new moon to crescent to first quarter to full moon?
1
The Moon does not shine on its own; it looks bright by reflecting sunlight.
2
As the Moon orbits the Earth, the relative positions of the Sun, Moon, and Earth change, so the bright side we see changes shape.
▸ Because the Moon's orbit changes the Sun-Moon-Earth positions, so the bright side appears different
A phase change is not the Moon changing shape but the visible bright side changing.
Example 2
About how many days does it take from one new moon to the next (a synodic month), and around which day is the full moon seen in that period?
1
The time from one new moon to the next is the synodic month, about 29.5 days.
2
The full moon is seen around day 15 from the new moon (about halfway through the synodic month).
▸ Synodic month ≈ 29.5 days / the full moon around day 15
A lunar-calendar month is set by this synodic month of about 29.5 days.
Unit Test
Which statement about the phases of the Moon is correct?
①The Moon makes its own light and so looks bright
②The phases of the Moon change because of the Moon's rotation
③The phases change because the Moon's orbit shifts the Sun-Moon-Earth positions
④A synodic month is about 365 days
⑤At full moon the Sun, Moon, and Earth line up in that order
▸ ③ The phases change because the Moon's orbit shifts the Sun-Moon-Earth positions
1
The Moon only reflects sunlight and does not shine on its own (① wrong), and the cause of the phases is the Moon's orbit, not its rotation (② wrong).
2
A synodic month is about 29.5 days (④ wrong), and at full moon the order is Sun-Earth-Moon while new moon is Sun-Moon-Earth (⑤ wrong); the phases change because of the Moon's orbit (③ correct).
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