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Grade 9 / Middle 3 (age 14-15)

grade 9 Phases of the Moon

The Moon makes no light of its own; we see it because it reflects sunlight, and at any moment exactly half of it is lit. As the Moon circles the Earth over about a month, the share of that lit half turned toward us changes day by day, so it seems to go from new moon to a thin crescent, a half moon, a full moon and back again. One round from new moon to new moon takes about 29.5 days, and seen from the northern hemisphere the Moon fills in from the right and then shrinks from the right. The picture below shows the Moon's place in its orbit seen from above the North Pole, together with how it looks from Earth that day.

Why the Moon changes shape

🌙 Half of the Moon is always lit
①The Moon shines by reflected sunlight, so the half facing the Sun is always bright and the far half is dark
②As the Moon orbits the Earth, the angle between Sun, Earth and Moon changes every day
③That changes how much of the lit half faces us, and this is what we call the phases of the Moon
④In the picture the Moon seen from above is always lit on the left, the side facing the Sun, but the small view at the top right shows it taking a different shape from Earth each day
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Eight shapes in one month

🔄 Growing from the right, shrinking from the right
①New moon: the Moon is on the same side as the Sun and its lit face points away from us, so it cannot be seen
②Waxing crescent: a thin sliver lit on the right, seen low in the west for a while after sunset
③First quarter: the right half is lit, and it stands in the south at sunset
④Waxing gibbous: more than half is lit
⑤Full moon: the whole disc is lit; it rises in the east at sunset and is up all night
⑥Waning gibbous: the dark part grows in from the right
⑦Last quarter: the left half is lit, and it is in the south at sunrise
⑧Waning crescent: a thin sliver lit on the left, seen low in the east before dawn
New moon

How long one round of shapes takes

📅 The synodic month, about 29.5 days
①From one new moon to the next takes about 29.5 days, a period called the synodic month
②Lunar calendars set each month to 29 or 30 days so that it keeps pace with this cycle
③First quarter comes about 7 days after new moon, full moon about 15 days, and last quarter about 22 days
④The Moon rises about 50 minutes later each day, so if you look at the same time each evening it has moved further east

Summary

Why the phases change
Moon orbits Earth → positions of Sun, Earth and Moon shift
The part of the Moon's sunlit half that faces Earth keeps changing
Synodic month
Synodic month ≈ 29.5 days
The time from one new moon to the next
Order of the phases
New → Waxing crescent → First quarter → Full → Last quarter → Waning crescent → New
Seen from the northern hemisphere it grows from the right and shrinks from the right
🎯 Points that often come up
①The Moon gives off no light of its own; it reflects sunlight
②The phases change because the Moon orbits the Earth
③At new moon the line-up is Sun, Moon, Earth, with the Moon in the middle
④At full moon the line-up is Sun, Earth, Moon, with the Earth in the middle
⑤The first quarter is lit on the right and is seen in the south in the early evening; the last quarter is lit on the left and is seen in the south before dawn

Worked examples and unit test

Example 1
Why does the Moon seem to change a little every day, from new moon to crescent, to half moon, to full moon?
1
The Moon is seen by reflected sunlight, so the half facing the Sun is always lit.
2
As the Moon orbits the Earth, the positions of Sun, Earth and Moon shift, and the part of the lit half that faces Earth changes from day to day.
▸ Because the Moon's orbit changes the positions of Sun, Earth and Moon, so the lit part we can see from Earth changes
Move the day slider in the picture above: from overhead the Moon is always lit on its left half, and only the view from Earth changes.
Example 2
About how many days does it take from one new moon to the next (the synodic month)? And about how many days after new moon is the full moon seen?
1
The time from one new moon to the next is the synodic month, about 29.5 days.
2
The full moon comes about halfway through, roughly 15 days after new moon.
▸ The synodic month is about 29.5 days, and the full moon comes about 15 days after new moon
That is also why, in a lunar calendar that starts each month at new moon, the full moon falls around the 15th.
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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