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Moon phases are the changing shapes of the Moon's sunlit half as seen from Earth. They matter because they help explain the motion of the Earth Moon Sun system and connect directly to tides, calendars, eclipses, and night sky observation. The Moon does not make its own light, so every phase depends on reflected sunlight. As the Moon moves around Earth, we see different fractions of its illuminated side.
The full cycle of phases takes about 29.5 days, called a synodic month. Half of the Moon is always lit by the Sun, but our viewing angle from Earth changes throughout the orbit. When the Moon is between Earth and the Sun, the lit half mostly faces away from us and we see a new moon. When Earth is between the Sun and the Moon, the lit half faces us and we see a full moon.
Key Facts
- Moonlight is reflected sunlight, not light produced by the Moon.
- One complete phase cycle is about 29.5 days.
- Half of the Moon is always illuminated by the Sun.
- New Moon occurs when the Moon is roughly between Earth and the Sun.
- Full Moon occurs when Earth is roughly between the Sun and the Moon.
- The main phase order is New Moon -> Waxing Crescent -> First Quarter -> Waxing Gibbous -> Full Moon -> Waning Gibbous -> Third Quarter -> Waning Crescent.
Vocabulary
- Phase
- A phase is the apparent shape of the Moon's lit portion as seen from Earth.
- Waxing
- Waxing means the visible illuminated part of the Moon is increasing.
- Waning
- Waning means the visible illuminated part of the Moon is decreasing.
- Synodic month
- A synodic month is the time from one new moon to the next, about 29.5 days.
- Quarter moon
- A quarter moon is when we see half of the Moon's disk illuminated because of the viewing angle from Earth.
Common Mistakes to Avoid
- Thinking Earth's shadow causes the regular moon phases, which is wrong because normal phases come from our viewing angle of the Moon's sunlit half, not from a shadow crossing it. Earth's shadow is involved only during a lunar eclipse.
- Assuming the Moon produces its own light, which is wrong because the Moon shines by reflecting sunlight. Without sunlight, the Moon would not be visible.
- Believing a quarter moon means one quarter of the Moon is lit, which is wrong because a quarter moon shows half of the Moon's visible disk illuminated. The word quarter refers to the Moon being one quarter of the way through its orbit phase cycle.
- Mixing up waxing and waning, which is wrong because waxing means the lit portion grows and waning means it shrinks. Track whether the bright part is increasing from night to night.
Practice Questions
- 1 If one full phase cycle takes 29.5 days, about how many days pass from new moon to full moon?
- 2 A student observes the Moon 7.4 days after a new moon. Which major phase should the student expect to see?
- 3 Explain why we can still have a full moon every month even though a lunar eclipse does not happen every month.