IGCSE Physics · Simulation · 6.1.1 Core
Phases of the Moon
Why does the Moon have phases?
The Moon has phases because the half facing the Sun is always lit, and as the Moon orbits the Earth about once a month we see a changing amount of that lit half. At new moon the lit half faces away from us; at full moon it faces us. The phases are not caused by the Earth’s shadow.
Half the Moon is always lit by the Sun. The phase is how much of that lit half is turned towards us. (Seen from above the North Pole, not to scale.)
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What to take away
- The Moon shines by reflecting sunlight, and the half of it facing the Sun is always lit.
- The Moon orbits the Earth about once a month, so we see a changing amount of its lit half: its cycle of phases.
- The Earth turns on its axis about once every 24 hours, which gives day and night.
Common mistakes
- Thinking the phases are the Earth’s shadow falling on the Moon: half the Moon is always lit, and we see different amounts of that half.
- Placing the full Moon between the Earth and the Sun: there it is a new moon, with its lit half facing away from us.
Questions
Why does the Moon have phases?
The Moon has phases because the half facing the Sun is always lit, and as the Moon orbits the Earth about once a month we see a changing amount of that lit half. At new moon the lit half faces away from us; at full moon it faces us. The phases are not caused by the Earth’s shadow.
Are the Moon’s phases caused by the Earth’s shadow?
No. The Earth’s shadow only reaches the Moon in a lunar eclipse. The phases come from where the Moon is in its orbit: half of it is always lit by the Sun, and we see different amounts of that lit half.
Where is the Moon at new moon?
Between the Earth and the Sun. Its lit half faces away from the Earth, so we see almost nothing of it.
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