Where a month came from
Tonight the Moon is a thin curve. Some days later it is half. Later still it is round. Why?
Step 1 of 4
- New Moon: it is between Earth and Sun. We see its dark side.
- A week later we see half of its lit side.
- Full Moon: we see the whole lit side.
- Then the lit part shrinks back to new.
New words
- Phases — the shapes of the lit Moon that we see.
- Waxing — the lit part grows (Shukla Paksha).
- Waning — the lit part shrinks (Krishna Paksha).
The rule
The Moon sends back sunlight. Its shape does not change. We see more or less of its lit half.
The Moon shines by reflecting sunlight. Half of the Moon is always lit. We see only the part of the lit half that faces us. So the Moon seems to change shape. The cycle takes about 29.5 days.
New Moon is Amavasya. Full Moon is Purnima. A shape less than half is a crescent. More than half is gibbous.
Watch out!
The Earth's shadow does not cause the phases. It causes a lunar eclipse, when the Moon moves into that shadow.
Slide the Moon round the Earth.
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