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SESK: Stuff Everyone Should Know · sesk.info/aero-moon-phases-cause

What causes the phases of the Moon?

We see different amounts of its sunlit half as it orbits Earth.

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  1. Earth's shadow covering part of the Moon
  2. Clouds blocking part of the Moon
  3. Seeing more or less of its sunlit half ✔

Why: The Sun always lights half of the Moon. As the Moon travels around Earth, we see a changing slice of that lit half. Earth's shadow only causes eclipses, not phases.

Huh, didn't know that: The full cycle of phases repeats about every 29.5 days.

The dad joke
Why did the Moon skip dessert? It was already full.
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References

Answer: Seeing more or less of its sunlit half. The Sun always lights half of the Moon. As the Moon travels around Earth, we see a changing slice of that lit half. Earth's shadow only causes eclipses, not phases.

  1. NASA Science: Moon Phases science.nasa.gov/moon/moon-phases
    how much we are able to see of that illuminated half changes as the Moon travels through its orbit.
    Checked 2026-10-10.

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