SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/astro-lunar-phases-geometry

What causes the Moon's phases?

Our view of the Moon's sunlit half changes as it orbits Earth.

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  1. Earth's shadow covers it nightly
  2. We see different parts of its sunlit half ✔
  3. The Moon produces varying light

Why: The Sun always illuminates half of the Moon. As the Moon moves around Earth, we see different fractions of that illuminated hemisphere.

Huh, didn't know that: Earth's shadow causes a lunar eclipse, not the ordinary monthly phase cycle.

The dad joke
The Moon changes phases, but it never loses its sunny half.
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References

Answer: We see different parts of its sunlit half. The Sun always illuminates half of the Moon. As the Moon moves around Earth, we see different fractions of that illuminated hemisphere.

  1. Moon Phases science.nasa.gov/moon/moon-phases
    The Sun always illuminates half of the Moon while the other half remains dark.
    Checked 2026-10-10.

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