Question

Which of the following contributes to the phases of the Moon as seen from Earth?

the Sun lighting different parts of the Moon at different times
varying amounts of the Moon's lit surface being visible from Earth
the Moon orbiting the Earth

Answer 1

All three.

Explanation (brief):
- The Moon orbits the Earth — this changing geometry is the root cause of phases.
- Because of that orbit, the Sun lights different parts of the Moon as seen from Earth (the Sun always lights the hemisphere facing it, but which portion of that lit hemisphere faces Earth changes).
- Consequently, we see varying amounts of the Moon’s lit surface (new, crescent, quarter, gibbous, full, etc.).