Explain how the phases of the Moon occur, describing the positions of Earth, Moon, and Sun, and why we always see the same side of the Moon.
Answer
Phases occur because the Moon orbits Earth while the Sun illuminates different portions, and tidal locking keeps the same side facing Earth.
Step-by-step solution
- State that the Moon revolves around Earth once every 27.3 days.
- Explain that the Sun illuminates the near side of the Moon at all times.
- Clarify that as the Moon moves in its orbit, the portion of the illuminated side visible from Earth changes.
- Describe the sequence: new moon (illuminated side away from Earth), waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, new moon again.
- Mention that Earth’s shadow does not affect the phases but causes lunar eclipses.
- Explain tidal locking: the Moon’s rotation period equals its orbital period, so the same hemisphere always faces Earth, allowing us to see only phases of the same side.
Common mistake: Attributing phases to Earth’s rotation rather than Moon’s orbit
Concept tested: Moon phases and tidal locking
Published · How this solution was prepared and checked
← All The Earth in the Solar System questions