Describe how the phases of the Moon are caused by the Moon's orbit around the Earth.

Cambridge International IGCSE Physics (0625) — 6.1 Earth and the Solar System · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The Moon is a natural satellite of the Earth with an orbital period of approximately one month. During this time, observers on Earth see the Moon's appearance change from a new Moon to a full Moon and back again.

Model answer (4 marks)

The Moon is visible because it reflects light from the Sun.
As the Moon orbits the Earth, the proportion of the illuminated surface that is visible from Earth changes.
At new Moon the illuminated half of the Moon faces away from Earth, so the Moon appears dark.
At full Moon the illuminated half of the Moon faces towards Earth, so the whole disc appears lit.

Examiner tips

Use the word "reflects" for light from the Sun. Explain the change in visible illuminated area. Mention new and full Moon specifically. Keep sentences short and clear.

Common mistakes

Forgetting to state that the Moon reflects sunlight. Confusing the direction of the illuminated half. Using vague terms like "changes" without specifying new and full Moon.

Mark scheme (4 marks)

The Moon is visible because it reflects light from the Sun
As the Moon orbits the Earth, the proportion of the illuminated surface visible from Earth changes
At new Moon, the illuminated half of the Moon faces away from Earth, so the Moon appears dark
At full Moon, the illuminated half of the Moon faces towards Earth, so the whole disc appears lit

Key terms in this question

orbit