Explain Moon phases using changing viewing geometry
Relate the approximately monthly lunar orbit to the visible fraction of the sunlit Moon.
Core and Extended; Physics 0625, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked.
Before you begin
- Recognise reflected light.
- Distinguish an illuminated surface from a viewing direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes the Moon visible in ordinary optical observations?
What you will learn
- Relate the approximately monthly lunar orbit to the visible fraction of the sunlit Moon.
- Explain new, quarter and full phases without invoking ordinary Earth-shadow covering.
Separate illumination from what Earth sees
The Moon orbits Earth in approximately one month. Sunlight illuminates about half its spherical surface at a time. During the orbit, an observer on Earth sees different fractions of this illuminated half: the changing visible shape is called the phase. The Moon does not produce its own visible light.
Near new Moon, its sunlit half mostly faces away from Earth; near full Moon, its sunlit half mostly faces Earth. Quarter phases show approximately half the visible disc illuminated. These are viewing relationships, not a Moon physically changing its shape.
Do not turn every phase into an eclipse
Ordinary lunar phases do not result from Earth’s shadow falling on the Moon. A lunar eclipse requires a special near-alignment in which the Moon passes into Earth’s shadow. A phase diagram is often drawn flat for clarity, but the real orbital planes are tilted, so eclipses do not occur at every full Moon.
The syllabus uses approximately one month for both the orbital and phase-cycle descriptions. More precisely, the orbit relative to distant stars is about 27.3 days and new Moon to new Moon is about 29.5 days because Earth also moves around the Sun. These are different reference cycles; exact values are supporting context rather than required recall here.
Pause and explain
What is the ordinary cause of changing Moon phases?
Worked example
Why is a quarter Moon only partly bright if sunlight still illuminates half the Moon?
Show the worked solution
- Sunlight lights one hemisphere.
- Earth views that hemisphere from the side at quarter phase.
- Only about half the Earth-facing disc then belongs to the sunlit hemisphere.
Answer Viewing geometry changes the visible illuminated fraction.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the geometry of a full Moon compared with a new Moon.
Give me a hint
Ask which side of the illuminated Moon faces Earth.
Compare my reasoning
- At full phase most of the lit half faces Earth.
- At new phase the lit half mostly faces away.
- The same sunlight mechanism operates at both positions.
Full: lit half toward Earth; new: lit half mostly away.
Look for these in your work
- State both viewing relationships.
- Keep reflected sunlight as the source.
A quarter-phase photograph shows half a bright lunar disc. Does this mean sunlight lights only a quarter of the sphere?
Give me a hint
Distinguish the whole surface from the visible disc.
Compare my reasoning
- Sunlight illuminates approximately half the spherical surface.
- Earth sees a projection of that surface.
- Quarter phase refers to position in the cycle, not that surface fraction.
No; about half the sphere is lit, with half the visible disc bright.
Look for these in your work
- Separate surface illumination from appearance.
- Explain the naming trap.
A learner predicts an eclipse at every full Moon. Explain what their flat drawing omits.
Give me a hint
Look for a condition beyond the ordinary full-phase viewing relationship.
Compare my reasoning
- An eclipse needs near-alignment with Earth’s shadow.
- The real lunar orbit is tilted to Earth’s orbital plane.
- Most full Moons pass above or below the shadow.
Full phase alone does not guarantee the special eclipse alignment.
Look for these in your work
- Identify the extra alignment condition.
- Retain the distinction between phase and eclipse.
Common mistakes
- Phases are not normally Earth’s shadow.
- The Moon remains a sphere.
- A quarter phase does not mean one quarter of the visible disc is bright.
What can you explain now?
What is the ordinary cause of changing Moon phases?
Compare with the explanation
The changing fraction of the sunlit half visible from Earth
The Moon’s orbit changes the Sun–Moon–Earth viewing geometry while sunlight still lights about half its surface.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain new, quarter and full phases without invoking ordinary Earth-shadow covering.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.