Explain lunar phases
Explain lunar phases using supplied evidence and a solved model.
About 25 minutes, plus any extended practice
Supplied materials
- All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
- Paper or a browser document for a labeled drawing, table or explanation.
- Worked problem inputs: A half-lit visible lunar disk appears without Earth’s shadow touching the Moon. Explain the observation.
- Assessment case 1: A quarter-phase Moon is visible while Earth’s shadow misses it. Which explanation fits? Which inference or prediction is best supported?
- Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Near-side disk appears fully lit.
- Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Half visible disk illuminated.
- Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: Observer sees varying lit portions during orbit.
- Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: Moon appears half illuminated.
- Assessment case 6: A full phase and a quarter phase occur with sunlight reaching half the Moon in both models. What changed? Which inference or prediction is best supported?
Learn the idea
The Moon reflects sunlight. Normally half its surface is illuminated, and Earth observers see changing portions of that lit half. Ordinary phases are not caused by Earth’s shadow.
New phase means near-side illuminated portion is least visible. Full phase means near-side illuminated portion is most visible. Quarter phase means about half the visible disk is lit. Phase cause means changing viewing geometry of sunlit Moon.
For the supplied mechanism problem, A half-lit visible lunar disk appears without Earth’s shadow touching the Moon. Explain the observation. A quarter phase comes from the viewing angle on the Moon’s sunlit half. Ordinary phases do not require Earth’s shadow; that shadow is part of a lunar eclipse.
- Read the supplied problem and mark the structures, processes or quantities named: A half-lit visible lunar disk appears without Earth’s shadow touching the Moon. Explain the observation.
- Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
- Write a causal explanation or calculation, showing how each observation supports the result.
- Compare your reasoning with the worked solution; name a condition that would change or limit the inference.
See the method in action
Worked supplied problem: A half-lit visible lunar disk appears without Earth’s shadow touching the Moon. Explain the observation. Solution: A quarter phase comes from the viewing angle on the Moon’s sunlit half. Ordinary phases do not require Earth’s shadow; that shadow is part of a lunar eclipse. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place.
Check your reasoning
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Helpful tips
- Use the feature “near-side illuminated portion is least visible” to justify new phase; do not sort by the name alone.
- Distinguish full phase (near-side illuminated portion is most visible) from a different process occurring in the same situation.
See the answer and explanation
The viewing angle shows half of the apparent disk illuminated; no Earth shadow is needed.
Ordinary phase is a view of the Moon’s generally half-lit sphere, not shadow coverage by Earth.
Take it further
New model task: place the Sun far right of Earth; put the Moon left of Earth for full phase and above Earth for a quarter phase. Draw the Sun-facing lit halves, with a slightly tilted orbit so Earth’s shadow misses the Moon. Expected: the observer sees all versus about half of the illuminated visible disk; ordinary phases require viewing geometry, not Earth’s shadow.
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More practice: try new cases
Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.
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Check the subject sources
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