How Astronomers Know — Evidence Lab
Moon Phases from Sun–Earth–Moon Geometry
- Estimated time
- ~20 min
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Learning goals
- The Moon's phase is caused by which part of its sunlit side we can see from Earth. Geometry, not the Moon's own light, drives the pattern.
- MS-ESS1-1 — Students match Earth–Sun–Moon geometry to observed lunar phases and eclipse conditions.
Help & glossary
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Question 1 of 3
Match each Moon phase (left) with the correct position of the Moon relative to Earth and Sun (right).
Imagine looking down on the Earth-Moon system from above. The Sun's light comes from the same direction for all four positions.
- New Moon
- First Quarter
- Full Moon
- Last Quarter
Sources
- Moon Phases — NASA Science — NASA · NASA Science · Accessed 2026-09-01