Moon Phases
Elementary School
Definition
The changing appearance of the Moon over about a month as different portions of its sunlit half face the Earth. The cycle runs from new moon to full moon and back, caused by the Moon's orbit around the Earth.
Worked examples
New moon \(\)→\(\) waxing crescent \(\)→\(\) first quarter \(\)→\(\) waxing gibbous \(\)→\(\) full moon
The Moon appears to grow from dark to fully lit as more of its sunlit half faces Earth.
Full moon \(\)→\(\) waning gibbous \(\)→\(\) third quarter \(\)→\(\) waning crescent \(\)→\(\) new moon
After full moon, the visible lit portion shrinks back to dark, completing the cycle in about 29.5 days.
Common mistakes
- The Moon phases are caused by Earth's shadow falling on the Moon → Moon phases are caused by the changing angle between the Sun, Moon, and Earth as the Moon orbits Earth's shadow only touches the Moon during a lunar eclipse, not during the regular monthly cycle.
- The Moon only reflects sunlight during a full moon → The Moon always reflects sunlight; we just see different amounts of the lit half from Earth Half the Moon is always sunlit — our view of that lit portion changes as the Moon orbits.
- A full moon happens when the Moon is closest to Earth → A full moon happens when the Moon is opposite the Sun from Earth's perspective Distance and phase are independent — full moons occur at all orbital distances.
Where you'll use it next
You'll apply moon phases when studying tides and their connection to lunar position, predicting eclipses, and understanding orbital mechanics and how perspective affects what we see from Earth.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026