The Moon appears to change shape because we see different amounts of its sunlit half during its orbit around Earth. The Moon itself is not changing shape, and Earth’s shadow is usually not involved. Moon phases matter because they connect everyday sky observations to geometry, motion, and light.
They also help explain calendars, tides, and the timing of eclipses.
Key Facts
- The Moon is always half illuminated by the Sun, except during a lunar eclipse.
- Moon phases are caused by the changing angle between the Sun, Earth, and Moon.
- One complete cycle of phases takes about 29.5 days, called a synodic month.
- New Moon occurs when the Moon is between Earth and the Sun, so the lit side faces mostly away from us.
- Full Moon occurs when Earth is between the Sun and Moon, so the lit side faces mostly toward us.
- Waxing means the visible lit fraction is increasing, while waning means it is decreasing.
Vocabulary
- Moon phase
- A Moon phase is the apparent shape of the Moon’s sunlit portion as seen from Earth.
- Waxing
- Waxing means the visible bright part of the Moon is growing larger from night to night.
- Waning
- Waning means the visible bright part of the Moon is shrinking from night to night.
- Synodic month
- A synodic month is the time it takes for the Moon to return to the same phase, about 29.5 days.
- Terminator
- The terminator is the boundary line between the bright and dark parts of the Moon’s visible disk.
Common Mistakes to Avoid
- Thinking Moon phases are caused by Earth’s shadow, which is wrong because Earth’s shadow only causes a lunar eclipse on rare occasions.
- Drawing the Sun’s light from different directions in the same diagram, which is wrong because sunlight reaching Earth and the Moon comes from nearly one direction at this scale.
- Confusing the Moon’s orbit time with its phase cycle, which is wrong because the Moon orbits Earth in about 27.3 days but returns to the same phase in about 29.5 days.
- Assuming a crescent Moon means most of the Moon is dark, which is wrong because half of the Moon is still sunlit but only a small part of that lit half faces Earth.
Practice Questions
- 1 If the Moon’s phase cycle is 29.5 days, about how many days pass from a new Moon to a full Moon?
- 2 A student observes a first quarter Moon. About how many days later should the next new Moon occur if the full cycle is 29.5 days?
- 3 Explain why a full Moon rises around sunset, using the positions of the Sun, Earth, and Moon.