Click image to open full size
The Moon appears to change shape because we see different portions of its sunlit half as it orbits Earth. These changing shapes are called lunar phases, and they repeat in a regular monthly pattern. Understanding Moon phases helps explain calendars, tides, eclipses, and the timing of nighttime observations. The key idea is that the Moon does not make its own light, but reflects sunlight.
Key Facts
- One full cycle of Moon phases takes about 29.5 days, called a synodic month.
- The Moon is always half lit by the Sun, except during a lunar eclipse.
- New Moon occurs when the Moon is between Earth and the Sun, so the lit side faces mostly away from Earth.
- Full Moon occurs when Earth is between the Sun and the Moon, so the lit side faces Earth.
- Waxing means the visible lit portion is increasing, and waning means it is decreasing.
- Approximate time per main phase step = 29.5 days / 8 = 3.7 days.
Vocabulary
- Lunar phase
- A lunar phase is the 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 29.5 day cycle from one New Moon to the next New Moon.
- Terminator
- The terminator is the boundary line between the bright and dark parts of the Moon's face.
Common Mistakes to Avoid
- Thinking Moon phases are caused by Earth's shadow, which is wrong because Earth's shadow causes lunar eclipses, not the normal monthly phases.
- Forgetting that the Moon is always half illuminated by the Sun, which is wrong because the phase depends on viewing angle, not on how much of the Moon receives sunlight.
- Mixing up waxing and waning, which is wrong because waxing means the lit area grows after New Moon and waning means it shrinks after Full Moon.
- Assuming a phase cycle is exactly 28 days, which is wrong because the average New Moon to New Moon cycle is about 29.5 days.
Practice Questions
- 1 The Moon phase cycle is about 29.5 days. About how many days pass from New Moon to Full Moon?
- 2 If the Moon is at First Quarter on day 7.4 of the cycle, about what day will it reach Third Quarter?
- 3 A student says a crescent Moon happens because Earth's shadow covers most of the Moon. Explain why this is incorrect using the positions of the Sun, Earth, and Moon.