Phases of the Moon
Explain Moon phases as changing views of the Moon's sunlit half as it orbits Earth, and describe how solar and lunar eclipses occur
What a learner can do afterwards
- Explains that the phases of the Moon arise from the changing geometry of Sun, Earth, and Moon, not Earth's shadow
- Describes the sequence of Moon phases over a cycle averaging about 29.5 days
- Distinguishes between a solar eclipse (Moon between Sun and Earth) and a lunar eclipse (Earth between Sun and Moon)
1 · Read
The Moon reflects sunlight; it does not make its own light. Except during a lunar eclipse, the Sun lights about half its surface. As the Moon orbits Earth, our view of that sunlit half changes. At new Moon, the sunlit half faces away from us. At full Moon, it faces us. The Moon stays the same shape. Ordinary phases come from this changing view, not Earth's shadow.
Waxing means the sunlit part visible from Earth is increasing; waning means it is decreasing. A crescent has less than half its disk lit; gibbous has more than half. At either quarter phase, half the visible disk is lit. A full phase cycle averages about 29.5 days. Starting at full Moon, last quarter is about 7 days later, new Moon about 15, and first quarter about 22. These are approximate times.
A solar eclipse happens at new Moon when the Moon passes directly between the Sun and Earth. Its shadow reaches part of Earth, where the Sun appears partly or fully covered. Most new Moons are not eclipses because the Moon's orbit is tilted and its shadow misses Earth. Never look directly at the Sun. For this lesson, use videos or models instead of direct Sun viewing.
Solar-eclipse alignment. The cells show body order, not sizes, distances, or a shadow diagram.
A lunar eclipse happens at full Moon when Earth lies directly between the Sun and Moon and Earth's shadow falls on the Moon. Most full Moons miss Earth's shadow because the Moon's orbit is tilted. During a total lunar eclipse, the Moon can look reddish because Earth's atmosphere bends some sunlight into the shadow. Unlike a solar eclipse, a lunar eclipse is safe to watch directly.
Lunar-eclipse alignment. The cells show body order, not sizes, distances, or a shadow diagram.
Phases are changing views of the Moon's sunlit half and repeat about every 29.5 days. Eclipses need close alignment so the Moon's or Earth's shadow reaches the other body.
Lesson, questions, and flashcards written for this dataset. Reference facts were studied in the supplied Astronomy and Moon-motion records.
Readings and flashcards written for this dataset. Linked videos retain their Khan Academy source credit; publisher terms have not been verified.
More to read
Watch the illuminated part
A phase describes how much of the Moon's sunlit surface we see from Earth. The rocky Moon does not change shape. At new Moon, its sunlit half faces away from us. At full Moon, that half faces us. Other phases give partial views of it. Earth's shadow does not make this ordinary sequence. A lunar eclipse is a different event: Earth's shadow reaches the Moon and changes the light falling on it.
Keep a dated Moon record
Compare drawings of the Moon made on several dates. If the sunlit portion is increasing, the Moon is waxing. If it is decreasing, the Moon is waning. A crescent has less than half the visible disk lit; a gibbous Moon has more than half. Both quarter phases show a half-lit disk. Do not rely on a universal rule about which side looks bright. The apparent orientation depends on your location and viewing direction. Dated observations help you follow the change through a cycle averaging about 29.5 days.
An orbit and a phase cycle differ
Relative to distant stars, the Moon completes an orbit of Earth in about 27.3 days. One rotation takes the same time, so nearly the same side faces Earth. A phase cycle averages about 29.5 days. During the orbit, Earth also moves around the Sun. The Moon must travel farther to return to the same phase relative to sunlight. The two numbers measure different cycles. Neither means that the Moon changes its physical shape.
Surface features stay between phases
Dark plains and bright highlands are parts of the Moon's surface. Phases change which sunlit parts we can see, not the surface itself. Many craters formed when objects struck the Moon. Without Earth-like wind and rain, craters can remain for a long time. The Moon is dry compared with Earth, but water ice exists in some permanently shadowed craters near its poles. Those lasting shadows are different from the changing view that produces monthly phases.
Model the view from Earth
With an adult, use a cool lamp and a pale ball. Your eyes represent the view from Earth, the lamp represents the Sun, and the ball represents the Moon. Hold the ball at arm's length, slightly above your head. Move it around you and compare how much of its bright side you can see. Keep it outside your head's shadow to model ordinary phases. Letting your head block the lamplight models lunar shadowing instead. This model does not show real sizes, distances, or cycle timing. Use the lamp, never the Sun, and do not touch the bulb.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
Learn first
Optional connections
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.