Phases of the Moon
Explain the phases of the Moon as the changing angle of sunlight on the lunar surface as seen from 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 approximately 28 days
- Distinguishes between a solar eclipse (Moon between Sun and Earth) and a lunar eclipse (Earth between Sun and Moon)
The lesson
The Moon does not make its own light. You see it because sunlight bounces off it and reaches your eyes. At any moment, half the Moon is lit by the Sun and half is dark, just like Earth has day and night. As the Moon orbits Earth, the angle between the Sun, the Moon, and you keeps changing. That changing angle decides how much of the lit half you can actually see, and that is why the Moon seems to change shape.
Two weeks ago, Amara saw a thin crescent Moon low in the sky. Tonight she looks up and sees a full circle of light. Nothing happened to the Moon itself. Over those two weeks it moved along its orbit, so the sunlit part facing Earth grew bigger each night until she could see the whole lit half: a full moon.
It's tempting to think Earth's shadow causes the phases, covering part of the Moon each night. That's not it. Earth's shadow only matters during an eclipse. The nightly phases come from the angle of sunlight and the Moon's position, nothing else.
Sometimes the Sun, Moon, and Earth line up almost exactly, and that's when eclipses happen. In a solar eclipse, the Moon slides between the Sun and Earth, and its shadow blocks sunlight from a small part of Earth during the day. In a lunar eclipse, Earth slides between the Sun and Moon, and Earth's shadow falls across the Moon, often turning it a dim reddish color.
Moon phases come from the changing angle of sunlight as the Moon orbits Earth, and eclipses happen only when the Sun, Moon, and Earth line up in a row.
Watch it
Where it sits
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.