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Moon Phases Come From Sunlight and Position · Earth in Motion · Moon phases · lunar orbit

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Moon Phases Come From Sunlight and Position

with Nova

Nova stands on a rooftop at dusk pointing a glowing flashlight at a white foam ball held at arm's length, rotating slowly around herself while a crescent Moon rises in the darkening sky behind her.

Explain why the Moon appears to change shape over the course of a month.
Identify the positions of the Sun, Moon, and Earth that produce a new moon, full moon, and quarter moon.
Compare the lit and dark sides of the Moon as seen from Earth at different points in its orbit.
Predict what phase comes after a first quarter moon.
Describe why the Moon itself does not produce its own light.

Key terms

Phase: How much of the Moon we see lit.
Orbit: The path the Moon takes around Earth.
New moon: When the lit side faces away from us.
Full moon: When the whole lit side faces us.

The Moon Borrows Light

The Moon does not make its own light at all. It is like a giant mirror that bounces sunlight back to us. The Sun always lights up one half of the Moon. The Moon is always a round ball and never really changes shape. We only see different amounts of its sunny side as it moves around Earth each month.

Why The Shape Changes

The Moon travels around Earth about every 29 or 30 days. As it moves, we see more or less of its lit side. When the Moon is between us and the Sun, the lit side faces away, so we see a new moon. When Earth is between the Moon and the Sun, the full lit side faces us, so we see a bright full moon.

Worked examples

It is a first quarter moon tonight. What phase comes next?

After first quarter, more of the lit side shows.
The growing phase between half and full is waxing gibbous.

Answer: A waxing gibbous moon comes next, on the way to full moon.

Hey, stargazer! I'm Nova, and tonight we are cracking one of the sky's coolest puzzles — why does the Moon seem to change shape? Here is the big secret: the Moon does NOT change shape at all. It is always a sphere, just like Earth. And the Moon does NOT make its own light. It is more like a giant mirror that bounces sunlight back toward us. As the Moon travels around Earth every 29 or 30 days, we see different amounts of its sunlit side. Picture a flashlight shining in a dark room. The lit side of the Moon always faces the Sun. When the Moon is between Earth and the Sun, the sunlit side faces away from us — we can barely see it. We call that a new moon. When Earth is between the Moon and the Sun, the full sunlit side faces us — that is a full moon! Halfway in between, we see exactly half the lit side — that is a quarter moon. The phases go in order every single month: new moon → waxing crescent → first quarter → waxing gibbous → full moon → waning gibbous → third quarter → waning crescent → back to new moon. 'Waxing' means growing bigger; 'waning' means getting smaller. 'Gibbous' means more than half the lit side is showing — almost, but not quite, a full circle. The Moon's position in its orbit is the ONLY reason the phase changes. If you get stuck on the activity below, think about which side of the Moon is always facing the Sun — that is the lit side we see from Earth!

Activity

Drag each Moon phase label to the correct position on the orbit diagram showing the Sun, Earth, and Moon.

Practice

Explain why the Moon looks like it changes shape.

Tell where the Moon is during a new moon.

Common mistakes to avoid

The Moon makes its own lightThe Moon reflects sunlight and makes no light of its own.

Check your understanding

Why does the Moon appear to change shape throughout the month?

A student says the Moon makes its own light, just like the Sun. What would you tell them?

During which phase is the Moon positioned between Earth and the Sun?

Recap

The Moon is always round and reflects sunlight. As it orbits Earth each month, we see different amounts of its lit side, which makes the phases.

Reflect

Which phase would you most like to see, and why?