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Science · Solar system

Moon & phases

Why the moon changes shape.

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The Moon Does Not Change Shape

The Moon is a big round ball of rock in space. It always stays round. We see different amounts of the lit-up side from Earth. That is why it looks like the Moon changes shape.

Sunlight Lights Up the Moon

The Moon does not make its own light. The Sun shines on the Moon. We only see the parts of the Moon that sunlight hits. The Moon travels around Earth, so different parts get lit up.

The Moon's Journey Creates Phases

The Moon takes about one month to go all around Earth. As it moves, we see different amounts of the bright side. These different shapes we see are called phases. The phases repeat every month.

Main Moon Phases

A New Moon happens when the lit side faces away from us. A Full Moon happens when the whole lit side faces us. A Half Moon shows half the lit side. Crescent moons show just a thin slice.

Why We See Phases

Imagine holding a ball near a lamp in a dark room. Walk around the lamp. You see different amounts of the bright side of the ball. The Moon does the same thing around Earth and the Sun.

Worked examples

  1. 1. Why does the Moon look different every week?

    1. The Moon travels around Earth.
    2. Sunlight always hits one half of the Moon.
    3. We see different amounts of the lit half as the Moon moves.
    4. This makes the Moon appear to change shape each week.

  2. 2. Does the Moon make its own light like the Sun?

    1. No, the Moon does not make light.
    2. The Sun shines light onto the Moon.
    3. The Moon reflects sunlight like a mirror.
    4. We see the Moon because sunlight bounces off it to our eyes.

  3. 3. What is happening during a Full Moon?

    1. The Moon is on the opposite side of Earth from the Sun.
    2. Sunlight shines on the side of the Moon facing Earth.
    3. We see the whole bright side of the Moon.
    4. This is called a Full Moon and happens once a month.

Try it yourself

If you see half the Moon lit up tonight, what might you see in two weeks?

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