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Why does the Moon change shape?

The Moon is always a round ball. What changes is the part of the Moon's sunlit side that we can see from Earth. Drag the Moon, watch the light move, and learn the phase names.

The Sun lights it The Moon does not glow by itself.
Half is always lit The bright half always faces the Sun.
We see an angle Different positions show us different amounts of light.
Start exploring Try the Moon game

Full Moon

A full Moon happens when the Moon is on the far side of Earth, so we see almost all of its lit half.

Section 1

Move the Moon around Earth

This is the big idea: the Moon goes around Earth, the Sun lights one side, and our view changes.

How to use this picture

  1. Drag the small Moon around its path.
  2. Watch the big Moon on the right.
  3. Notice which side is bright and how much of it you can see.

The Sun is on the left. Earth is in the middle. The Moon goes around Earth in about a month.

About day 15 of 29.5

Sun, Earth, and Moon from above

Drag the Moon around its circle. The sunlight always comes from the left.

What you see from Earth

Full Moon

Waning starts after this

The whole face looks bright because the lit half is facing us.

Bright part we can see: 100%

Section 2

Meet the 8 main Moon phases

Click any phase card to jump there in the explorer.

Section 3

Waxing means growing. Waning means shrinking.

These are the two words that make the whole cycle easier to remember.

Waxing

The bright part is getting bigger night after night.

New Moon → Crescent → First Quarter → Gibbous → Full Moon

Waning

The bright part is getting smaller night after night.

Full Moon → Gibbous → Last Quarter → Crescent → New Moon

Quarter does not mean tiny

A quarter Moon looks like half a circle. The word quarter means the Moon is a quarter of the way around Earth.

Section 4

Moon phases are not Earth's shadow

A common mix-up is thinking phases happen because Earth blocks the Sun. Usually it does not.

On most nights, the Moon is a little above or below Earth's shadow, so the phase comes only from sunlight and our viewing angle.

Earth's shadow matters during a lunar eclipse, not during the ordinary monthly phase cycle.

Section 5

Moon phase game

Look at the Moon. Can you pick its name?

Score: 0

What phase is this?

Pick an answer to check.

Remember this

Three takeaways

The Moon stays round. It is the sunlight and our viewing angle that change.
Half is always lit by the Sun. We just cannot always see all of that lit half.
Waxing grows. Waning shrinks. That one pair of words helps you follow the whole cycle.