Why does the Moon change shape?

The Moon does not shine by itself

On some nights the Moon is a bright, round circle. On others it is a thin silver curve, and some nights you cannot see it at all. But the Moon itself never changes shape. It is a round ball of rock, and it is always the same size.

The Moon makes no light of its own. In our whole solar system, the only object that shines with its own light is the Sun. Moonlight is really sunlight bouncing off the Moon's grey, dusty surface.

The Sun always lights up exactly half of the Moon: the half facing the Sun. That half is having daytime, and the other half is having night. What changes is how much of the sunny half we can see from Earth. These changing shapes are called the Moon's phases.

A month-long journey

The Moon travels around Earth in a path called an orbit. On average it is about 384,400 kilometres away, a gap wide enough to fit about 30 Earths in a row.

It takes about 27.3 days to go once around Earth. But the full set of phases, from one new Moon to the next, takes about 29.5 days. That is because Earth is moving around the Sun at the same time, so the Moon needs a couple of extra days to get back into the same position compared with the Sun.

The Moon also spins as it travels, turning exactly once for every trip around Earth. That is why we always see the same side of the Moon, with the same dark patches and craters.

From new Moon to full Moon and back

At new Moon, the Moon is in the same part of the sky as the Sun. Its sunlit side faces away from us and its dark side faces us, so we cannot see it at all, even though it is up during the day.

Over the next few days a thin crescent appears and grows. About a week after new Moon we see a first quarter Moon, which looks like half a circle. Next comes a gibbous Moon, when more than half of its face is lit. About two weeks after new Moon, the Moon is on the opposite side of Earth from the Sun, and we see its whole sunny side: a full Moon. A full Moon rises around sunset and sets around sunrise.

After that, the lit part shrinks again, through gibbous, last quarter and a final thin crescent, before the next new Moon. When the lit part is growing, the Moon is waxing. When it is shrinking, it is waning.

Many people think the dark part of the Moon is Earth's shadow, but it is not. It is simply the Moon's own night side. Earth's shadow only falls on the Moon during a lunar eclipse.

Upside down from Australia

Everyone on Earth sees the same phase at the same time. But people in the Southern Hemisphere, like us, see the Moon upside down compared with people in the Northern Hemisphere.

So from Australia, a waning crescent in the early morning sky is lit on its right-hand side, while people in countries such as Canada or England see it lit on the left. When the Moon is waxing, Australians see the light appear on the left side first and spread across to the right.

Look closely at a thin crescent Moon and you may see the dark part glowing faintly. This is called earthshine. It is sunlight that has bounced off Earth, travelled to the Moon, and bounced back to your eyes.

The Moon and the tides

The Moon does more than light up the night. Its gravity tugs on Earth's oceans, pulling the water into two bulges: one on the side of Earth facing the Moon, and one on the opposite side.

As Earth spins, coastlines pass through these bulges. That is why most beaches get two high tides and two low tides each day, a little over 12 hours apart. The Moon keeps moving along its orbit, so each day it takes Earth about 50 extra minutes to catch up with it, and the tides come a little later each day too.

The Sun's gravity helps as well. Around new Moon and full Moon, when the Sun, Earth and Moon line up, we get spring tides, with extra-high high tides and extra-low low tides. Around the quarter Moons, when the Sun and Moon pull at right angles, we get gentler neap tides. Spring tides have nothing to do with the season!

📚 Words to know

Phase

The shape of the lit part of the Moon, as we see it from Earth on a particular night.

Orbit

The curved path one object follows around another in space.

Waxing

Growing. The lit part of a waxing Moon gets bigger each night.

Waning

Shrinking. The lit part of a waning Moon gets smaller each night.

Gibbous

A Moon phase when more than half, but not all, of the Moon's face is lit.

Tide

The regular rise and fall of the sea, caused mostly by the Moon's gravity.

🧪 Try this: make Moon phases in your lounge room

1. In a darkened room, put a lamp or torch on a table at about head height, shining towards you. This is the Sun. Don't stare straight into the light.
2. Hold a ball, such as a tennis ball or an orange, at arm's length and a little above your head. You are Earth, and the ball is the Moon.
3. Face the lamp so the ball is between you and the light. The side of the ball facing you is dark. That is a new Moon.
4. Keep your arm straight and turn slowly on the spot. Watch a thin crescent of light appear on the ball, grow into a half, then a full circle when the lamp is behind you. Keep turning to see it shrink again.
5. Now check the real thing. Starting a few days after a new Moon, draw the Moon each evening for two weeks in a Moon diary, and note which side is lit. Only ever look at the Moon, never at the Sun.