The Phases of the Moon

A shape that keeps changing

Watch the Moon over a couple of weeks and you will notice something strange. One night it is a thin curve, like a fingernail clipping. A week later it looks like a half-circle. A week after that it is a bright, full circle lighting up the whole sky. Then it slowly shrinks away again.

These different shapes are called the phases of the Moon. People have watched them for thousands of years. In fact, our word "month" comes from "Moon", because the Moon's changing shapes were one of the first calendars humans ever used.

But here is the surprising part: the Moon is not actually changing shape. It is always the same round, rocky ball. So why does it look so different from one night to the next? To answer that, we need to know one important secret about the Moon.

The Moon is a giant mirror

The Sun and the stars make their own light. They are blazing hot balls of glowing gas. The Moon is completely different. The Moon is cold rock, and it makes no light of its own at all.

So why does it shine? Because sunlight hits it. The Moon acts like a huge mirror in the sky. Sunlight travels across space, lands on the Moon, and bounces back down to Earth and into our eyes. When you see the Moon "glowing", you are really seeing reflected sunlight.

This is the key to everything. The Sun can only ever light up one half of the Moon at a time — the half that faces the Sun. The other half is always in darkness. (It is just like Earth: one side has day, the other has night.) The half of the Moon facing the Sun is always lit. What changes is how much of that lit half we can see from Earth.

Why the phases happen

The Moon does not sit still. It travels all the way around the Earth in a path called an orbit, taking about a month to go round once. As it moves, we look at its sunlit half from different angles.

• When the Moon is between the Earth and the Sun, its lit side points away from us. We see its dark side, so the Moon almost disappears. This is a new Moon.
• When the Moon is on the opposite side of Earth from the Sun, we see its whole lit face shining straight at us. This is a full Moon.
• In between these, we catch the lit half at an angle, so we see only part of it — a curve or a half-circle.

So the phases are really about position and angle, not about anything covering the Moon or eating it. The same amount of Moon is always lit; we just see a changing slice of it.

The eight phases in order

Here are the main phases, in the order they happen:

1. New Moon — almost invisible; the lit side faces away from us.
2. Waxing Crescent — a thin curve appears and starts to grow.
3. First Quarter — we see exactly half the Moon lit (it looks like a half-circle).
4. Waxing Gibbous — more than half is lit and still growing.
5. Full Moon — the whole face is bright and round.
6. Waning Gibbous — the bright part starts to shrink.
7. Last Quarter — half-lit again, but the other side this time.
8. Waning Crescent — a thin curve that fades back towards new Moon.

Two useful words: waxing means growing (getting brighter, heading to full), and waning means shrinking (getting smaller, heading to new). A handy memory trick is that "waning" and "wane" both contain a sound like "gone" — a waning Moon is on its way to being gone.

The whole cycle, from one new Moon to the next, takes about 29.5 days. That is why a calendar month is roughly the length of one Moon cycle.

Why this matters here on Earth

The Moon is not just pretty to look at — it tugs on our planet. The Moon's gravity pulls on Earth's oceans and helps create the tides, the daily rise and fall of the sea. Around full Moon and new Moon, the Sun and Moon line up and pull together, giving especially big tides called spring tides. Sailors, fishers and beach-walkers have used the Moon to predict the sea for centuries.

The Moon also lights up the night. Before electric lights, a full Moon meant people could travel, work and harvest after dark. That is why a bright autumn full Moon is still nicknamed the "Harvest Moon".

Try it yourself: keep a Moon diary

You can watch the whole cycle with your own eyes — no telescope needed.

1. Pick a notebook and a spot where you can see the sky. Around sunset on a clear evening is a good time.
2. Each night, draw the shape of the Moon you can see and write the date. Shade in the dark part with a pencil.
3. Try to look on as many nights as you can over about four weeks.
4. Look back through your drawings. Can you see the Moon grow from a thin crescent to a full circle and back again?

If you miss some nights because of cloud, do not worry — that is exactly what real astronomers deal with too. You can also model the phases at home: stand in a dark room, hold a ball at arm's length, and have a friend shine a torch on it from across the room. As you turn slowly on the spot, watch how the lit part of the ball seems to change shape. That ball is your "Moon", and the torch is your "Sun"!

Want to explore more of the sky? See Planets of the Solar System to meet the Moon's neighbours, and The Sun, the Moon and the Stars for a gentle first look at space.