Look up on a clear evening and you might see a thin curve of light. A week later, the same Moon is a bright half circle. A week after that, it is a full, round disc. Nothing about the Moon itself has changed. So what did?

This short course answers that question. You need no science background. Every new word is explained the first time it shows up. Tap any word with a dotted line to see what it means.

The whole idea in one sentence

The Sun always lights one half of the Moon. As the Moon travels around Earth, we see more or less of that lit half.

That is all a phase is: the shape of the lit part we see on a given night.

What you will learn

Light and shadow. Where moonlight comes from, and why half of the Moon is always lit.
The monthly cycle. Why the shape changes, the eight phases, and how to tell if the Moon is growing or shrinking.

Four words you will meet

Reflect: to bounce light back, like a mirror does.
Orbit: the path the Moon follows as it travels around Earth.
Waxing: growing. The lit part gets bigger each night.
Waning: shrinking. The lit part gets smaller each night.

No need to memorize them now. Each one comes back later, with a picture.

Explain where the Moon's light comes from.
Explain why one half of the Moon is always lit.

Where does the Moon's light come from?

The Moon glows by itself, like a lamp.The Moon is cold rock. It makes no light of its own.
Sunlight bounces off the Moon and reaches our eyes.Right. The Moon only bounces back light from the Sun.
Light from cities on Earth lights it up.City lights are far too weak to light up something so far away.
Starlight lights it up.Stars are so far away that their light is much too faint.

The Moon is a mirror, not a lamp

The Moon is a huge ball of grey rock. It makes no light at all. We see it for one reason: sunlight hits it and bounces back toward Earth. In other words, the Moon reflects sunlight.

Think of a bike reflector at night. It seems to glow when a car passes. But it only shines while the car's headlights hit it. The Moon works the same way, with the Sun as the headlights.

It is night where you live, and the Sun has set. Why can you still see the Moon?

The Sun still shines on the Moon, and the Moon reflects that light to you.Yes. It is night where you stand, but the Moon is still in sunlight.
The Moon stores sunlight during the day and gives it off at night.The Moon stores no light. Like a mirror, it reflects light the moment the light arrives.
At night, the Moon makes its own light.The Moon never makes its own light, day or night.

Half lit, always

Sunlight comes from one direction. So it can only light the side of the Moon that faces the Sun. The other side is in darkness.

Earth works the same way. Right now, half of Earth has daytime and half has night. The Moon also always has a day side and a night side.

Sunlight comes from the left. The half of the Moon that faces the Sun is lit. The other half is dark.

At any moment, the Sun lights exactly one half of the Moon. Always half. Never more, never less.

Stand next to a single lamp in a dark room and hold up a ball. The side that faces the lamp is bright. The side that faces away is dark. Walk around the lamp and the bright side keeps facing it. The Sun and the Moon do exactly this.

Explain it another way

Picture a parked car under a single streetlight at night. The side of the car that faces the streetlight is lit. The other side is in shadow. Nothing about the car changed. Only which side faces the light.

"The dark side of the Moon never gets any sunlight."

No side of the Moon stays dark. The Moon slowly turns, so every part of it gets daytime and nighttime. The far side, the side we never see from Earth, gets as much sunlight as the side we do see.

In your own words: why is exactly half of the Moon lit at any moment?

Sunlight comes from one direction, so it only reaches the side of the Moon that faces the Sun. The side facing away stays dark. It is the same as day and night on Earth.

Explain to a 10-year-old where moonlight comes from. Use something they can find at home.

A good explanation says three things:

The Moon makes no light. It is rock.
Sunlight bounces off it to our eyes, the way light bounces off a mirror or a bike reflector.
It shows this with a home object: a torch and a ball, a mirror, or a reflector.

The Moon makes no light. It reflects sunlight.
Sunlight comes from one direction, so it lights only the half of the Moon facing the Sun.
So one half of the Moon is always lit and one half is always dark. As the Moon turns, which half that is changes.

Where does the Moon's light come from?

From the Sun. Sunlight bounces off the Moon to our eyes. The Moon makes no light of its own.

How much of the Moon does the Sun light at any moment?

Exactly half: the half that faces the Sun.

Does the far side of the Moon ever get sunlight?

Yes, as much as the near side. No part of the Moon stays dark.

Why can you see the Moon at night, when the Sun has set where you are?

The Moon is still in sunlight. It reflects that light down to you.

Explain why we see different shapes as the Moon travels around Earth.
Say where the Moon is at new moon and at full moon.
Explain why Earth's shadow is not the cause.

What do you think makes the Moon change shape?

Earth's shadow covers part of the Moon.This is the most common guess, and it is wrong. You will see why in this lesson.
As the Moon travels around Earth, we see its lit half from a different angle.Right. The lit half stays the same size. Our view of it changes.
Clouds in space block parts of the Moon.There are no clouds in space that could do this.
The Moon spins to show us its dark side.The Moon does turn, but that is not what changes the shape we see.

The Moon travels around us

The Moon does not stay in one place. It travels around Earth on a path called an orbit. One trip around takes about a month.

As it travels, its shape in our sky goes through a set of changes, then starts again. That repeating set of shapes is the Moon's cycle of phases. One full cycle takes 29.5 days.

Why 29.5 days, if one trip around Earth takes 27.3?

The Moon goes once around Earth in 27.3 days. But Earth is moving around the Sun at the same time. So the Moon needs a little over two more days to line up with the Sun the way it did at the start. That is why the shapes repeat every 29.5 days, not every 27.3.

You see a full moon tonight. About when will you see the next one?

In about a weekA week takes the Moon only a quarter of the way through its cycle.
In about a month (29.5 days)Yes. The whole cycle of shapes takes 29.5 days.
Tomorrow nightThe shape changes a little every night, so tomorrow it will already be slightly less than full.
In about a yearThe cycle is much shorter. There are about 12 full moons in a year.

Same lit half, different view

Here is the heart of it. The Sun always lights the half of the Moon that faces the Sun. But as the Moon moves around Earth, that lit half faces us more, or less.

Follow the Moon through one trip. Before you reveal each step, try to picture what we see from Earth.

The Moon is between Earth and the Sun. Its lit half faces the Sun, so it faces away from us. We look at the dark half and see almost nothing. This is the new moon.
A quarter of the way around. Now we see the lit half from the side. Half of the round shape we see is lit. This is the first quarter.
The Moon is on the far side of Earth from the Sun. The whole lit half faces us. We see a bright, round disc. This is the full moon.
Three quarters of the way around. We see the lit half from the other side. Half the disc is lit again, but the other half this time. This is the last quarter.

Seen from far above Earth's North Pole. The Moon is always half lit on the side facing the Sun. What changes is how much of that lit half faces Earth.

The lit half never changes size. What changes is how much of it faces us.

Try it tonight with a lamp and a ball (an orange works too). In a dark room, stand facing the lamp. Hold the ball at arm's length, a little above your head. Turn slowly on the spot, all the way around. Watch the lit part of the ball: dark, then half, then full, then half, then dark again. You just made the Moon's phases.

"The phases happen because Earth's shadow falls on the Moon."

Earth's shadow only reaches the Moon during a lunar eclipse. That can only happen at full moon, when the Sun, Earth and Moon line up almost exactly. It does not happen every month. At every other phase, Earth's shadow points off into space, away from the Moon.

At full moon, where is the Moon?

Between Earth and the SunThat is where it is at new moon, when its lit half faces away from us.
On the other side of Earth from the SunYes. From there, its whole lit half faces us.
Right next to the Sun in the skyThen we would be looking at its dark half, as at new moon.
Inside Earth's shadowOnly during a lunar eclipse. At a normal full moon, the Moon is in full sunlight.

Why can't we see the new moon?

Its lit half faces the Sun, which means away from us, so we look at its dark half. It is also in the sky close to the Sun, during the day, so any faint edge is lost in the Sun's glare.

A friend says, "The phases are Earth's shadow on the Moon." In two or three sentences, explain what really happens.

A good answer says:

The Sun always lights half of the Moon.
As the Moon travels around Earth, we see more or less of that lit half.
Earth's shadow only reaches the Moon in a lunar eclipse, and only at some full moons.

The Moon travels around Earth. Its cycle of shapes repeats every 29.5 days.
Half the Moon is always lit. The phase is how much of that lit half faces us.
New moon: the Moon is between Earth and the Sun. Full moon: it is on the far side of Earth.
Earth's shadow does not make phases. It only reaches the Moon in a lunar eclipse.

What makes the Moon's shape change over a month?

As the Moon travels around Earth, we see more or less of its sunlit half.

How long is the Moon's cycle of phases?

29.5 days, from one new moon to the next.

Where is the Moon at new moon?

Between Earth and the Sun. Its lit half faces away from us.

Where is the Moon at full moon?

On the far side of Earth from the Sun. Its whole lit half faces us.

When does Earth's shadow fall on the Moon?

Only during a lunar eclipse, which can only happen at full moon, and not every month.

Name the eight phases in order.
Tell from one look if the Moon is growing or shrinking.
Know roughly when the main phases are in the sky.

You live in Europe or North America. Just after sunset, you see a thin curve of Moon, lit on its right side. Is the Moon growing or shrinking?

GrowingRight. Seen from the northern half of Earth, a Moon lit on the right is growing.
ShrinkingNot this time. A Moon lit on the left would be shrinking.
You can't tell from one lookYou can. This lesson shows you how.

Growing, then shrinking

From new moon to full moon, the lit part we see grows a little each night. A growing Moon is waxing. After full moon, the lit part shrinks each night. A shrinking Moon is waning.

Two more words describe the shape. A crescent is less than half lit: a thin curve. A gibbous Moon is more than half lit, but not full.

The eight phases

Put those words together and you get the eight phase names. Here they are in order, starting from new moon, as seen from the northern half of Earth:

The eight phases, left to right, as seen from the Northern Hemisphere. The cycle then starts again.

Phase	What you see	When it is in the sky
New moon	Nothing	Up during the day, next to the Sun
Waxing crescent	Thin curve, lit on the right	Low in the west, just after sunset
First quarter	Right half lit	Afternoon and evening
Waxing gibbous	Mostly lit, dark sliver on the left	Most of the evening
Full moon	Fully lit	All night: rises around sunset, sets around sunrise
Waning gibbous	Mostly lit, dark sliver on the right	Late at night and early morning
Last quarter	Left half lit	After midnight and in the morning
Waning crescent	Thin curve, lit on the left	Low in the east, just before sunrise

Which phase comes right after the first quarter?

Waxing gibbousYes. After the half Moon, the lit part keeps growing past half: that is gibbous, and it is still waxing.
Waning gibbousThat comes after the full moon, when the lit part is shrinking.
Full moonFull moon comes one step later, after the waxing gibbous.
Waxing crescentThat comes before the first quarter, not after.

Read it in one look: D, O, C

In the Northern Hemisphere, read the Moon like the letters D, O and C, in that order:

Lit part shaped like a D, lit on the right: growing.
O: full.
Lit part shaped like a C, lit on the left: shrinking.

You walk the dog at 7 in the morning and see a half Moon, lit on its left side. Lit on the left means C: shrinking. So it is a last quarter Moon. The timing fits too, because a last quarter Moon is up in the morning.

"The D-O-C trick works everywhere on Earth."

It only works in the Northern Hemisphere. Seen from the Southern Hemisphere, the Moon looks upside down. There, a growing Moon is lit on the left. It is the same Moon and the same phase, seen from the other side of the planet.

You are in Canada. At 8 in the evening you see a half Moon, lit on its right side. Which phase is it?

First quarterYes. Half lit, lit on the right (D), so growing. And a first quarter Moon is up in the evening.
Last quarterA last quarter Moon is lit on the left (C) and shows up after midnight and in the morning.
Waxing gibbousGibbous means more than half lit. This Moon is exactly half lit.
Full moonA full moon is lit all the way across.

List the eight phases in order, starting from new moon.

New moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent.

Your cousin in Australia says a growing Moon is lit on the left. You live in Spain and say it is lit on the right. Who is right? Explain.

You are both right. A good answer says:

Everyone on Earth sees the same phase on the same night.
From the Southern Hemisphere, the Moon looks upside down compared with the north.
So the same growing Moon is lit on the right in Spain and on the left in Australia.

Waxing means growing, waning means shrinking. Crescent is less than half lit, gibbous is more than half.
The order: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent.
In the Northern Hemisphere, D (lit right) is growing and C (lit left) is shrinking. In the south it flips.

What does "waxing" mean?

Growing: the lit part gets bigger each night.

What is a gibbous Moon?

More than half lit, but not full.

Name the eight phases in order.

New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent.

Northern Hemisphere: the Moon is lit on the right. Growing or shrinking?

Growing (D shape). Lit on the left (C shape) means shrinking.

Why does a growing Moon look lit on the left from Australia?

From the Southern Hemisphere, the Moon looks upside down compared with the north. Same phase, flipped view.

These questions mix everything from the course. Some ask you to use an idea in a new situation. Answer from memory first, then check.

It is 6 in the morning in Germany. You see a Moon that is more than half lit, lit on its left side. Which phase is it?

Waning gibbousYes. More than half lit means gibbous. Lit on the left (C side) means shrinking. And a waning gibbous Moon is up in the early morning.
Waxing gibbousA waxing gibbous Moon is lit on the right and is seen in the evening.
Last quarterA last quarter Moon is exactly half lit. This one is more than half.
Full moonA full moon is lit all the way across.

Which of these is a lunar eclipse, not a phase?

On a full-moon night, the Moon goes dark red for about an hour, then turns bright again.Yes. Earth's shadow is passing over the full moon. That only happens in a lunar eclipse.
A thin curve of Moon, low in the west after sunset.That is a waxing crescent, an ordinary phase.
A half Moon high in the morning sky.That is a last quarter Moon, an ordinary phase.

Why is half of the Moon always lit?

Sunlight comes from one direction, so it lights the half of the Moon that faces the Sun. The other half faces away and stays dark.

The Moon goes once around Earth in 27.3 days. How long is its cycle of phases?

27.3 days, the sameClose, but Earth moves around the Sun meanwhile, so the Moon needs a little over two extra days to line up with the Sun again.
29.5 daysYes. The extra two days come from Earth's own trip around the Sun.
7 daysA week is only a quarter of the cycle.

Explain to someone who has never thought about it why the Moon changes shape. Use the lamp-and-ball idea.

A good explanation covers:

The lamp is the Sun, the ball is the Moon, and your head is Earth.
The lamp always lights half of the ball.
As the ball goes around your head, you see more or less of that lit half: dark, half, full, half, dark.