EARTH & SPACE · ONE LESSON
Why does the Moon change shape?
Tonight the Moon might be a thin silver curve. In a week it will be a half. In two weeks, a full bright circle. A week after that you may not find it at all.
Here is the strange part: the Moon is not changing. It is the same ball of rock it was yesterday, the same size, in the same place in its journey. Nothing about it grows or shrinks.
So what is changing? That is the whole question, and by the end of this lesson you will be able to answer it — and prove it to someone else.
WHAT THIS LESSON DOES
Read, then try, then check
This lesson has three parts. First you read and look at some pictures. Then you take control of a simulator and move the Moon yourself. Then you answer eight questions to see what stuck.
- Say where the Moon's light actually comes from.
- Explain why we see a different shape each night.
- Name the eight phases, in order, in English and in Sanskrit.
- Look at tonight's Moon and say whether it is growing or shrinking.
- Explain why this is not the Earth's shadow — and prove it.
Thirty-one screens. Take about forty minutes. Keep your notebook open — you will need it three times.
BEFORE WE START
What do you think right now?
There is no mark for this and no right answer yet. Just pick what you honestly believe. We will come back to your choice near the end, and you may be surprised.
On Amavasya (new moon night), the Moon is completely dark. Why?
Choose one to continue.
STEP 1
The Moon makes no light of its own
The Sun is a star. Deep inside it, hydrogen is being crushed together so hard that it pours out light and heat — enough to reach us across 150 million kilometres. The Sun is a light source.
A candle is a light source. A tubelight is. A phone screen is. Each one makes light.
The Moon does not. The Moon is a ball of grey rock about 3,475 kilometres across — roughly the distance from Delhi to Chennai and back. Rock does not glow. Left alone in the dark, the Moon would be completely invisible.
Which raises an obvious problem. If the Moon makes no light, why can you see it?
STEP 2
You see most things by bounced light
Look at this page. Look at your hand, your desk, the wall. None of those things make light. You can see them because light from somewhere else — a window, a tubelight — lands on them and bounces off into your eyes.
That bouncing is called reflection. Switch off every light in a windowless room and your hand does not disappear. It is still there. But you cannot see it, because there is no light left to bounce.
The Moon works exactly the same way. Sunlight travels to the Moon, bounces off its grey rock, and some of it lands in your eye. That bounced sunlight is everything we call moonlight.
Moon rock is actually rather dull — it reflects only about 12% of the light that hits it, roughly like old asphalt on a road. It looks brilliant to us only because the night sky around it is so black.
STEP 3
One half of the Moon is always lit
Hold a ball up in sunlight. One side is bright. The other is in shadow. You cannot light the whole ball at once, however you turn it — and you cannot make both sides dark either.
The Sun never switches off, so this is true of the Moon at every moment of every day. Half of the Moon is in sunlight right now. Half is in darkness. That is true tonight, tomorrow, on Amavasya and on Purnima.
Drag the Sun anywhere you like. Watch the lit fraction. It never budges from one half.
There is no permanently dark side. Every part of the Moon gets sunlight at some point in the month. What there is is a far side — the half that always faces away from Earth. That is a completely different idea, and we will come back to it.
QUICK CHECK
Before you go on
On the night of Amavasya, when we see no Moon at all, how much of the Moon is lit by the Sun?
Answer to continue.
STEP 4
The Moon goes around the Earth
The Moon travels in a great loop around the Earth. That loop is called an orbit. One complete trip, from new moon back to new moon, takes about 29½ days.
That number should feel familiar. A month is about 29 or 30 days, and that is not a coincidence — the word "month" comes from "moon". Long before anyone had a calendar on a wall, people counted time by watching the Moon go through its shapes. Many calendars still do.
The Moon travels about 3,680 kilometres every hour. It is moving fast. But it is so far away — about 384,000 kilometres, thirty Earths laid side by side — that from down here it seems to drift gently across the sky.
STEP 5
We only ever see part of the lit half
Put those two facts side by side. Half the Moon is always lit. The Moon keeps moving around us. What follows?
Our viewing angle keeps changing. Some nights we are looking almost straight at the lit half, and we see a bright full circle. Some nights we are looking almost entirely at the dark half, and we see nothing. Most nights we catch the lit half at an angle, and we see a slice of it — a curve, a half, a fat oval.
Here is a way to feel it. Picture a classmate standing near a bright window, facing it. From where you sit you see their face lit up. Now they turn slowly on the spot. As they turn away you see less and less of their lit face, until you are looking at the back of their head and see none of it.
Their face has not changed. It is still there and still lit. What changed is where you are standing relative to them.
TRY IT · 3 ROUNDS
Can you predict the shape?
The picture below is the view from far above the North Pole. Sunlight comes from the left, so the Moon's left side is always the lit side. You are standing on the Earth in the middle.
From where the Moon is now, which shape would you see?
Round 1 of 3 — choose a shape.
STEP 6
The eight shapes, always in this order
Astronomers give the eight main shapes names. They always come round in the same order, month after month, year after year.
The words behind the names
Crescent means a thin curve — less than half lit. Gibbous is an old word meaning "humped" or bulging — more than half lit, but not yet full. Quarter is the one that confuses people: a "first quarter" Moon looks like a half circle. It is called a quarter because the Moon has finished one quarter of its journey around the Earth, not because a quarter of it is lit.
Crescent comes from a word meaning "growing" — and a crescent is the smallest sliver. Gibbous sounds heavy and fat, and a gibbous Moon is fat. If it is less than half, it is a crescent; if more than half, gibbous.
STEP 7
Growing and shrinking: waxing and waning
Two words worth owning. When the lit part is growing night after night, the Moon is waxing. When it is shrinking, it is waning.
In India these same two halves of the month have their own names, and you may already know them from home.
| What you see | English | In the Indian calendar |
|---|---|---|
| No Moon at all | New moon | Amavasya |
| Growing, night by night | Waxing | Shukla Paksha — the bright fortnight |
| Full bright circle | Full moon | Purnima |
| Shrinking, night by night | Waning | Krishna Paksha — the dark fortnight |
Each fortnight is divided into fifteen days called tithis. So a date like "Shukla Paksha, tithi 8" means the eighth day of the growing half — which is exactly the first quarter Moon, the half circle.
This is why so many festivals move around the calendar each year. Diwali falls on the Amavasya of Kartik. Holi falls on a Purnima. Eid begins when the thin crescent is first sighted after a new moon. All of them are tied to the Moon's shape, not to a fixed date.
STEP 8
Tonight: is it growing or shrinking?
You can work this out from a single glance, and it is a genuinely useful skill.
From India and anywhere else in the northern half of the world:
Lit on the right → the Moon is waxing. It is growing. It will be fuller tomorrow, and it sits in the western sky in the early evening.
Lit on the left → the Moon is waning. It is shrinking. It will be thinner tomorrow, and it rises late at night or is still up in the morning.
So a crescent with its horns pointing left — the bright curve on the right — is a young growing Moon a few days after Amavasya. A crescent with horns pointing right is an old Moon, a few days before the next Amavasya.
In Australia or South Africa this rule flips, because you are standing "upside down" relative to someone in India and see the Moon the other way up. The Moon has not changed. Your point of view has — which is the theme of this entire lesson.
NOW YOU TAKE CONTROL
Move the Moon yourself
Nobody has ever seen the left-hand picture with their own eyes — you would have to travel far above the Earth to get it. The right-hand picture is what you would see standing in your own street the same night. Watch them together.
Looking down from high above
Sunlight comes from the left. Drag the Moon, or use the slider.
What you see from Earth
The same Moon, the same night.
INVESTIGATION 1
Find the two extremes
What to do
- Go back one screen to the simulator.
- Drag the Moon until it is directly opposite the Sun, with the Earth in between. Read the "lit part we can see" number.
- Now drag it until it sits between the Sun and the Earth. Read the number again.
- Write both numbers in your notebook, with the phase name beside each.
Check your answer
Opposite the Sun → 100% → Full Moon, Purnima. The Moon's whole lit face is pointing at us.
Between Sun and Earth → 0% → New Moon, Amavasya. The lit face is pointing at the Sun, away from us, so we see only the dark half.
Notice what did not change: in both positions, exactly half the Moon was lit. Only our view of it changed.
INVESTIGATION 2
Watch the little Moon, not the big one
What to do
- Go back to the simulator and look only at the small Moon in the left panel — not the big disc on the right.
- Drag it slowly all the way around the orbit, one full lap.
- Watch the little white half on it. Does the amount of white ever change?
- Watch where that white half points. What is it always pointing at?
Check your answer
The amount of white never changes — it is always exactly half. What changes is the direction it faces relative to us.
And it always points at the Sun. It has to: that is the side the sunlight is falling on. The Moon cannot choose to light up a different half.
This is the single most important thing in the lesson. If you can explain this to someone at home tonight, you have understood Moon phases.
INVESTIGATION 3
Time the cycle
What to do
- Press Reset, then Play.
- Watch the "day of month" counter and the shape at the same time.
- Note the day number when the Moon is exactly full.
- Note the day number when it returns to new. How many days is one complete cycle?
Check your answer
Full at about day 14–15, and back to new at about day 29–30. The full cycle is roughly 29½ days.
Full moon lands halfway through the cycle — which is exactly why Purnima falls in the middle of the month in a lunar calendar, with Amavasya at each end.
STEP 9
"Isn't it just the Earth's shadow?"
This is the most common wrong answer in the world, and it is a good wrong answer — it shows someone is thinking. The Earth is between the Sun and the Moon at full moon, the Earth does cast a shadow, so why not?
Because of one fact that destroys the idea completely. Think it through with me.
If the Earth's shadow falling on the Moon caused the phases, then the Moon would have to pass through that shadow every single month — because it goes round us every month. So we would get a lunar eclipse every month, twelve times a year, as regular as clockwork.
But we don't. Eclipses are rare. They make the news. People travel across countries to see one.
So what stops the Moon hitting the shadow most months? Turn the page.
STEP 10
The orbit is tilted
Every flat drawing of the Sun, Earth and Moon — including the simulator you just used — tells a small lie. It draws everything on one flat sheet, as though the Moon must pass exactly through the shadow each month.
In reality the Moon's orbit is tilted by about 5° compared with the Earth's path around the Sun. Five degrees sounds tiny. Across 384,000 kilometres, it is enough for the Moon to pass thousands of kilometres above or below the Earth's shadow and miss it entirely.
Slide through a year of full moons. Only at two points does the Moon line up well enough for an eclipse.
Those two special points — where the tilted orbit crosses the flat plane — are called nodes. An eclipse needs the Moon to be at a node and full (for a lunar eclipse) or new (for a solar one) at the same time. That combination only happens a few times a year.
STEP 11
Two kinds of eclipse, and how to tell them apart
Since we are here, let us sort out the two properly — they get mixed up constantly.
| Lunar eclipse — चंद्र ग्रहण | Solar eclipse — सूर्य ग्रहण | |
|---|---|---|
| What blocks what | Earth's shadow falls on the Moon | The Moon blocks the Sun from us |
| Order of the three | Sun — Earth — Moon | Sun — Moon — Earth |
| Only possible at | Full moon (Purnima) | New moon (Amavasya) |
| Who can see it | Everyone on the night side of Earth | Only a narrow strip of land |
| Safe to look at? | Completely safe | Never look directly — it can permanently damage your eyes |
A solar eclipse is the one genuinely dangerous thing in astronomy. Sunglasses are not enough. Smoked glass is not enough. An X-ray film is not enough. Use only certified eclipse glasses, or watch the Sun's image projected onto paper through a pinhole.
STEP 12
Why we always see the same face
Here is something you may never have noticed. The patterns of dark patches on the Moon — the ones people in India see as a rabbit, or as a woman spinning — are always in the same place. They never rotate away.
That is because the Moon turns on its own axis in exactly the same time it takes to go around us: 27.3 days. So as it travels round, it turns to match, and keeps one face pointed at Earth the whole time. This is called being tidally locked, and it is very common among moons.
Walk around a chair while always facing it. By the time you are back where you started, you have turned around once yourself — and the chair never saw your back. The Moon does that to us, permanently.
So there is a far side we never see from Earth. No human eye saw it until a Soviet spacecraft photographed it in 1959.
BACK TO THE BEGINNING
What you thought at the start
If you picked the shadow, you are in enormous company — most adults would pick it too, and it is the answer that feels most obvious. Changing your mind when the evidence says so is not a weakness. It is the entire method of science.
BEFORE THE QUESTIONS
The five things that matter
1. The Moon makes no light. It reflects sunlight, like everything else you see.
2. Half the Moon is lit at every moment, and that half always faces the Sun.
3. A phase is how much of that lit half is turned towards us tonight.
4. One full cycle takes about 29½ days, which is where our month comes from.
5. Phases are not the Earth's shadow — the proof is that eclipses are rare and phases are not.
Eight questions next. You will see the answer straight after each one.
FINISHED
Well done
Words you now own
Your homework: keep a Moon diary
For ten nights, go outside at about the same time and find the Moon. Write the date, the time, and draw its shape. Small drawings are fine — this is data, not art.
Add one column: waxing or waning? Use the rule from Step 8 — lit on the right means growing. By night four you will be predicting tomorrow's shape before you go out, which is the real test of whether you have understood this.
If it is cloudy, write "cloudy". Scientists record what they could not measure, not only what they could.
Bring the diary to class. When thirty students put their drawings side by side and they all agree, you will have proved this yourselves — without taking anyone's word for it.
Look for the Moon in daylight. Most people insist it only comes out at night. In the week after first quarter it is easily visible in the afternoon sky, pale against the blue. Find it once and you will keep spotting it.