Earth, Moon, and the Sun
Why This Matters
One morning in Kanniyakumari, Tamil Nadu, a 12-year-old girl named Rashmika was cycling to school. On the way she noticed something. The shadows of the coconut trees were long in the morning. But on her way back in the afternoon, the same shadows were short.
She thought about it. “The shadows change because the Sun moves across the sky during the day,” she said to herself. But then she stopped. In Grade 6 she had learnt that it is the Earth that moves around the Sun. So which is it? Does the Sun move in the sky? Or does the Earth move?
This is a brilliant question. And it is the same question people have asked for thousands of years.
In this chapter we will answer it. We will find out why the Sun seems to rise and set. Why we have day and night. Why the Moon keeps changing its shape, from a thin curve to a full round disc. And why, on some special days, the Sun or the Moon suddenly goes dark. None of these will be “just facts to remember”. We will see the reason behind each one — with pictures.
The Big Idea
The Sun does not really cross the sky, and the Moon does not really change its shape. It is the Earth that is moving — and from a moving Earth, the sky seems to move. The Earth spins on itself (this gives us day and night) and also travels around the Sun (this gives us the year). The Moon travels around the Earth. The Sun is the only one that makes its own light. The Earth and the Moon do not glow — they only catch the Sun’s light. Once you hold these few simple motions in your head, every puzzle in the sky — sunrise, moonlight, the Moon’s changing shape, even an eclipse — falls neatly into place.
Let’s Break It Down
The Sun, the Earth, and the Moon
Let us first meet the three main characters of this chapter.
The Sun is a star. A star is a huge ball of hot, glowing gas. The Sun makes its own light and heat. It is by far the biggest of the three. If the Sun were the size of a big watermelon, the Earth would be smaller than a peppercorn. The Sun is also very, very far away — about 150 million kilometres from us. That is so far that its light takes about 8 minutes to reach us.
The Earth is the planet we live on. A planet is a large round body that goes around a star. The Earth does not make its own light. It only catches light from the Sun.
The Moon is a natural satellite of the Earth. A satellite is something that goes around a bigger body. “Natural” means it is not man-made. The Moon is much smaller than the Earth, and it goes around the Earth. Like the Earth, the Moon makes no light of its own. The moonlight you see at night is really sunlight bouncing off the Moon.
Here is the key fact to carry through the whole chapter: only the Sun shines on its own. The Earth and the Moon are just lit up by the Sun. Keep this in mind — it explains almost everything that follows.
Rotation, and why we have day and night
Have you ever sat on a merry-go-round facing outwards? As the merry-go-round turns, the trees and buildings around you seem to swing past in the opposite direction. They are not moving. You are. But because you are turning, the still world seems to move.
The very same thing happens with the Earth and the Sun. The Earth is like a giant merry-go-round. It is turning. So the Sun, which is actually still, appears to move across our sky.
This turning of the Earth on itself has a name. Rotation is the motion of an object spinning around an imaginary line that passes through it. That imaginary line is called the axis. Think of a spinning top, or a spinning fan, or a ball you spin on your finger. The Earth spins in the same way. Its axis runs from the North Pole to the South Pole.
The Earth makes one full rotation in about 24 hours — one day and one night.
Now, the big “why”. Why does spinning give us day and night? Here is the reason. The Sun’s light can only fall on the half of the Earth that faces it. The other half is turned away, so it stays dark. The lit half has day. The dark half has night. Figure 12.1 below shows this clearly.
So as the Earth spins, your spot on it keeps moving from the lit half into the dark half and back again. When your spot turns into the light, you see the Sun appear — that is sunrise. When it turns away into the dark, the Sun disappears — that is sunset. You are not standing still while the Sun travels. You are being carried around on a spinning Earth.
Which way does the Earth spin? It spins from West to East. That is why the Sun always seems to rise in the East and set in the West. Your eastern edge swings into the light first, so you see the Sun appear in the East. Hours later your spot swings out of the light on the western side, so the Sun seems to vanish in the West. The Moon and the stars seem to do the same thing, and for the same reason — the spinning Earth, not them.
The “nothing is a given” point here is worth saying plainly. The Sun is not racing across your sky. It just looks that way because you are riding a slowly spinning planet.
Two friends, one in eastern India and one in western India, are watching for sunrise. Who sees the Sun first, and why?
The friend in eastern India sees the Sun first. The Earth spins from West to East, so the eastern edge of the country turns into the Sun’s light before the western edge does.
Revolution — the Earth’s trip around the Sun
The Earth does two motions at the same time. It spins on its axis (that is rotation, which we just saw). And it also travels around the Sun. This second motion is called revolution.
Revolution is the motion of one object travelling around another object. The path it takes is called its orbit. The Earth’s orbit around the Sun is nearly a circle, with the Sun in the middle.
How long does one full trip take? The Earth completes one revolution around the Sun in about 365 days — and that is exactly what we call a year. So a year is just one full lap of the Earth around the Sun.
It is easy to mix up rotation and revolution, because both are “the Earth going round”. So let us lay them side by side. Figure 12.2 below shows the two motions next to each other.
Here is a clean comparison you can keep in your notes.
| Rotation | Revolution | |
|---|---|---|
| What it is | The Earth spinning on its own axis | The Earth travelling around the Sun |
| Goes around | An imaginary line inside itself (its axis) | The Sun (a body outside it) |
| Time for one | About 24 hours (one day) | About 365 days (one year) |
| What it gives us | Day and night | The year (and, with the tilt, the seasons) |
A quick note on seasons. The Earth’s axis is not perfectly upright — it is tilted. As the Earth travels around the Sun, this tilt makes one half of the Earth lean toward the Sun for part of the year and away from it for the rest. The half leaning toward the Sun gets stronger sunlight for longer hours — that is summer. The half leaning away gets weaker sunlight for shorter hours — that is winter. So the seasons come from the tilt plus the revolution, not from the Earth being closer to or farther from the Sun.
The Moon’s motion
While the Earth spins and travels around the Sun, the Moon is doing its own thing: it travels around the Earth. The Moon is the Earth’s natural satellite, and its orbit goes round the Earth (you can see this small grey ball circling the Earth on the right side of Figure 12.2).
The Moon takes about a month to go once around the Earth. Like the Sun, the Moon also seems to rise in the East and set in the West each night — and again, that is because of the spinning Earth, not the Moon racing across the sky.
One more time, the big idea: the Moon makes no light of its own. When you see a bright Moon at night, you are seeing sunlight that hit the Moon and bounced off toward you.
Phases of the Moon — why the Moon seems to change shape
Look at the Moon over a few weeks. Some nights it is a thin curve, like a fingernail clipping. Some nights it is half a disc. Some nights it is a full bright circle. A few nights you can hardly see it at all. These different shapes are called the phases of the Moon.
Now, the Moon is a solid round ball. It does not actually grow and shrink. So why does its shape seem to change? Here is the whole secret, and it rests on the idea we keep repeating: the Sun lights only one half of the Moon at a time — the half that faces the Sun. The other half is always dark.
As the Moon travels around the Earth, we on Earth look at it from different angles. So we see different amounts of the lit half. Sometimes we see the whole lit half — that looks like a full round Moon. Sometimes we see only a sliver of the lit half — that looks like a thin curve. Sometimes the lit half is turned completely away from us, and we see only the dark side — so we can barely see the Moon at all. Figure 12.3 below puts the whole story in one picture.
So the phases are not the Moon shrinking and growing. They are just how much of the lit half we happen to see from where we stand. The full set of phases — from no Moon, to a thin crescent, to half, to full, and back again — takes about a month. That is one trip of the Moon around the Earth.
Here is a useful word check before the worked example. Some students wonder whether the dark part of the Moon is the Earth’s shadow. It is not. The dark part is simply the half of the Moon that the Sun is not lighting up at that moment. No shadow of the Earth is involved in ordinary Moon phases.
Sometimes when we lean on an idea from an earlier class, it helps to refresh it. The phases rely on the simple idea that an object is only bright where light falls on it — let us recap that.
Let us use everything so far on a real “why” question.
On a clear night you see a perfectly round, full bright Moon. Using what you know, where must the Moon be — on the same side of the Earth as the Sun, or on the opposite side? And is the Moon making its own light?
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First, the light. The Moon never makes its own light. So the brightness you see is sunlight bouncing off the Moon. That is true for every phase.
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A full Moon means you are seeing the whole lit half of the Moon. The lit half is always the half facing the Sun.
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For you on Earth to see that entire lit half, the Sun must be lighting the side of the Moon that is turned toward you. That can only happen when the Moon is on the opposite side of the Earth from the Sun — Sun, then Earth, then Moon. The Sun lights the far side of the Moon, which is the side facing us, so we see a full bright disc.
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Answer: The Moon is on the opposite side of the Earth from the Sun, and it is not making its own light — it is reflecting sunlight. (This is also why a full Moon rises around sunset: as the Sun goes down on one side of you, the full Moon comes up on the other side.)
Eclipses — shadows in space
We have seen that the Earth and the Moon are lit by the Sun. Anything lit by a light also casts a shadow — the dark space behind it where its body blocks the light. Out in space, these shadows are huge. When one body’s shadow falls on another, we get an eclipse.
There are two kinds, depending on whose shadow falls on whom.
Solar eclipse — the Moon’s shadow falls on the Earth. Sometimes the Moon comes exactly between the Sun and the Earth. When it does, the Moon blocks the Sun’s light and throws its shadow onto a small patch of the Earth. People standing in that dark patch see the Sun get covered up in the middle of the day. That is a solar eclipse (“solar” means of the Sun). Figure 12.4 below shows how this lines up.
You might wonder: the Moon is far smaller than the Sun, so how can it cover the Sun at all? The trick is distance. The Moon is much closer to us than the Sun. A small thing held close to your eye can hide a big thing far away — just as your thumb held at arm’s length can hide a friend’s whole head across the room. The Moon and the Sun happen to look about the same size in our sky, so the Moon can just cover the Sun.
One safety warning, very important: never look straight at the Sun during a solar eclipse, not even through sunglasses, binoculars, or a telescope. The Sun is strong enough to damage your eyes and even cause blindness. Watch only through proper viewing arranged by a science club or planetarium.
Lunar eclipse — the Earth’s shadow falls on the Moon. Now flip the order. Sometimes the Earth comes exactly between the Sun and the Moon. The Earth then blocks the Sun’s light and throws its shadow into space. When the full Moon moves into the Earth’s shadow, it suddenly goes dark. That is a lunar eclipse (“lunar” means of the Moon). Figure 12.5 below shows this.
A neat way to keep the two straight: in a solar eclipse the Moon hides the Sun from us (Moon in the middle); in a lunar eclipse the Earth hides the Sun from the Moon (Earth in the middle). One simple test — what is in the middle? If the Moon is in the middle, it is a solar eclipse. If the Earth is in the middle, it is a lunar eclipse.
During a total lunar eclipse, the Moon is in the Earth's shadow. Can you safely watch it with your eyes?
Yes. A lunar eclipse is safe to watch directly, because you are only looking at the dimmed Moon, not the Sun. It is the solar eclipse — where you would be looking toward the Sun — that you must never view directly.
Common Mistakes
These are the slippery ideas that trip up almost everyone. Spotting them now will save you marks and confusion.
The Moon makes its own light, like the Sun does.
The Moon glows brightly at night and is the brightest thing in the night sky, so it really looks like a light source switched on in the dark.
The Moon makes no light of its own. It is lit by the Sun, and what we see is sunlight bouncing off the Moon. That is exactly why the Moon shows phases — we are seeing different amounts of the half the Sun lights up. If the Moon made its own light, it would always look fully round.
The Sun really moves across the sky — it goes around the Earth each day.
With your own eyes you clearly see the Sun rise in the East, climb the sky, and set in the West, day after day. It genuinely looks like the Sun is doing the travelling.
The Sun stays put. The Earth spins from West to East, and from a spinning Earth the still Sun only seems to move — just as trees seem to swing past when you turn on a merry-go-round. One spin of the Earth takes about 24 hours, which is why the Sun appears to cross the sky once a day.
The phases of the Moon are caused by the Earth's shadow falling on the Moon.
Both involve the Moon and darkness, so it is tempting to think the curved dark part of the Moon is the Earth's shadow creeping across it.
Ordinary Moon phases have nothing to do with the Earth's shadow. The dark part of the Moon is simply the half the Sun is not lighting at that moment, and we see different amounts of the lit half as the Moon moves around us. The Earth's shadow on the Moon is a separate, rare event — that is a lunar eclipse, not a phase.
Rotation and revolution are just two names for the same motion of the Earth.
Both are the Earth 'going round and round', so the two words feel like they must mean the same thing.
They are different motions. Rotation is the Earth spinning on its own axis, and one spin takes about 24 hours (this gives day and night). Revolution is the Earth travelling all the way around the Sun, and one trip takes about 365 days (this gives the year). The Earth does both at the same time.
Quick Check
Test yourself before moving to the practice problems.
Why do we have day and night on the Earth?
A full, perfectly round Moon means we are seeing:
In which kind of eclipse does the Earth's shadow fall on the Moon?
The Earth completes one revolution around the Sun in about:
Practice Problems
Easy
Name the two motions of the Earth, and say roughly how long each one takes.
The two motions are rotation and revolution.
- Rotation is the Earth spinning on its own axis. One rotation takes about 24 hours (one day and night).
- Revolution is the Earth travelling around the Sun. One revolution takes about 365 days (one year).
Which of these makes its own light: the Sun, the Earth, or the Moon? What about the other two?
Only the Sun makes its own light — it is a star, a giant ball of hot glowing gas.
The Earth and the Moon make no light of their own. They only catch and reflect the Sun’s light. That is why the Moon shines at night — it is bouncing sunlight toward us.
Medium
The Sun appears to rise in the East and set in the West every day. Explain why this happens, even though the Sun is not really moving across the sky.
The Sun stays still. It is the Earth that spins, from West to East, completing one spin in about 24 hours.
Because the Earth turns toward the East, your spot on the Earth swings into the Sun’s light on its eastern side first. So you see the Sun appear in the East — that is sunrise. Hours later, as the Earth keeps turning, your spot swings out of the light on its western side, and the Sun seems to disappear in the West — that is sunset.
So the Sun’s daily journey across the sky is an illusion created by a spinning Earth — like the trees that seem to swing past when you turn on a merry-go-round.
A student says, 'The dark curved part of a half Moon is the shadow of the Earth.' Is the student right? Explain.
No, the student is not right.
The dark part of the Moon during its normal phases is simply the half of the Moon that the Sun is not lighting at that moment. The Sun always lights one half of the Moon; the other half is dark. As the Moon moves around the Earth, we see different amounts of the lit half, which gives the phases.
The Earth’s shadow is not involved in ordinary Moon phases. The Earth’s shadow only falls on the Moon during a rare event called a lunar eclipse, which is different from a phase.
Challenge
Suppose tonight there is a solar eclipse happening somewhere on Earth. Using the positions of the Sun, Moon, and Earth, explain (a) why it can only be seen from a small area, and (b) why it is dangerous to look at directly, while a lunar eclipse is safe.
During a solar eclipse, the order is Sun, then Moon, then Earth, all in a line. The Moon comes between the Sun and the Earth and blocks the Sun’s light.
(a) Why only a small area sees it: The Moon is small compared to the Earth, so the dark shadow it throws onto the Earth is also small — it covers only a little patch of the Earth’s surface. Only people standing inside that small dark patch see the Sun hidden. Everyone outside the patch still sees a normal Sun. (The patch is also moving, because the Earth spins and the Moon moves, so each place sees it for only a few minutes.)
(b) Why a solar eclipse is dangerous but a lunar eclipse is safe: In a solar eclipse you are looking toward the Sun. Even when partly covered, the Sun is strong enough to damage your eyes and even cause blindness, so you must never look at it directly. In a lunar eclipse, the Earth is in the middle and its shadow falls on the Moon. You are only looking at the dimmed Moon, not the Sun, so it is completely safe to watch with your naked eyes.
The simple test: a solar eclipse points your gaze at the Sun (unsafe); a lunar eclipse points your gaze at the Moon (safe).
Summary
- The Sun is a star and makes its own light. The Earth is a planet, and the Moon is the Earth’s natural satellite — neither the Earth nor the Moon makes its own light; both are lit by the Sun.
- Rotation is the Earth spinning on its own axis. One spin takes about 24 hours and causes day and night — the half of the Earth facing the Sun has day, the half turned away has night.
- The Earth spins from West to East, which is why the Sun, Moon, and stars all seem to rise in the East and set in the West. They are not really moving — the Earth is.
- Revolution is the Earth travelling around the Sun. One trip takes about 365 days, which is one year. The tilt of the Earth’s axis, combined with this trip, gives us the seasons.
- The Moon travels around the Earth in about a month. The phases of the Moon happen because the Sun lights only one half of the Moon, and from Earth we see different amounts of that lit half — not because the Moon changes shape.
- A solar eclipse happens when the Moon comes between the Sun and the Earth and its shadow falls on the Earth (never look at it directly).
- A lunar eclipse happens when the Earth comes between the Sun and the Moon and the Earth’s shadow falls on the Moon (safe to watch).
What’s Next
That is the end of this chapter — and it is also the last chapter of your Class 7 Science journey. Well done for making it all the way here.
Take a moment to look back at how far you have come. You began with what science really is, explored substances and metals, followed electricity through a circuit, watched changes happen around you, learnt about your own growing body, and now you have travelled all the way out to the Earth, the Moon, and the Sun. From the tiniest particle to the whole sky — you have looked at it with a curious eye and asked why.
That habit — asking “but why is that true?” and looking for the answer — is the real prize. It will carry you through Class 8 and far beyond. The sky you can now explain with your own words tonight is the same sky that puzzled Rashmika on her cycle ride. Now you are the one who can explain it.
Keep wondering, keep looking up, and revisit any chapter from the chapter list whenever you want to refresh an idea. Congratulations on finishing Class 7 Science.
Frequently Asked Questions
Why do we have day and night on Earth?
The Earth is always spinning on its own axis — one full spin takes 24 hours. The side of the Earth that faces the Sun gets sunlight and experiences day, while the side facing away from the Sun is in darkness and experiences night. As the Earth spins, every place on its surface moves in and out of the sunlight, giving us day and night one after another.
Why does the Moon seem to change its shape every night?
The Moon does not actually change shape. It is always a round ball. What changes is how much of the sunlit side of the Moon we can see from Earth, as the Moon moves around the Earth each month. When we see the fully lit half it is a full Moon; when we see none of it the sky is dark (new Moon); and in between we see crescent, half or gibbous shapes. These are called the phases of the Moon.
What is the difference between the rotation and revolution of the Earth?
Rotation is the Earth spinning on its own axis (like a top), which takes about 24 hours and causes day and night. Revolution is the Earth travelling in its orbit around the Sun, which takes about 365 days (one year). The tilted axis of the Earth during its revolution also causes the seasons.
What is a solar eclipse and how is it different from a lunar eclipse?
A solar eclipse happens when the Moon comes directly between the Earth and the Sun, so the Moon's shadow falls on part of the Earth and blocks sunlight — people in that shadow see the Sun disappear partially or completely. A lunar eclipse happens when the Earth comes between the Sun and the Moon, so the Earth's shadow falls on the Moon, making it go dark or turn reddish. Solar eclipses happen at new Moon, lunar eclipses at full Moon.
Does the Moon make its own light or does it borrow light from the Sun?
The Moon does not make any light of its own. It is a solid rocky body with no glow. The moonlight you see at night is really sunlight that has hit the Moon's surface and bounced off (reflected) towards the Earth. The same is true of the Earth — it does not shine by itself either. Only the Sun, being a star, produces its own light and heat.