Class 6 Science Chapter 12: Beyond Earth
- 5 topics
- 100 MCQs
- 30 short answers
- 15 long answers
- 75 flashcards
- 25 games
- ~160 min read
- English
Joining the Dots in the Night Sky
The patterns people see among the stars are invented, and different cultures invented different ones. That did not stop them being useful enough to steer a ship by.
What you will be able to do
- I can explain what a constellation is and how the modern meaning of the word differs.
- I can name stars and constellations that are easy to pick out, and say what marks them.
- I can explain why the Pole Star is useful for finding direction.
- I can give Indian names for some stars and star patterns.
Key points
Stars shine with their own light, and some of them look brighter than others.
Groups of stars appear to form patterns that look like familiar animals or objects.
Different cultures gave those patterns different names, out of their own stories.
Recognising the patterns let sailors and travellers find direction before compasses.
A constellation now means a defined region of the sky, not just a pattern in it.
The International Astronomical Union fixed 88 of them, covering the whole sky.
Sirius, over in Canis Major, outshines every other star at night.
The Pole Star stays nearly still in the north, which is what makes it useful.
Full explanation
A Sky Worth Looking At
Yangdol and Dorjay are eleven-year-old twins living in a village in Nubra, in Ladakh. They like the mountain peaks and the glaciers, but the night sky is the favourite. The weather there is almost cloudless and there is next to no air or light pollution, so thousands of stars come out clearly.
Stories They Grew Up With
Their elders have always told them about the stars. One story is about caravans passing through Nubra in ancient days, and how certain stars helped them find direction. The twins wonder how far away the stars are and how big, and they look for shapes among them.
Try It Yourself First
Take a photograph of a patch of night sky and join the bright points with lines, the way you would join dots in a drawing. Give each shape a name. Then compare with somebody else. The patterns will not match, the names will not match, and the stories will not match either.
What the Stars Actually Are
Look up on a clear night and there are a great many of them, some bright and some dim. Stars shine with their own light. That is worth saying early, because not everything in the night sky does.
Inventing the Pictures
Watching the night sky was a favourite pastime for our ancestors, and they matched the patterns to animals, to objects, and to characters out of their stories. Many cultures had their own names, built out of their own tales. The shapes were imaginary, and they made the stars easier to recognise.
Not Only for Fun
The patterns had a real job. Before modern technology, and even before the magnetic compass was invented, recognising stars helped people find directions at sea and on land. Sailors and travellers depended on it. It is still kept as a backup method for emergencies.
The Word Changed Meaning
In earlier times a constellation meant a group of stars forming a pattern. That is not quite what it means now. A constellation today is a defined region of the sky, and it often contains one of those patterns. Because the pattern is the noticeable part, people still use the word for the group of stars.
Agreeing on the Boundaries
Different cultures drew the boundaries differently, so in the early twentieth century the International Astronomical Union settled an agreed set. Eighty-eight constellations were officially listed, and between them they divide the entire sky into 88 regions.
Orion the Hunter
Orion is often pictured as a hunter. Three stars in the middle make his belt, and those three are the easiest thing to find in the whole sky. Some people imagine Orion followed by his dog, the constellation Canis Major, battling a bull, the constellation Taurus.
The Brightest Star There Is
Inside Canis Major sits Sirius, and nothing else out at night shines harder. Once you have found Orion it is easy to find, which makes the pair of them a good place to start for anybody learning the sky.
Indian Names for the Same Stars
In Indian astronomy the word nakṣhatra is used for a star or a group of stars. Ārdrā is the star called Betelgeuse, in Orion. Kṛittikā is the group called Pleiades, in Taurus. Aldebaran, another star in Taurus, is known as Rohiṇī.
The Two Dippers
Two more patterns are worth knowing: the Big Dipper and the Little Dipper. The Big Dipper sits inside the constellation Ursa Major, and the Little Dipper inside Ursa Minor. In India it goes by the name Saptarṣiḥi.
The Star That Stays Put
The Pole Star, also called Polaris, is part of the Little Dipper. It appears nearly stationary in the north while everything else wheels about. That is what makes it useful: it shows north to anybody in the Northern hemisphere. In India it is called Dhruva tārā.
Other People, Other Pictures
Beyond the common regional stories, many forest-dwelling communities and tribes of India have their own. Tribes in Central India see four stars of the Big Dipper as a grandmother's cot, with the other three as thieves stealing it. Fishermen on the Konkan coast see those same four as a boat, with the last three as its neck.
The Point Behind All of It
Every one of these pictures is invented, and none of them is wrong. The stars are really there, and the lines are not. What the patterns do is make a crowded sky memorable, which is the whole reason anybody could ever steer by them.
Concepts and terms
Once it meant a group of stars forming a pattern. Today it means a defined region of the sky, and the sky is divided into 88 of them. The patterns are still the most noticeable thing in each region.
The lines are imaginary, so what anybody sees depends on the stories they grew up with. Many cultures named the same stars differently, which is why an agreed set of boundaries was needed.
Before the magnetic compass, sailors and travellers used star patterns to find their way at sea and on land. It is still kept as a backup method for emergencies.
Part of the Little Dipper, and nearly stationary in the north. Everything else wheels about while it stays put, so it marks north for anybody in the Northern hemisphere.
Watch out!
Myth or fact?
Case Study
Scenario:
A class joins the dots of a star group on paper and each child names the shape they see. The answers differ: a plough, a bear, a question mark, a saucepan. Meera says most of them must be wrong. Their teacher explains that the same group has carried different names in different parts of the world for centuries. The class works out why. The stars are fixed and the picture is not, because joining dots is something people do rather than something the sky contains. Sana notes that the names still work, because any of them lets one person point out the group to another.
Analysis:
Four names and one group of stars. The pattern is a human aid to finding them again, so differing names cost nothing.
Activity:
Join the stars of one group into your own shape and name it. Then find out two names other cultures have used for it.
Try it at home
Title:
Invent your own constellations
Safety:
Nothing here is dangerous. If you go on to look at the real sky, do it with an adult and never go to a dark open spot at night by yourself.
What You Notice:
No two sheets match, and that is the finding. The same dots became different animals and different objects, and each person can see their own shape instantly and nobody else's. That is exactly what happened across the world, which is why an agreed set of boundaries had to be settled in the end.
Did you know
The International Astronomical Union settled the boundaries in the early twentieth century, dividing the sky into 88 regions.
Tribes in Central India see four stars of the Big Dipper as a grandmother's cot, with the other three as thieves stealing it.
Along the Konkan coast the fishermen make those same four into a boat, and the remaining three into its neck.
Memory tricks
Finding Orion — Three bright stars in a short straight row — That is the belt, and it is the easiest way in.
What the Pole Star is for — Everything turns, it does not — Staying still is exactly what makes it useful.
Practice MCQs with answers (20)
- A shine with their own light
- B reflect light from the Earth
- C are lit by street lamps
- D reflect light from the Moon
Show answer
Why:
Not everything in the night sky can say that.
- A one of them has better eyesight
- B the lines are imaginary, so each person draws their own
- C the stars must have moved about between the two drawings
- D only one of the shapes is correct
Show answer
Why:
Different cultures did exactly the same thing.
- A measuring how far away a star is
- B telling the time of year
- C finding direction at sea and on land
- D predicting the weather ahead
Show answer
Why:
Sailors and travellers depended on it.
- A a star that changes brightness
- B a pattern nobody has named yet
- C any group of three bright stars
- D a defined region of the sky
Show answer
Why:
The pattern is usually the noticeable part of it.
- A 12
- B 88
- C 50
- D 360
Show answer
Why:
Between them they divide the entire sky.
- A three bright stars in a short straight row
- B a wide open circle of very dim stars indeed
- C a red star on its own
- D its brightest single star
Show answer
Why:
Those three make the belt of the hunter.
- A Taurus
- B Canis Major
- C Ursa Minor
- D Pleiades
Show answer
Why:
And the bull he battles is Taurus.
- A the Pole Star
- B Betelgeuse
- C Sirius
- D Aldebaran
Show answer
Why:
It lies in Canis Major.
- A the Moon only
- B the whole of the sky at once
- C a planet only
- D a star, or a group of stars
Show answer
Why:
The word covers both a single star and a group.
- A Betelgeuse
- B Aldebaran
- C Polaris
- D Sirius
Show answer
Why:
It lies in Orion.
- A just one single very bright star on its own
- B the group of stars called the Pleiades
- C the Big Dipper
- D the Pole Star
Show answer
Why:
It lies in Taurus.
- A Canis Major
- B Ursa Minor
- C Ursa Major
- D Orion
Show answer
Why:
The Little Dipper lies in Ursa Minor.
- A rises exactly at midnight
- B changes its colour right through the night
- C is the brightest star of all
- D appears nearly stationary in the north
Show answer
Why:
Everything else in the sky wheels about.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is the distinction. You use the Big Dipper to find the Pole Star, which is why the two get confused.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: the Big Dipper lies in Ursa Major, and it is the Pole Star that sits in the Little Dipper.
- A a bull
- B a boat
- C a grandmother's cot
- D a hunter holding a bow
Show answer
Why:
And the other three stars as thieves stealing it.
- A a plough
- B a kite
- C a cot
- D a boat
Show answer
Why:
The last three stars are its neck.
- A different cultures drew them in different ways
- B there were too few stars to share
- C nobody could ever see them at all clearly enough
- D the stars kept moving about
Show answer
Why:
An agreed set was settled in the early twentieth century.
- A right, they are faint but visible
- B wrong, the lines are drawn only to help us identify
- C right, they appear on clear nights
- D wrong, because there are no stars out there at all ever
Show answer
Why:
The stars are real and the lines are not.
- A they tell us how far each star is
- B no other method of navigation exists anywhere in the whole world at all now
- C they predict what the weather will do
- D they make a crowded sky memorable, and serve as a backup for emergencies
Show answer
Why:
Useful, even with modern technology about.
Important short questions (6)
A defined region of the sky, which often contains a pattern of stars. The International Astronomical Union listed 88 of them.
The lines joining stars are imaginary. Each culture drew its own out of its own stories, and named them accordingly.
It is pictured as a hunter. Three bright stars in a short straight row near its middle form his belt, and those are the easiest to spot.
It appears nearly stationary in the north while the rest of the sky wheels about. So it marks north in the Northern hemisphere.
Saptarṣiḥi is the Big Dipper. Dhruva tārā is the Pole Star. Kṛittikā is the Pleiades.
Different cultures marked the boundaries in different ways. An agreed set fixed one map that everybody could use.
Long answer questions (3)
In earlier times a constellation meant a group of stars that appeared to form a pattern. That worked well enough for storytelling, but it was not precise. Different cultures joined the same stars into different shapes and drew the boundaries between them in different ways, so no two sky maps agreed. In the early twentieth century the International Astronomical Union settled the matter by defining regions instead. Eighty-eight constellations were officially listed, and they divide the entire sky between them. The word is still used for the pattern, because the pattern is the part anybody notices.
Orion is pictured as a hunter. Three bright stars in a short straight row near his middle make his belt. Those three are the easiest thing to recognise in the whole sky, which is why people start there. Following him is his dog, the constellation Canis Major. Inside it lies Sirius, the brightest star in the night sky. Ahead of him is the bull, the constellation Taurus, which holds the Pleiades and the star Aldebaran. One story links three constellations, and finding the belt leads to all of them.
The lines are not in the sky at all. They are drawn by people, which is why two friends joining the same stars will produce different shapes. It is also why different cultures named the same stars from their own stories. Tribes in Central India see a cot in four stars of the Big Dipper, and Konkan fishermen see a boat in the same four. None of that makes the patterns useless. A shape is easy to remember and a scatter of points is not. So the patterns let people recognise stars night after night, and that is what let sailors find direction before the magnetic compass existed.
Flashcards (15)
Play this topic
- Star, pattern, or region?
- Indian name and English name
- Fact or myth?
- Finding your way around Orion
- Night sky sprint
Why the City Hides the Stars
The same sky is overhead in a city and in a village, and one of them shows a handful of stars and the other shows thousands. Most of the difference is our own lighting.
What you will be able to do
- I can explain what light pollution is and why it hides stars.
- I can say why a star or constellation is not visible from everywhere on every night.
- I can plan a night sky watching session safely and sensibly.
- I can describe how to find the Pole Star from the Big Dipper, and Sirius from Orion.
Key points
Given a clear night with no cloud about, a great many stars come out.
In a big city only a few are seen, because of light pollution, smoke and dust.
Light pollution means artificial light thrown about in excess after dark.
Tall buildings and trees around a house block the view as well.
Open unlit ground gives the best view, and artificial light at night keeps increasing.
No single place, and no single night, shows every star and every constellation.
Sky mapping apps and online resources show when and where to look from your own location.
Never set off for unlit open ground after dark without an adult along.
Full explanation
The Same Sky, Two Views
Give it a clear night with no cloud about and a great many stars come out. Yangdol and Dorjay in Nubra see thousands. Somebody standing in a big city on the same night may see only a handful. Nothing has changed overhead, so the difference has to be down here.
What Is in the Way
Three things account for most of it: light pollution, smoke and dust. Villages, and any other place with fewer lamps burning, show a good many more stars than that. That is not a small effect, and it is the reason Nubra is such a good place to look.
The Term Itself
Light pollution is the name for artificial light thrown about in excess after dark. Street lamps, hoardings, floodlights and windows all add to it. None of it was meant to reach the sky, and a great deal of it does.
And the Ordinary Obstacles
There is a simpler problem as well. A house often has tall buildings and trees all round it, and those block the view whatever the sky is doing. Between them, the two difficulties give the same conclusion. Open unlit ground gives much the best view of a night sky.
It Is Getting Worse
Light pollution is growing sharply across the world. That reduces our ability to enjoy objects in the night sky, and also our ability to study them, which matters to people doing research.
Places Kept Dark on Purpose
Reserves and parks for dark skies have been set aside in several countries. Lamplight inside a reserve is held down, so that dark skies survive for research. There are also organisations working to educate people about reducing it.
The Sky Is Not the Same Everywhere
Even on a perfect night, no single spot on Earth and no single night of the year shows every one of them. The Pole Star, for example, cannot be seen at all from the southern half of the world.
So You Need Two Things
Naming one of them takes two things: the shape it makes, and the part of the sky it will be in. Pictures of the constellations take care of the first. The second depends on your location and the date.
Apps Can Answer the Second
Sky mapping apps can be downloaded onto a mobile phone, and there are online resources too. They tell you when a star or constellation will be visible from your location, and in which portion of the sky. Sky Map is one handy example. Stellarium is another, and its computer version is free to download.
Getting Ready
With an adult guiding you, pick out unlit open ground well clear of lamps, of tall buildings and of trees. Use an app to choose a date and time that suits what you want to identify. Pick a night with neither the Moon nor clouds about, especially if the Pole Star is on your list, because it is not very bright.
What to Carry
A mobile app with a sky map, or printed pictures of whichever constellations you mean to look at. A magnetic compass for directions. A notebook to note or draw what you see. And an adult, without exception: never go to a dark open spot at night by yourself.
The Half Hour That Matters
After reaching the place, wait about half an hour before deciding what is visible. Your eyes need that long to adjust to the darkness, and the sky you see afterwards is not the sky you saw on arrival.
Finding the Big Dipper
In summer, at about nine o'clock at night, face north and look up. The Big Dipper is a large, obvious pattern and it is usually the first thing anybody picks out.
And Then the Pole Star
Take the pair sitting at the outer end of the cup, and picture a straight line drawn through the two of them going north. Travel about five times their separation along that line and you reach another star, not a very bright one. That is the Pole Star.
Orion, and Sirius From It
In India Orion is best viewed from December to April, after sunset. Find the three bright stars in a short straight row around its middle first, because that is the easiest way in. Then imagine a line through those three and follow it towards the east. It leads to Sirius, which is very bright and lies close by.
Concepts and terms
Artificial light thrown about in excess after dark. It washes the fainter stars out, so a city sky shows only a few. Smoke and dust in the air add to the same effect.
No single spot on Earth, and no single night of the year, shows every one of them. The Pole Star, for instance, cannot be seen at all from the southern half of the world.
Tracts set aside in several countries where lamplight is held down, so that dark skies survive for research. Organisations also work to teach people about cutting it.
After reaching a dark place, wait about half an hour before judging the sky. Eyes take that long to adapt, and the difference in what you can see afterwards is large.
Watch out!
Myth or fact?
Case Study
Scenario:
Two cousins compare the night sky. One lives in a city and counts about twenty stars. The other is visiting a village and counts far more than she can keep track of. The city cousin decides the village sky is different. Their teacher asks whether the stars could have moved. They could not, in one night, between two places so close. The class lists what the city has that the village does not: street lights, lit windows, smoke and dust. All of them brighten the sky itself. The children see that the stars were above the city too, and what changed was the background they had to be seen against.
Analysis:
Both counts were honest and the sky was the same. A brighter background hides faint stars without removing any of them.
Activity:
Count the stars you can see in ten minutes from your home. Note the nearest lights, then compare with a darker spot if you can.
Try it at home
Title:
Count the stars from two places
Safety:
Never go to a dark open spot at night without an adult. Stay away from roads, water and rooftop edges, and tell somebody at home where you are going.
What You Notice:
The second number is always larger, and usually several times larger. Two separate things caused that: the lights went away, and your eyes had half an hour to adjust. Try it again without the wait to see how much of the difference each one accounts for.
Did you know
Dark sky reserves keep their lighting under control so that research can go on, and India has one at Hanle in Ladakh, where the air is thin, dry and very clear.
Sky Map is a handy app for identifying stars, constellations and planets from a mobile phone.
The computer version of Stellarium is free to download and has many features.
Memory tricks
Finding the Pole Star — Two stars of the cup, then five times the gap — The line through them leads north to a star that is not very bright.
Finding Sirius — Along the belt, and east — The three middle stars of Orion point the way.
Practice MCQs with answers (20)
- A artificial light in excess after dark
- B smoke rising from a factory
- C dust blowing about in the air
- D sunlight that is too strong by day
Show answer
Why:
Street lamps and floodlights all add to it.
- A a real difference between the stars over each of them
- B light pollution, along with smoke and dust
- C the Earth turning faster over a city than over a village
- D cloud that forms over cities and nowhere else
Show answer
Why:
Nothing has changed overhead.
- A his own balcony must face the wrong way entirely
- B the stars are behind the Moon
- C tall buildings and trees are blocking his view
- D stars are not out that early
Show answer
Why:
And the street lights are not helping either.
- A under a large tree
- B a lit balcony facing north
- C a rooftop in the middle of a city
- D an open dark area away from lights
Show answer
Why:
Both of the usual problems point the same way.
- A control light pollution and preserve dark skies
- B store large telescopes safely through the night
- C grow plants that will only come up in darkness
- D keep visitors away from the countryside at night
Show answer
Why:
Research depends on them.
- A yes, on any clear night
- B no, it depends on the place and the date
- C yes, but only in summer
- D no, only the Sun can be seen anywhere
Show answer
Why:
The Pole Star is one clear example.
- A the northern hemisphere
- B India
- C the southern hemisphere
- D any village
Show answer
Why:
Which follows from what it marks.
- A how old it is
- B how many stars it holds exactly
- C how far away it is
- D where to look for it in the sky
Show answer
Why:
The second part depends on your location and the date.
- A apps that help identify objects in the sky
- B two large telescopes
- C two dark sky reserves
- D the names of two of the main constellations
Show answer
Why:
The computer version of one is free to download.
- A the stars need a while before they come out
- B your eyes need that long to adjust to the darkness
- C the sky always turns clearer once midnight has gone
- D a sky app takes about that long to finish loading
Show answer
Why:
What you see afterwards is quite different.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is what gets in the way. The Pole Star is not a bright star, and a lit sky washes it out.
- A east
- B west
- C south
- D north
Show answer
Why:
Around nine in the evening is a good time.
- A the two stars at the end of the cup
- B the brightest star only
- C the whole pattern at once
- D the three stars making up the handle
Show answer
Why:
About five times the gap between those two.
- A twice the distance between the two stars
- B five times the distance between the two stars
- C half the distance between them
- D ten times the distance between them
Show answer
Why:
And it is not a very bright star when you get there.
- A at midday in any month
- B from June to September, before sunrise
- C from December to April, after sunset
- D only during the monsoon
Show answer
Why:
Look for the three belt stars first.
- A north
- B south
- C west
- D east
Show answer
Why:
Sirius is very bright and lies close by.
- A a bright torch pointed upwards
- B a pair of sunglasses
- C a loudspeaker
- D an adult
Show answer
Why:
Never go to a dark open spot at night alone.
- A a ruler
- B a magnetic compass
- C a thermometer
- D a weighing scale
Show answer
Why:
A notebook for observations is useful too.
- A found only in villages
- B slowly disappearing
- C growing sharply
- D exactly the same as a century ago
Show answer
Why:
Which reduces our ability to enjoy and study the sky.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: the Pole Star stands over the north pole of the sky and never rises for a southern observer.
Important short questions (6)
Artificial light in excess after dark. It washes the fainter stars out, so far fewer of them are visible.
Villages have less light pollution. Cities also have more smoke and dust, and tall buildings that block the view.
A magnetic compass, for finding directions. A notebook, for noting or drawing what you observe.
Your eyes need that long to adjust to the darkness. Afterwards you can see the night sky far better.
Take the two stars at the end of the cup and imagine a line through them running north. About five times their gap along it lies the Pole Star.
The Pole Star is not very bright. Moonlight, and cloud, would make it hard to pick out at all.
Long answer questions (3)
Nothing about the sky differs between them, so the reasons are all on the ground. The city has light pollution, which means artificial light in excess after dark, and that washes the fainter stars out. It usually has more smoke and dust in the air as well. On top of that, tall buildings and trees round a house block whatever is left. The village has far less of all this, so a good many more stars show. That is why open unlit ground gives much the best view, and why reserves for dark skies are set aside.
Beforehand, work with an adult to pick a dark open area. It should be away from lamps, tall buildings and trees. Use a sky app to choose a date and time to suit what you want to identify. Pick a night with neither the Moon nor clouds if the Pole Star is on the list, since it is not very bright. Carry a sky map on a phone, or printed pictures of the constellations. Take a magnetic compass for directions and a notebook. On the night, go with an adult, allowing about half an hour on arrival for your eyes to adjust before judging what is visible.
Start with the Big Dipper, looking north in the early part of a summer night. Take the two stars at the end of its cup and imagine a straight line through them going north. Follow that line about five times the distance between those two stars and you reach the Pole Star, a faint thing to be looking for. For Sirius, start with Orion, best viewed in India from December to April after sunset. Find the three bright stars in a short straight row at its middle, then imagine a line through them and follow it towards the east. That leads to Sirius, which is very bright.
Flashcards (15)
Play this topic
- Helps or hinders sky watching?
- Term and meaning
- Fact or myth?
- Planning a night of sky watching
- Sky watching sprint
A Star Close Enough to Feel
The Sun is an ordinary star that happens to be nearby, and that single fact explains why it fills the sky while the others are points, and why they vanish by day.
What you will be able to do
- I can explain why the Sun looks so much bigger than any other star.
- I can say what an astronomical unit is and use it to compare distances.
- I can explain why other stars cannot be seen during the daytime.
- I can define a planet, and distinguish rotation from revolution.
Key points
The Sun is a star, and it is the star closest to us.
It is a fiercely hot ball of gases, roughly spherical, pouring out an enormous quantity of energy.
The distance from the Earth to the Sun is about 150 million kilometres.
That distance is called one astronomical unit, written au.
Measured across, the Sun is roughly a hundred times the Earth, and still looks small.
Other stars look like points because they are very much farther away.
The extreme brightness of the Sun makes other stars impossible to see by day.
A planet is a large body, close to spherical, travelling round the Sun.
Full explanation
Which Star Is Nearest?
Of all the stars anybody can see, one is far closer than the rest. It is the Sun. That one sentence is the key to most of this topic, because nearly everything odd about the Sun turns out to be a consequence of how near it is.
What the Sun Is Made Of
The Sun is a ball of gases, roughly spherical and fiercely hot. The quantity of energy it pours out is enormous, and that is why it glows so. Nothing about that description is unusual for a star, which is exactly the point.
What It Does for Us
The Sun produces heat and light, and it is the chief supplier of energy here. Its warmth holds this planet at a temperature where life is possible at all. Sunlight lets plants grow, and plants provide food and oxygen to animals, ourselves included.
And Rather More Than That
The Sun is responsible for climate, for seasons, for weather, for the water cycle and for winds. Every one of those matters for keeping life going on Earth. It is hard to name anything about the surface of this planet that the Sun has no hand in.
A Deity in Most Places
As the brightest thing overhead, and the supplier of warmth and light here, it was made a deity in most ancient civilizations. In India it is worshipped as Sūrya.
How Far Away
The Earth sits about 150 million kilometres from the Sun. That number is awkward to use, so a shorter unit exists. One astronomical unit, written au, is roughly how far the Earth sits from the Sun, and it is handy for talking about distances inside the solar system.
How Big
Measured across, the Sun comes to roughly a hundred times the Earth. Set the two side by side and the Earth would be a speck. And still the Sun looks small in the sky, because it is so very far away.
The Question Worth Asking
Our Sun is a star. So why does it appear big and light up the whole sky? Why do the other stars look like bright dots at night, and vanish altogether during the day? It is a fair question, and the answer is short.
Distance Does All of It
Ours sits far nearer than any of the others, so it shows up far larger than they do. The rest lie so far off that they appear as points, even though some of them are much bigger than our Sun. Apparent size is not real size.
Why the Day Hides Them
During daytime the Sun is extremely bright, and that brightness makes it impossible to view other stars from Earth. They have not gone anywhere. They are simply lost in the glare, in the same way a candle is lost beside a floodlight.
How Far the Next One Is
Next nearest after ours is Proxima Centauri, lying some 269000 au off. Put another way, it sits 269000 times as far from us as ours does. Nothing about the night sky looks that far away, which is part of why the sky is deceptive.
More Than Stars Up There
Stars are not the only objects in the sky. This planet, some of those other objects, and the Sun make up between them what we call the Solar System. Most of them travel round the Sun, and travelling round it in that way is what the word revolution names.
What a Planet Is
A planet is a large body, near enough round in shape, that travels about the Sun. The Earth counts as one, on exactly that ground. A single trip round takes it close to a year, and other planets are doing the same.
The Other Motion
All the while it travels round the Sun, the Earth is turning on its own axis as well. One full turn takes about 24 hours, and that is what we call a day. Like the Earth, other planets rotate about their own axes while they revolve.
Two Motions, Two Units of Time
It is worth keeping the two apart. The spin about the axis gives us the day. The journey around the Sun gives us the year. Both are going on at once, all the time, and neither can be felt from where we stand.
Concepts and terms
A fiercely hot ball of gases, roughly spherical, pouring out an enormous quantity of energy, which is why it glows so. Its only special feature is that it is the star closest to us.
Roughly how far the Earth sits from the Sun, about 150 million kilometres. It is a useful unit for stating distances inside the solar system, and it is written au.
Travelling round the Sun goes by the name revolution. Separately, the Earth spins about its own axis, and one full turn takes about 24 hours, which is a day.
A large body, near enough spherical, which travels round the Sun. The Earth is one, and a single trip round takes it close to a year.
Watch out!
Myth or fact?
Case Study
Scenario:
A class is told the Sun is a star. Rohit objects that stars are small points and the Sun is a huge disc. Their teacher asks what else looks small at a distance. A lamp at the end of a long street, or a person across a field. The class works out that apparent size depends on distance as well as actual size. The Sun looks large because it is the nearest star, and the others are so far away that even a vast ball of gas arrives as a point. Meera puts it plainly. The Sun is not a different kind of object, it is the same kind of object nearby.
Analysis:
Rohit was comparing how things look rather than what they are. The Sun's disc is the nearest example of what every point in the sky actually is.
Activity:
Hold a coin at arm's length and then close to your eye. Draw both and write one sentence on apparent size and distance.
Try it at home
Title:
Make distance change apparent size
Safety:
Choose a path with no traffic, and agree on a stopping point before you start. Do not do this near a road or a staircase.
What You Notice:
The two coins are the same size throughout, and one of them looks half or a quarter as wide. That is the whole of the argument about the Sun. Now imagine the second coin at 269000 times the distance of the first and you have the situation with Proxima Centauri.
Did you know
Proxima Centauri is the next one along after ours, some 269000 au off, which is to say 269000 times as far as ours sits.
As the brightest thing overhead, and the supplier of warmth and light, it was made a deity in most ancient civilizations, and India worships it as Sūrya.
Other planets rotate about their own axes too, while they revolve around the Sun.
Memory tricks
Two different motions — Spin for a day, orbit for a year — Rotation gives the day; revolution gives the year.
Why the Sun looks big — Not bigger, just nearer — Some stars are much larger and still look like dots.
Practice MCQs with answers (20)
- A a star
- B a comet
- C a satellite of the Earth
- D a planet
Show answer
Why:
And it is the one closest to us.
- A a solid ball of rock
- B an extremely hot spherical ball of gases
- C a flat round disc made out of burning wood
- D a very large cloud of dust
Show answer
Why:
And it gives out a huge amount of energy.
- A 150 billion kilometres
- B 150 thousand kilometres
- C 150 million kilometres
- D 1500 kilometres
Show answer
Why:
That distance has a unit named after it.
- A the distance out to the very nearest star only
- B the width of the solar system
- C the diameter of the Earth
- D the distance between the Sun and the Earth
Show answer
Why:
It is written au.
- A 10
- B 100
- C 2
- D 1000
Show answer
Why:
And it still looks small from here.
- A wrong, some stars that look like dots are much bigger
- B right, because all of the other stars are really quite tiny
- C wrong, because the Sun is not a star
- D right, nothing is bigger than the Sun
Show answer
Why:
Apparent size is not real size.
- A much smaller than the Sun in every case
- B very much farther away than the Sun
- C covered by dust clouds
- D not really giving out any light
Show answer
Why:
Distance does all of it.
- A clouds always cover them by day
- B they move to the other side of the Earth
- C the extreme brightness of the Sun hides them
- D they stop shining while it is light
Show answer
Why:
Like a candle beside a floodlight.
- A the star Betelgeuse
- B Polaris
- C Sirius
- D Proxima Centauri
Show answer
Why:
About 269000 au away.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is what an astronomical unit is for. Counting in au keeps the numbers small enough to picture.
- A rotation
- B revolution
- C reflection
- D radiation
Show answer
Why:
Most objects in the solar system do it.
- A any object which happens to give out a light of its very own
- B any bright object seen at night
- C a large body, roughly spherical, travelling round the Sun
- D a star that has stopped shining
Show answer
Why:
The Earth fits that description.
- A day
- B sunrise
- C rotation
- D revolution
Show answer
Why:
That is where the year comes from.
- A 24 hours
- B one month
- C one week
- D one year
Show answer
Why:
And that is what we call a day.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: rotation gives the day, and one revolution round the Sun comes close to a year.
- A blocks out all of the other stars quite permanently from our view
- B stops the Earth from rotating
- C drives climate, seasons, weather, the water cycle and winds
- D prevents plants from growing
Show answer
Why:
Hard to find anything it has no hand in.
- A it keeps their eyes working
- B it makes them move faster
- C the animals simply eat the sunlight directly themselves
- D it lets plants grow, and plants give food and oxygen
Show answer
Why:
The link runs through the plants.
- A Sūrya
- B Dhruva
- C Mangala
- D Chandra
Show answer
Why:
Most ancient civilizations raised it to the status of a deity.
- A only the Earth
- B other planets too
- C only the Moon
- D nothing besides the Sun
Show answer
Why:
It is a general pattern, not a special case.
- A it only ever shines out in the one direction
- B it is not really bigger
- C it has gone out recently
- D it is enormously farther away than the Sun
Show answer
Why:
Apparent size depends on distance.
Important short questions (6)
It is a star, and the one closest to us. A fiercely hot ball of gases, roughly spherical, pouring out an enormous quantity of energy.
Roughly how far the Earth sits from the Sun, about 150 million kilometres. It makes distances inside the solar system easier to state.
It is much closer to us. Other stars are so far away that they appear as points, even when they are larger than the Sun.
The Sun is extremely bright by day. That brightness makes it impossible to view other stars from the Earth.
Rotation is the spin about its own axis, taking about 24 hours. Revolution is the movement around the Sun, taking nearly a year.
A large body, roughly spherical, travelling round the Sun. The Earth does exactly that, so it qualifies.
Long answer questions (3)
The difference is distance and nothing else. The Sun is the star closest to us, so it appears much bigger than any other and lights up the whole sky. The other stars are very much farther away, so they appear as points, even though some of them are much bigger than ours. Next nearest after it comes Proxima Centauri, some 269000 au off, or 269000 times as far as ours sits. During the daytime the extreme brightness of the Sun makes it impossible to view any of them, so they seem to disappear. They have not moved at all.
The Earth does two separate things at once. It travels round the Sun, and that travelling round is what the word revolution names. One complete circuit takes nearly one year, which is where the year comes from. At the same time it rotates about its own axis, and one full rotation takes about 24 hours, which is what we call a day. Neither motion can be felt from where we stand. Other planets do exactly the same, rotating about their own axes while revolving around the Sun.
The Sun produces heat and light, and it is the chief supplier of energy here. The warmth it gives holds this planet at a temperature where life can exist. Sunlight lets plants grow, and those plants provide food and oxygen to animals, ourselves included. The Sun is also responsible for climate, for seasons, for weather, for the water cycle and for winds. Every one of those matters for sustaining life. That is why most ancient civilizations raised it to the status of a deity, and why in India it is worshipped as Sūrya.
Flashcards (15)
Play this topic
- Rotation or revolution?
- Term and meaning
- Fact or myth?
- Why a huge star can look like a dot
- Sun sprint
Eight Planets, and One That Stopped Being One
Four small rocky worlds, then a belt of rubble, then four giants of gas and ice. And one object that was a planet for seventy-six years until the definition was tightened.
What you will be able to do
- I can name the eight planets in order of increasing distance from the Sun.
- I can describe what the inner four have in common and how the outer four differ.
- I can explain why Venus is hotter than Mercury even though it is farther out.
- I can say how a planet can be told from a star in the night sky.
Key points
Counting outward: Mercury first, then Venus and Earth, then Mars, Jupiter, Saturn, and Uranus with Neptune last.
The inner four are smaller, and have solid surfaces with rocks on them.
The four outermost are much larger than the Earth, and are mostly gas and ice.
Each giant planet carries large flat rings of dust particles and rocky material.
A planet draws nearly all its energy from the Sun, so as a rule the farther out, the colder.
An atmosphere can trap heat, which is why Venus is hotter than Mercury.
Pluto and similar small objects are now called dwarf planets, after a 2006 decision.
Stars appear to twinkle a lot, and planets do not, which is how you tell them apart.
Full explanation
More Objects Than Stars
The Earth, some other objects besides, and the Sun make up our Solar System between them. Most of those objects travel round the Sun. Among them are the planets, and there are eight of them.
The Eight, in Order
Outward from the Sun they run Mercury, Venus, Earth and Mars, then Jupiter, Saturn, Uranus and Neptune. Learning them in that order is worth the small effort, because almost every other fact in this topic hangs off the position of a planet in the list.
Names From Closer to Home
Various names have been used across India from ancient times for those planets that can be seen without any instrument. Budha is Mercury, Śhukra is Venus, Pṛithvī is Earth, Mangala is Mars, Bṛihaspati or Guru is Jupiter, and Śhani is Saturn.
The Inner Four
Mercury, Venus, Earth and Mars are the ones nearest the Sun, and they are smaller in size. All four have solid surfaces with rocks on them. If you could stand anywhere, it would be on one of these.
The One That Is Not a Star
Venus is generally spotted blazing away at dawn or at dusk, and the names it picked up are Morning Star and Evening Star. It is not a star. The naming is old, and it has stuck despite being wrong.
Two Planets With Colour Names
Mars carries the name Red Planet for the plain reason that it looks red, and it looks red because its soil has a reddish colour. Water covers a great part of the surface of the Earth, so from outside it looks blue, and that is where the name Blue Planet came from.
The Outer Four
Jupiter, Saturn, Uranus and Neptune are the four outermost, and they are much larger compared to the Earth. They are mostly made of gas and ice rather than rock. Each of these giants wears a broad flat hoop, built out of dust grains and of stony matter.
The Belt in Between
Between the inner four and the outer four lies the asteroid belt, marked on any drawing of the solar system. It sits between the paths of Mars and Jupiter, and it is a useful landmark for remembering where the rocky planets stop.
Cold Farther Out, Mostly
A planet draws nearly all its energy from the Sun. So as a rule, the farther out one lies, the colder it is. That is a sensible rule and it holds for most of the list.
The Exception Worth Knowing
Where a planet carries an atmosphere, that atmosphere holds heat in, and the temperature can shift a great deal as a result. Venus is the example. It is farther from the Sun than Mercury is, and it is hotter. Distance sets the general trend, and an atmosphere can override it.
Pluto, and a Change of Rules
Farther out than Neptune lies Pluto, which travels round the Sun as well. It is smaller than the Moon of the Earth. On its discovery it was counted a planet of the Solar System, and for a long time everybody learnt nine.
Why the Number Changed
Later, more small objects like Pluto were discovered. In 2006 the International Astronomical Union redefined the requirements for an object to be called a planet. Under that definition these smaller objects, Pluto among them, are now called dwarf planets. Nothing about Pluto changed. The definition did.
Which Ones You Can See
Among the planets Venus is the easiest to identify, because it is very bright. Only the Sun and the Moon outshine it. Mercury, Mars, Jupiter and Saturn can be picked out without any instrument as well. All of them sit so far off that they show as shining dots, exactly as the stars do.
So How Do You Tell?
If a planet and a star both look like dots, something has to separate them. Stars appear to twinkle a great deal, and planets do not. Watch a dot steadily for a while: if it twinkles it is a star, and if it shines steadily it is probably a planet.
Looking Closer
A great deal up there can be seen without any instrument, but binoculars or a telescope make them brighter and larger. A telescope also shows many dim objects that cannot be seen directly at all. Higher education institutions, amateur astronomy clubs, museums and planetariums all organise sky watching events, and that is usually the easiest way to get a look through one.
Concepts and terms
The four nearest the Sun are smaller and have solid surfaces with rocks on them. The four outermost are much larger than the Earth, mostly made of gas and ice, and each carries rings.
A planet draws nearly all its energy from the Sun, so as a rule the farther out it lies the colder it is. An atmosphere holds heat in and can upset that, which is why Venus runs hotter than Mercury.
Pluto was called a planet when it was discovered. After more small objects like it were found, the International Astronomical Union redefined the requirements in 2006, and these are now dwarf planets.
Both look like shining dots, because planets are far away too. Stars appear to twinkle a lot, while planets do not, and that is the practical test.
Watch out!
Myth or fact?
Case Study
Scenario:
A class lists the eight planets in order from the Sun and then sorts them by size. The two orders do not match. Imran says one of the lists must be wrong. Their teacher asks the class to look for a pattern across both. The inner four are the smaller ones and have solid rocky surfaces, and the outer four are the larger ones. So the orders are not in conflict; they agree about a split. The class then asks why Pluto is absent and learns that the definition of a planet was revised. Nita writes the point down. Nothing about Pluto changed, the rule it was being measured against did.
Analysis:
Two different orders describing one arrangement, and together they show a split rather than a contradiction. Pluto moved because the definition did.
Activity:
Write the eight planets in order of distance, then in order of size. Mark where the two lists split and what the split tells you.
Try it at home
Title:
Catch Venus at dusk
Safety:
Never look at the Sun, even as it sets. Go with an adult, stay away from roads and rooftop edges, and come back before it is fully dark.
What You Notice:
The first thing out is usually Venus, and it does not twinkle. That steadiness is the test, and having a star in view at the same time makes it obvious. Comparing two evenings shows something else: the position against the horizon shifts a little from night to night, which a star would not do.
Did you know
Indian names for those planets that can be seen without any instrument include Budha, Śhukra, Pṛithvī, Mangala, Bṛihaspati or Guru, and Śhani.
Only the Sun and the Moon outshine Venus.
Pluto is smaller than the Earth's Moon, and lies farther out than Neptune.
Memory tricks
The eight in order — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — Small and rocky first, then the belt, then the giants.
Star or planet? — Twinkle means star — Planets shine steadily.
Practice MCQs with answers (20)
- A Mercury, Venus, Earth and Mars, then Jupiter, Saturn, Uranus and Neptune
- B Earth, Mars, Mercury and Venus, and then Jupiter, Uranus, Saturn and Neptune
- C Mercury, Earth, Venus, Mars, Saturn, Jupiter, Uranus, Neptune
- D Venus, Mercury, Earth, Mars, Saturn, Jupiter, Neptune, Uranus
Show answer
Why:
Four small ones, then four giants.
- A a great deal larger than the Earth is
- B smaller, with solid rocky surfaces
- C made mostly of gas and ice
- D all surrounded by rings
Show answer
Why:
You could stand on any of them.
- A far too small to have been discovered yet at all
- B smaller than the Earth and rocky
- C much larger than the Earth, mostly gas and ice
- D all the same size as the Earth
Show answer
Why:
And each of them carries rings.
- A pure water only
- B gas escaping from the planet
- C solid metal sheets
- D dust particles and rocky material
Show answer
Why:
Large, flat, ring-like structures.
- A a planet draws nearly all its energy from the Sun
- B they spin more slowly on their axes
- C they have no rings to keep them warm
- D they are a great deal older than the inner planets
Show answer
Why:
A general rule, with a famous exception.
- A Venus spins much faster
- B Its atmosphere traps heat
- C Mercury is made of ice
- D Venus is closer to Jupiter
Show answer
Why:
An atmosphere can override the general trend.
- A misleading, because Venus is a moon
- B correct, because Venus is a star
- C misleading, because Venus is a planet and not a star
- D correct, because it only ever shines just before dawn
Show answer
Why:
An old name that stuck despite being wrong.
- A it is closest to the Sun
- B it is covered in red gas
- C it is the very hottest planet of all
- D the soil on it is reddish in colour
Show answer
Why:
The colour is in the ground.
- A a large part of its surface is covered with water
- B its soil is blue in colour
- C it is a very long way away from the Sun altogether
- D the sky above it is blue
Show answer
Why:
Which is why it is called the Blue Planet.
- A Earth and Mars
- B Mars and Jupiter
- C Saturn and Uranus
- D Venus and Earth
Show answer
Why:
A useful landmark in the list.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is the size of it. Arun learnt the older list, which was revised in 2006.
- A it somehow stopped revolving right around the Sun altogether at some point
- B it broke into pieces
- C it moved farther from the Sun
- D the definition of a planet was changed after similar objects were found
Show answer
Why:
Nothing about Pluto itself changed.
- A smaller
- B exactly the same size
- C ten times larger
- D much larger
Show answer
Why:
And it lies farther out than Neptune.
- A Mars
- B Venus
- C Jupiter
- D Sirius
Show answer
Why:
Which makes it the easiest planet to identify.
- A the steady one of the two must certainly be a star of some sort or other
- B both are stars
- C the twinkling one is a star and the steady one is probably a planet
- D both are planets
Show answer
Why:
That is the practical test.
- A straight overhead
- B towards the north
- C towards the east
- D towards the west
Show answer
Why:
At dawn you would look east instead.
- A they give out no light at all
- B they are smaller than stars in every case
- C they move too quickly to see
- D they are so very far away from us
Show answer
Why:
The same reason the stars look like points.
- A makes objects move more slowly
- B shows objects brighter and larger, and shows dim ones too
- C stops the stars twinkling
- D brings all of the objects physically closer to the viewer's eye
Show answer
Why:
Binoculars help a little as well.
- A Venus
- B Mercury
- C Jupiter
- D Saturn
Show answer
Why:
Śhani is Saturn, and Mangala is Mars.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: Pluto is smaller than our own Moon, which is part of why it is no longer counted a planet.
Important short questions (6)
Mercury, Venus, Earth and Mars, then Jupiter, Saturn, Uranus and Neptune.
The near four are smaller, with firm rocky ground. The far four are much larger, chiefly gas and ice, and each one wears a hoop.
Venus has an atmosphere, and an atmosphere traps heat. That outweighs its being farther from the Sun than Mercury is.
Mars appears red, because the soil on it is reddish. The Earth appears blue from space, because so much of its surface is water.
More small objects like it were discovered. In 2006 the IAU redefined what counts as a planet, and Pluto became a dwarf planet.
Both look like shining dots. Stars appear to twinkle a lot, while planets shine steadily.
Long answer questions (3)
Nearest the Sun come Mercury, Venus, Earth and Mars. These four are smaller in size and have solid surfaces with rocks on them. Beyond Mars lies the asteroid belt, where many small rocky objects revolve, and that belt marks the boundary. Outside it come Jupiter, Saturn, Uranus and Neptune. These four are much larger than the Earth, are made chiefly of gas and ice, and each one wears a broad flat hoop of dust grains and stony matter. Farther out still lies Pluto, which is smaller than the Moon of the Earth and is now counted a dwarf planet.
The general rule comes from where the heat starts. A planet draws nearly all its energy from the Sun, so the farther out one lies, the less it receives, and the colder it tends to be. That rule holds for most of the list. What it leaves out is the atmosphere. A planet carrying an atmosphere has heat held in by it, and the temperature can shift a great deal. Venus has one and Mercury does not, so Venus stays hotter even though it lies farther from the Sun. Distance sets the general trend; an atmosphere can override it.
On its discovery Pluto was counted a planet of the Solar System, and it lies farther out than Neptune. Later, more small objects like it were found. That created a difficulty: either every one of them had to be called a planet, or the definition had to be tightened. In 2006 the International Astronomical Union redefined the requirements for an object to be called a planet. Under the new definition Pluto and the other small objects became dwarf planets. Nothing about Pluto changed at all. Only the definition did, which is how careful naming usually works in science.
Flashcards (15)
Play this topic
- Inner or outer planet?
- Planet and label
- Fact or myth?
- Outward from the Sun
- Planets sprint
Moons, Rubble, Visitors, and What Lies Beyond
Craters on the Moon stay for ages because nothing is there to wear them away. That one observation opens out, step by step, until the Solar System is a speck in a galaxy.
What you will be able to do
- I can explain what a satellite is and describe the Moon of the Earth.
- I can say how craters form and why they last so long on the Moon.
- I can describe asteroids and comets, and say where each is found.
- I can list what makes up the Solar System, and place it inside the Milky Way Galaxy.
Key points
Anything travelling round a planet is generally called a satellite, and moons are the natural ones.
The Earth has one Moon and Mars has two; the four giant planets have a large number.
One trip round the Earth takes the Moon roughly 27 days.
Measured across, the Moon comes to roughly a quarter of the Earth, and carries almost no air.
Craters are circular bowl-like structures, mostly formed by impacts from space.
Asteroids are rocky, irregular objects, many of them in a belt between Mars and Jupiter.
Comets are made of dust, gases, rocks and ice, and grow a tail near the Sun.
Nothing in the Solar System is larger or heavier than the Sun, and it makes nearly all the energy there.
Full explanation
Things That Go Round Other Things
Planets move around the Sun. So the obvious next question is whether anything moves around planets, and the answer is yes. Anything travelling round a planet is generally called a satellite, and such a thing is smaller than the planet it circles. Moons are the natural ones.
How Many Each Planet Has
The Earth keeps one Moon. Mars keeps two. Jupiter, Saturn, Uranus and Neptune each keep a great many. The word also has a wider sense. Anything travelling round a body much bigger than itself may be called a satellite, so the Earth itself counts as a satellite of the Sun.
Our Own Moon
The Moon is the natural satellite of the Earth, and our nearest neighbour in space. One trip round the Earth takes it roughly 27 days, and it lies about 3,84,000 kilometres off. Measured across, it comes to roughly a quarter of the Earth.
What Is Missing There
The Moon has almost no atmosphere, quite unlike the Earth. There is no water on it and nothing living. Those three absences matter more than they sound, as the next card shows.
The Bowls on Its Surface
The surface of the Moon shows circular bowl-like structures called craters. Most were dug by asteroids, or by rock out of space, striking it. On Earth such marks would weather away. On the Moon, with no atmosphere, water or life, these features stay for a very long time.
Going There to Look
Far off though the Moon is, spacecraft have been sent out to explore it. India has sent three Chandrayaan missions to study it. The first went in 2008 and the second in 2019.
The Third Mission
Chandrayaan-3 went up in July 2023. The Vikram lander it carried, with the Pragyan rover aboard, came down gently on the Moon on 23 August 2023. No country before India had put anything down near the barely explored southern pole of the Moon. To mark it, 23 August is now celebrated in India as National Space Day. A fourth mission is being planned, and its aim is to return samples of soil and rock.
Rocks That Never Became a Planet
Both the Sun and the planets are close to spherical in shape. There are also many small objects in the Solar System that are rocky and irregular in shape, and these are called asteroids. Their sizes run from about 10 metres up to about 500 kilometres.
Where Most of Them Are
Between the orbits of Mars and Jupiter lies a band where a great many asteroids circle the Sun; it is known as the asteroid belt. Now and then one of them passes very close to us.
Visitors With Tails
Sometimes objects arrive out of the far reaches of the Solar System with long tails, and these are comets. Dust, gases, rock and ice are what they consist of. That mixture is the key to why they look the way they do.
Where the Tail Comes From
Draw one in close to the Sun and the frozen matter inside it begins turning to vapour, and it is that vapour which makes the tail. Move away from the Sun again and a comet grows dim, and past a point the unaided eye loses it entirely.
What Happens to Them
A great many of them have been found travelling round the Sun, drawing in close at regular intervals. A few get away and leave the Solar System for good. Some break up, or fall into the Sun or into planets when they approach. Halley's Comet is the famous one, appearing every 76 years, and its last appearance was in 1986.
The Whole Membership
Now the list can be completed. Between them, the Sun, its eight planets, whatever moons those carry, and a host of smaller bodies such as asteroids and comets make up the Solar System. Nothing in it is larger or heavier than the Sun, and the Sun makes nearly all the energy there. Every other object shines by the sunlight thrown back off its surface.
Our Galaxy
On a moonless night, from a dark location away from city lights, an extended faint band of light can be seen running roughly north to south right across the sky. That is our home galaxy, the Milky Way Galaxy, also called Ākāśha Gangā. Anything from millions to billions of stars go to make a galaxy, and ours sits inside this particular one.
And Beyond That
Out past the Milky Way lie a great many more galaxies, which scientists study in order to understand stars, galaxies and the Universe. Whether life exists anywhere else is still unknown. Most of the looking has gone towards exoplanets, which are the planets found travelling round other stars within our own galaxy. No evidence has turned up so far, and the looking goes on.
Concepts and terms
Objects that move around planets, and smaller in size than planets. Moons are the natural satellites. In general any object moving around a much larger one can be called a satellite, so the Earth is a satellite of the Sun.
Circular bowl-like structures on the surface of the Moon, mostly formed by asteroids or rocks from space striking it. With no atmosphere, water or life there, they stay for a very long time.
Asteroids are small rocky objects, irregular in shape, many revolving in a belt between Mars and Jupiter. Comets are made of dust, gases, rocks and ice, and come from the outer regions.
Millions to billions of stars go to make a galaxy. Ours sits inside the Milky Way, also called Ākāśha Gangā, and a great many more galaxies lie out past it.
Watch out!
Myth or fact?
Try it at home
Title:
Make craters in a tray
Safety:
Do this outdoors or over newspaper, because flour goes everywhere. Do not throw the pebbles hard, and keep your face well away from the tray.
What You Notice:
Every mark is circular, whatever shape the pebble is, and that is the first finding. The bigger pebble and the greater height both make a wider mark. The last step is the important one: blowing and sprinkling spoil the marks quickly, and that is exactly what air and water do on Earth and cannot do on the Moon.
Case study: the object the school telescope keeps arguing about
Scenario:
A school astronomy club has borrowed a small telescope for a fortnight. On the first clear evening three members point it at a bright object low in the west. They cannot agree what it is. Anjali says it must be a star, since it is the brightest thing in that part of the sky. Farhan says it cannot be. He watched it for two minutes and it did not twinkle once. Devi has the log from last week, and says the object sat in a slightly different place then. The teacher refuses to name it. She asks the three of them to settle it from what they already know.
Analysis:
Each of the three has hold of a different piece of evidence, and only two of those pieces actually bear on the question.
Activity:
Keep a log of your own for a week. Each clear evening, note the brightest object low in the west, the time, roughly where it sits against a fixed landmark such as a roof or a tree, and whether it twinkles. At the end of the week, look at whether the position against the landmark stayed put or drifted, and write one sentence saying what you think the object is and why.
Did you know
Chandrayaan-3 went up in July 2023, and the Vikram lander it carried, with Pragyan aboard, came down gently on 23 August 2023.
No country before India had put anything down near the barely explored southern pole of the Moon, and 23 August is now National Space Day.
Asteroids range in size from about 10 metres up to about 500 kilometres.
Memory tricks
Why craters last — No air, no water, no life, no repair — Nothing on the Moon works to wear them away.
Zooming outward — Moon, planet, Solar System, galaxy, Universe — Five steps, each one very much larger than the last.
Practice MCQs with answers (20)
- A satellites
- B asteroids
- C galaxies
- D comets
Show answer
Why:
Moons are the natural ones.
- A none
- B two
- C ten
- D a hundred
Show answer
Why:
The giant planets have a large number each.
- A seven days
- B 24 hours
- C 27 days
- D one year
Show answer
Why:
Our nearest neighbour in space.
- A twice the size of the Earth
- B ten times the size of the Earth
- C the same size as the Earth
- D a quarter the size of the Earth
Show answer
Why:
And about 3,84,000 km away.
- A circular bowl-like structures
- B deep pools full of still water
- C patches of green plants
- D tall pointed mountains
Show answer
Why:
Most were made by impacts from space.
- A volcanoes erupting there
- B asteroids or rocks from space hitting the surface
- C water wearing the rock away
- D wind blowing all of the dust up into big heaps there
Show answer
Why:
The Earth gets hit too, but the marks do not last.
- A a layer of ice protecting them
- B far too much loose rock about for them to fill in
- C no atmosphere, water or life to wear them away
- D nobody there to clear them up
Show answer
Why:
Three absences, one consequence.
- A the Earth is far too large ever to be marked at all
- B craters only form on satellites
- C the Earth is never hit by anything
- D on Earth, air, water and life wear the marks away
Show answer
Why:
The difference is in what happens afterwards.
- A Vikram and Pragyan
- B Mangal and Chandra
- C Sarabhai and Kalam
- D Aryabhata and Bhaskara
Show answer
Why:
They soft-landed on 23 August 2023.
- A north pole of the Moon
- B south pole of the Moon
- C centre of the near side
- D far side of Mars
Show answer
Why:
And 23 August is now National Space Day.
- A stars that have gone out
- B large spherical planets
- C small rocky objects, irregular in shape
- D great balls of ice with very long tails on
Show answer
Why:
From about 10 m to about 500 km in size.
- A Venus and Earth
- B Saturn and Uranus
- C Earth and Mars
- D Mars and Jupiter
Show answer
Why:
The same belt marked on a solar system drawing.
- A dust, gases, rock and ice
- B hot glowing gases only
- C solid metal throughout
- D only frozen water
Show answer
Why:
And they come from the outer regions.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is where the tail comes from. Out in the cold there is nothing to evaporate and no tail to see.
- A gas escaping from the Sun
- B dust picked up from a planet
- C the frozen material evaporating from it
- D light being reflected off of its surface
Show answer
Why:
Which is why the tail appears near the Sun.
- A 760 years
- B 7600 years
- C 7 years
- D 76 years
Show answer
Why:
Its last appearance was in 1986.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: Sirius is a star in its own right, far outside the family of the Sun.
- A produces its own light
- B reflects sunlight from its surface
- C is heated up by all the nearby planets
- D burns gases inside itself
Show answer
Why:
The Sun produces almost all the energy there.
- A only dust and gas
- B millions to billions of stars
- C exactly eight planets
- D one star and its planets
Show answer
Why:
Ours is the Milky Way Galaxy.
- A Towards comets, because they pass close by the Earth now and then
- B Towards the asteroid belt, because so many rocks are gathered there
- C Towards the surface of the Moon, because it is the nearest body
- D Towards exoplanets, because they revolve around stars of their own
Show answer
Why:
Life as we know it needs a star to warm it. A planet going round another star is the nearest thing to our own arrangement.
Important short questions (6)
An object that moves around a planet, smaller than the planet. The Moon is the natural satellite of the Earth.
Measured across it comes to roughly a quarter of the Earth. One trip round the Earth takes it about 27 days.
Circular bowl-like structures, dug mostly by rock arriving out of space. Hardly any air is there, no water and nothing living, so nothing rubs them out.
Small rocky objects, irregular in shape. Many revolve around the Sun in the belt between the paths of Mars and Jupiter.
A comet consists of dust, gases, rock and ice. Close to the Sun the frozen matter inside turns to vapour, and that vapour makes the tail.
The Sun, its eight planets, whatever moons those carry, and a host of smaller bodies such as asteroids and comets.
Long answer questions (3)
The Moon is the natural satellite of the Earth, and our nearest neighbour in space. One trip round us takes it roughly 27 days. It lies about 3,84,000 kilometres off, and measured across it comes to roughly a quarter of the Earth. Across its surface are circular bowl-like structures called craters, dug mostly by asteroids or by rock out of space striking it. What keeps them there is what the Moon lacks. Hardly any air is there, no water and nothing living. So nothing rubs the marks out, and they stay for a very long time.
Asteroids are small objects of the Solar System, rocky and irregular in shape. They range from about 10 metres to about 500 kilometres. Many revolve around the Sun between the paths of Mars and Jupiter, in the asteroid belt. Occasionally one passes very close to us. Comets are visitors out of the far reaches, and they consist of dust, gases, rock and ice. Draw one in close to the Sun and its frozen matter turns to vapour, which makes the long tail. Away from the Sun it grows dim.
The Sun, its eight planets, the moons those carry, and a host of smaller bodies such as asteroids and comets make up the Solar System between them. Nothing in it is larger or heavier than the Sun, and the Sun makes nearly all the energy there. Every other object shines only by the sunlight thrown back off its surface. The Solar System itself is part of the Milky Way Galaxy. A galaxy holds millions to billions of stars. Beyond ours lie many more, which scientists study to understand the Universe.
Flashcards (15)
Play this topic
- In the Solar System, or not?
- Term and meaning
- Fact or myth?
- Zooming outward
- Beyond Earth sprint
Class 6 Science Chapter 12 — frequently asked questions
A defined region of the sky, which often contains a pattern of stars. The International Astronomical Union listed 88 of them.
The lines joining stars are imaginary. Each culture drew its own out of its own stories, and named them accordingly.
It is pictured as a hunter. Three bright stars in a short straight row near its middle form his belt, and those are the easiest to spot.
Artificial light in excess after dark. It washes the fainter stars out, so far fewer of them are visible.
Villages have less light pollution. Cities also have more smoke and dust, and tall buildings that block the view.
A magnetic compass, for finding directions. A notebook, for noting or drawing what you observe.
It is a star, and the one closest to us. A fiercely hot ball of gases, roughly spherical, pouring out an enormous quantity of energy.
Roughly how far the Earth sits from the Sun, about 150 million kilometres. It makes distances inside the solar system easier to state.
It is much closer to us. Other stars are so far away that they appear as points, even when they are larger than the Sun.
Mercury, Venus, Earth and Mars, then Jupiter, Saturn, Uranus and Neptune.
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