NCERT Solutions Curiosity Chapter 12 – 251Chapter exercise — Let us enhance our learning

Book page 249 Updated on2026-09-05

Q1.

Match the column: Column I — (i) Satellite of Earth, (ii) Red planet, (iii) Constellation, (iv) Planet which is commonly called an evening star. Column II — (a) Orion, (b) Venus, (c) Mars, (d) Moon.

Answer

Column I	Match	Column II	Why
(i) Satellite of Earth	(d)	Moon	The Moon is the Earth's only natural satellite; it revolves around the Earth in about 27 days.
(ii) Red planet	(c)	Mars	Mars looks red because the soil on its surface is reddish in colour.
(iii) Constellation	(a)	Orion	Orion is a constellation, imagined as a hunter with a three-star belt.
(iv) Planet commonly called an evening star	(b)	Venus	Venus shines brightly in the west after sunset — but it is a planet, not a star.

(i) → (d) (ii) → (c) (iii) → (a) (iv) → (b)

Tip: Venus is also called the Morning Star when it is seen in the east before sunrise. Same planet, different time of day.

Q2.

(i) Solve the following riddle. My first alphabet is in MAN but not in CAN / My second alphabet is in ACE and also in FAN / My third alphabet is in RAT and not in CAT / My fourth alphabet is in SUN but not in FUN / I am a planet that moves around the Sun. (ii) Make two similar riddles by yourself.

Answer

(i) Solving the riddle, letter by letter:

Clue	Working	Letter
In MAN but not in CAN	MAN = M, A, N · CAN = C, A, N → the odd one out is M	M
In ACE and also in FAN	ACE = A, C, E · FAN = F, A, N → common letter is A	A
In RAT but not in CAT	RAT = R, A, T · CAT = C, A, T → the odd one out is R	R
In SUN but not in FUN	SUN = S, U, N · FUN = F, U, N → the odd one out is S	S

M + A + R + S = MARS
Answer: Mars, the Red Planet — the fourth planet from the Sun.

(ii) Two riddles of my own. Solve them the same way.

Riddle 1
My first letter is in VAN but not in CAN
My second letter is in NET and also in ELM
My third letter is in NUT but not in CUT
My fourth letter is in USE but not in SEA
My fifth letter is in SUN but not in RUN
After the Sun and the Moon, I am the brightest object in the sky.
Answer: VENUS

Riddle 2
My first letter is in MAT but not in CAT
My second letter is in COT but not in CAT
My third letter is in TOP and also in DOG
My fourth letter is in NET and also in SUN
I am the Earth's natural satellite.
Answer: MOON

How to build your own: write the answer first, then for every letter find two short words — one that contains the letter and one that does not, but which are otherwise as similar as possible (RAT / CAT). For a letter you want to keep, use two words that share only that letter (ACE / FAN).

Q3.

Which of the following is not a member of our Solar System? (i) Sirius (ii) Comets (iii) Asteroids (iv) Pluto

Answer

Answer: (i) Sirius

Why: Sirius is a star — the brightest star in the night sky — belonging to the constellation Canis Major. It lies far outside the Solar System and does not revolve around the Sun.

(ii) Comets — members. They are icy-rocky bodies; many revolve around the Sun and return periodically.
(iii) Asteroids — members. Small rocky bodies, most of them in the asteroid belt between Mars and Jupiter.
(iv) Pluto — a member. It revolves around the Sun beyond Neptune; since 2006 it is classified as a dwarf planet, but it is still part of the Solar System.

Tip: the test for membership of the Solar System is simple — does it go round our Sun? Sirius does not; it is a sun of its own.

Q4.

Which of the following is not a planet of the Sun? (i) Jupiter (ii) Pluto (iii) Neptune (iv) Saturn

Answer

Answer: (ii) Pluto

Why: Pluto revolves around the Sun and lies farther out than Neptune, but it is not counted as a planet. It is smaller than the Earth's Moon. When many more small bodies like it were discovered, the International Astronomical Union (IAU) in 2006 redefined what an object must be in order to be called a planet. Under that definition Pluto and similar small bodies are now called dwarf planets.

Jupiter, Neptune and Saturn are three of the eight planets of the Sun.

Did you know? Pluto was discovered in 1930 and was called the ninth planet for 76 years. Science changed its mind when new evidence arrived — which is exactly how science is supposed to work.

Q5.

Which is the brighter star, the Pole Star or Sirius?

Answer

Sirius is the brighter star.

Sirius, in the constellation Canis Major, is the brightest star in the whole night sky. It is impossible to miss once you follow the line of Orion's belt to the east.
The Pole Star (Polaris, Dhruva tārā) is not very bright. The chapter itself warns you to choose a night with no Moon and no clouds precisely because the Pole Star is faint.

Why the Pole Star is still the more useful of the two: importance in the sky has nothing to do with brightness. The Pole Star matters because it is the one star that stays almost fixed in the north all night and all year, so it can be used to find direction. Sirius, for all its brilliance, moves across the sky like every other star.

Q6.

An artist's representation of the Solar System is given in Fig. 12.12. Is the order of the planets correct? If not, write the correct order in the boxes in the figure.

Answer

No, the order in Fig. 12.12 is not correct. Two pairs of planets have been swapped.

Reading the figure outward from the Sun, the artist has drawn: Mercury, Earth, Venus, Mars, Jupiter, Uranus, Saturn, Neptune. Compare that with the true order:

Box	Planet drawn in Fig. 12.12	Correct planet
1	Mercury	Mercury	correct
2	Earth (blue, with clouds)	Venus	swapped
3	Venus (orange-brown)	Earth	swapped
4	Mars	Mars	correct
5	Jupiter	Jupiter	correct
6	Uranus (pale blue, thin upright ring)	Saturn	swapped
7	Saturn (yellow, broad rings)	Uranus	swapped
8	Neptune	Neptune	correct

So the boxes should be filled, from the Sun outwards, as:

Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune

The correct order of the eight planets outward from the Sun. Sizes and distances are not to scale.

Tip — a sentence to remember the order: My Very Educated Mother Just Served Us Noodles → Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

How to spot the mistake yourself in a picture: Earth is the blue-and-white one (water and clouds); Venus is plain yellowish-brown; Saturn has broad, obvious rings lying flat; Uranus is a small pale blue-green ball with a thin ring standing almost upright. In Fig. 12.12 the blue-and-white ball sits at position 2 and the ringed yellow giant at position 7 — both are one place out of turn.

Q7.

A portion of night sky with stars is shown in Fig. 12.13. Look carefully and identify the groups of stars that form the patterns—the Big Dipper and the Little Dipper. Draw lines to connect the stars for these patterns and label them. Also, identify and label the Pole Star. You may refer to Fig. 12.4 for help.

Answer

How to work on the figure in your book:

Find the seven brightest dots in the upper-left part of Fig. 12.13. Four of them form a slightly slanted rectangle (the cup) and three trail away from one corner in a gentle curve (the handle). Join them — that is the Big Dipper (Saptaṛiṣhi).
Take the two stars at the outer edge of the cup, the edge away from the handle. Draw a light straight line through them, continuing away from the cup.
Measure the gap between those two stars and travel about five times that gap along the line. You reach a lone, fainter star — mark it Pole Star.
Starting from that star, join a short curved handle of three stars and then four stars forming a small cup. That smaller ladle is the Little Dipper. Note that the Pole Star is at the end of its handle.

Your finished figure should look like this:

The completed answer for Fig. 12.13 — Big Dipper, Little Dipper and the Pole Star at the end of the Little Dipper's handle.

Tip: use a pencil and a ruler and press lightly, so you can rub out and try again. Only the bright dots belong to the patterns; ignore the faint ones scattered in between.

Q8.

A portion of the night sky is shown in Fig. 12.14. Draw lines to connect the stars for Orion and label the star Sirius. You may refer to Fig. 12.3.

Answer

How to work on the figure in your book:

Look near the middle of Fig. 12.14 for three fairly bright dots very close together in a short, straight, evenly spaced row. That is Orion's belt — always find it first.
Above the belt, mark the two bright stars that form the hunter's shoulders (one of them, Betelgeuse, is the brighter, redder one).
Below the belt, mark the two bright stars that form the feet (the brilliant one is Rigel).
Join shoulder → belt end → foot on each side, and join the two shoulders across the top. You now have the tall hourglass shape of Orion.
Now extend the line of the three belt stars towards the east (in the figure, away from the belt on the lower side). The first really brilliant star you meet along that line is Sirius. Label it.

Your finished figure should look like this:

The completed answer for Fig. 12.14 — Orion joined up, and Sirius found by extending the line of the belt.

Why the belt is the key: three stars of almost equal brightness, almost equally spaced, in an almost perfect straight line, occur nowhere else in the sky. Once you have the belt, every other star of Orion is fixed relative to it — and so is Sirius.

Q9.

From Earth, you can see stars fading away at dawn and appearing at dusk. During the day we do not see the stars. Explain why.

Answer

The stars are shining all day long. They are simply drowned out by sunlight.

Once the Sun rises, its light falls on the air above us and is scattered in every direction by the gases and dust in the atmosphere. This scattered light is what makes the whole daytime sky glow bright and blue.
The light reaching us from any other star is extremely feeble, because every one of them is enormously far away.
A faint light cannot be picked out against a very bright background. So against the glowing daytime sky, the stars become invisible — not absent.
At dusk the Sun goes below the horizon, the scattered light dies away, the sky background darkens, and one by one the brightest stars begin to show. At dawn the reverse happens and they fade out.

An everyday parallel: switch on the torch of your mobile phone in a dark room and it looks dazzling. Take it out into bright noon sunlight and you can barely tell it is on. The torch has not changed — the background has.

Did you know? On the Moon there is almost no atmosphere, so there is nothing to scatter sunlight into a bright sky. The lunar sky stays black, and an astronaut can see the stars even while standing in full sunlight. That proves it is our air, not the Sun itself, that hides the stars from us.

Q10.

During a clear night, try to observe the Big Dipper 3–4 times at an interval of 2 to 3 hours. Also try to locate the Pole Star each time. Does the Big Dipper appear to move? Draw a rough sketch to illustrate this, mentioning the time in each case.

Answer

Observation: yes, the Big Dipper appears to move — but the Pole Star does not.

Over the night the whole Big Dipper appears to swing around the Pole Star in a circle, moving anticlockwise as you face north.
The shape of the pattern never changes; only its tilt changes. It may stand upright at one hour and lie almost on its side a few hours later.
The distance of each star from the Pole Star also stays the same — the whole pattern simply rotates about that one point.
The Pole Star stays in the same place, in the north, at the same height above the horizon, all night long.

The sky turns through 360° in about 24 hours
→ about 15° in one hour
→ about 30° to 45° in an interval of 2 to 3 hours

Rough sketch (three observations, 3 hours apart):

The same Saptaṛiṣhi seen at 9 p.m., midnight and 3 a.m. The shape is unchanged; only its position around the fixed Pole Star has turned, by about 45° every three hours.

Why it happens: the stars are not really moving across our sky — the Earth is rotating on its axis, once in about 24 hours. Standing on a spinning Earth, we see the whole sky appear to turn the opposite way. The Pole Star lies almost exactly on the line of the Earth's axis, so it is the one point that does not swing round, and every other star seems to circle about it.

Check it yourself: at each observation, hold your notebook the same way up and sketch the dipper together with the horizon and the Pole Star. Write the time beside each sketch. After three sketches the turning is unmistakable.

Q11.

Think about the night sky and write a poem or a story on it.

Answer

This is your own creative work, so there is no single right answer. A good piece should use things you have actually learnt in this chapter — the Saptaṛiṣhi, the Pole Star, Orion's belt, Sirius, the Milky Way, a comet, the craters of the Moon — rather than vague words like "twinkling diamonds".

What a good answer must contain:

One or two real objects of the night sky, named correctly.
Something true about them (the Pole Star stays in the north; Sirius is the brightest star; a comet grows a tail near the Sun).
A feeling — wonder, curiosity, a question you would like answered.

Sample answer — a poem
The Lamp That Never Moves

Seven sages carry a ladle of light,
slowly they circle the roof of the night.
They swing and they lean, they turn and they go,
but one small lamp in the north hangs low.

It is not the brightest, it is not the best,
yet sailors have trusted it more than the rest.
For Orion may march and Sirius may burn —
the Dhruva alone does not wander or turn.

And under that lamp, on a cold Ladakh night,
the Ākāśha Gangā spills over with light,
a river of stars that no bucket can hold,
and I am a speck on a planet of gold.

Sample answer — a story (in brief)
Yangdol could not sleep. She wrapped her goncha around her, climbed to the flat roof and lay down on the cold stone. Dorjay came up too, carrying his grandfather's old brass compass.
"Look," he said, "the ladle has moved." At nine o'clock the Saptaṛiṣhi had stood upright over the ridge; now, past midnight, it lay tilted on its side.
"It has not moved," said Yangdol. "We have. The Earth turned under us." She traced the two pointer stars with her finger, counted five gaps, and stopped at a faint, lonely star. Dorjay opened the compass. The needle and the star agreed: north.
Far above them the Ākāśha Gangā stretched from one mountain to the other. Somewhere in that band of light, Yangdol thought, is a star with a planet, and on that planet perhaps someone is lying on a roof, looking back at us. She fell asleep still wondering.

Tip for writing your own: begin with something you have really seen — the Moon behind a neem tree, a shooting star, the sky above your terrace during a power cut. A true observation always makes a better opening line than an invented one.

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