Class 7 Science — Chapter 12: Earth, Moon, and the Sun | Solutions
NCERT • Curiosity • Grade 7 Science
Chapter 12 — Earth, Moon, and the Sun
Complete, detailed solutions of every question in the chapter — In-Text (Activity) questions and Exercise questions, with the original NCERT diagrams, step-wise reasoning and clear explanations.
📘 Pages 169–186🔭 4 Activities✅ 12 Exercise Questions🖼️ Original Diagrams
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In-Text Questions & Activity Solutions
Questions asked inside the chapter — activities, figures, “Dive Deeper” boxes and discussion prompts
1
Page 169
Rashmika noticed that coconut tree shadows were long in the morning but shorter in the afternoon. She wondered — does the Sun move in the sky? Or does the Earth move?
✔ Answer
The Earth moves — the Sun only appears to move.
The Earth rotates on its own axis from West to East, completing one rotation in about 24 hours.
Because we are standing on this rotating Earth, the Sun seems to rise in the East, travel across the sky, and set in the West. This is an apparent motion, not real motion of the Sun.
As the Sun’s apparent position in the sky changes through the day, the direction and length of shadows also change — long shadows in the early morning and late evening (Sun low in the sky) and short shadows near noon (Sun almost overhead).
Rashmika’s observation was correct, but her explanation was not. Shadows change because the Earth is turning, not because the Sun is travelling across the sky.
2
Activity 12.1
While sitting on a merry-go-round turning anti-clockwise — do the objects around you appear to be moving? In which direction do they appear to move? In which direction do you find the tree turning around you? Is it in your view all the time?
✔ AnswerFig. 12.1: A girl observing objects around her while riding a merry-go-round
Observation 1Yes — the trees, buildings and other objects around you appear to be moving, even though they are actually standing still.
Observation 2 — direction
You are turning anti-clockwise, so the objects appear to turn around you in the opposite direction, i.e., clockwise.
Observation 3 — the tree
The tree appears to turn around you in the clockwise direction. No, it is not in your view all the time. It first enters your view from your left-hand side, crosses in front of you, and then moves out of your view on your right-hand side.
This is exactly what happens with the Sun. We are on the Earth, which rotates from West to East (anti-clockwise, seen from above the North Pole), so the Sun appears to move in the opposite direction — from East to West, and it goes out of our view (sunset) and comes back (sunrise).
3
Section 12.1 • Dive Deeper
In which way is the Earth turning around itself? What is rotation, and what is the Earth’s axis of rotation? How long does one rotation take?
✔ AnswerFig. 12.2: (a) A spinning top (b) A spinning fan (c) A spinning ball
Just like a spinning top, a fan or a ball spinning on a finger, the Earth also spins (rotates) on its own axis in space.
📖 Dive Deeper — What is rotation? Rotation is the motion of an object in which all its parts move in circles around an imaginary line that passes through it. This line is called the axis of rotation.
Fig. 12.3: Rotation of the Earth
The Earth’s axis of rotation passes through its geographic North Pole (NP) and South Pole (SP).
The Earth completes one rotation in about 24 hours.
When viewed from above the North Pole, the Earth rotates in the anti-clockwise direction — that is, from West to East.
Earth’s rotation: West → East, one turn in ≈ 24 hours, about an axis through the North and South Poles.
4
Activity 12.2
Using a globe and a torch — do you notice how half of the globe receives light while the other half stays dark? In India, sunrise first occurs in the eastern part. What is the direction of rotation when light falls on the eastern part of India first? While rotating the globe from West to East, does your location go through a cycle of day and night?
✔ AnswerFig. 12.4(a): Using a globe to understand rotation of the Earth
Step 1 — Mark your locationPlace a small sticker on the globe to mark your location. Viewing from above the North Pole, slowly rotate the globe anti-clockwise. Your location turns around and finally comes back to its original position — that is one complete rotation.
Fig. 12.4(b): Using a globe and a torchlight to understand day and night
Step 2 — Torch = the SunYes. The torch (placed about 1.5 m away) lights up only one half of the globe; the other half stays dark. This is because the Earth is opaque and spherical — sunlight can fall on only half of it at any moment.
➜ The lit half has day, the dark half has night.
Step 3 — Direction of rotation
The light falls on the eastern part of India first when the globe is rotated from West to East (with respect to the North–South axis). This matches the real fact that sunrise happens earlier in the eastern parts of India (e.g., Arunachal Pradesh) than in the western parts (e.g., Gujarat).
Step 4 — Day–night cycleYes. As the globe keeps rotating from West to East, your marked location repeatedly moves into the lit half and then into the dark half. Sunrise occurs as your location moves into the light, and sunset occurs as it moves into darkness — so it passes through a continuous cycle of day and night.
Fig. 12.5: Sunlight falls on half of the Earth’s surface
The Earth’s rotation from West to East causes the day–night cycle. The side facing the Sun has daytime; the other side has night.
5
Fig. 12.6
Imagine you are standing on the Equator and watching the sky during one rotation of the Earth while it rotates from West to East. What will you observe? Will your observation be the same as that of the girl shown in Fig. 12.6?
✔ AnswerFig. 12.6: A girl on the rotating Earth viewing the Sun from the Equator
Yes — my observation would be exactly the same as the girl’s. Standing on the Equator and facing North (back to the South), during one rotation I would see:
Morning: The Sun appears on my right-hand side, in the East — this is sunrise.
Around noon: I have rotated to a new position, and the Sun is now almost directly over my head.
Evening: I have rotated further, and the Sun starts to disappear from my view towards my left, in the West — this is sunset. The stars now become visible in the night sky.
Night: My location is on the dark side of the Earth, away from the Sun.
Because the Earth rotates from West to East, the Sun (and also the Moon and the stars) appears to rise in the East and set in the West.
6
Activity 12.3
“Since the Earth is rotating, shouldn’t the stars also appear to move in the sky like the Sun?” After observing the Big Dipper every two hours — has it moved? Do you observe that the Big Dipper appears to move around the Pole Star? Why does the Pole Star appear to stay still?
✔ AnswerFig. 12.7: Illustrative sketch of the Big Dipper (for Activity 12.3) by a student in Pune on the night of 1–2 April
Yes, the stars also appear to move — for exactly the same reason as the Sun: the Earth is rotating beneath them.
Observation
Sketch the Big Dipper (Saptarishi) with respect to the Pole Star (Dhruva Tara) at 7 PM, then again at 9 PM, 11 PM, 1 AM… Each time, the Big Dipper has changed its orientation and position. Joining these positions shows that the Big Dipper slowly moves in a circle around the Pole Star.
Why is the Pole Star nearly stationary?
The Earth’s axis of rotation points very close to the Pole Star in the Northern Hemisphere. A point lying almost exactly on the axis does not appear to swing around as the Earth spins. So the Pole Star appears fixed in the sky, while all the other stars appear to move in circles around it.
Just like the Sun, the Moon and the stars also appear to rise in the East and set in the West, because the Earth rotates from West to East.
🌌 Fascinating fact: In a long-exposure photograph, this apparent motion of stars gets recorded as arcs of circles, known as star trails — direct visual proof of the Earth’s rotation.
7
Section 12.2 & 12.2.1
Why do different stars appear in the night sky over the course of a year? What is revolution and what is an orbit? How long does the Earth take to complete one revolution?
✔ Answer
Key definitionsRevolution is the motion of an object around another object. The path an object takes while revolving around another object is called its orbit.
Viewed from the top, the Earth’s orbit around the Sun is nearly circular. The Earth completes one revolution in about 365 days and 6 hours (≈ 1 year).
Fig. 12.8: Revolution of the Earth around the Sun leads to a changing view of the night sky throughout the year (sizes and distances not to scale)
Why the night sky changes over a year
As the Earth revolves around the Sun, its position in the orbit keeps changing. So the night side of the Earth faces a different direction in space in different months. Since we look out towards different parts of the sky, we see different stars and constellations after sunset in different months.
For example, the stars visible after sunset in March are not the same as those visible after sunset in June, September or December. This is why in Grade 6 we were told to look for certain constellations at certain times of the year.
Rotation (24 hours) ⇒ the day–night cycle and the daily rising/setting of stars. Revolution (≈ 1 year) ⇒ the changing view of the night sky through the year, and the seasons.
8
Section 12.2.2 • Dive Deeper
Is the cycle of seasons related to the revolution of the Earth around the Sun? Why are days longer in summer than in winter? Also — what are the two incorrect reasons often given to explain the seasons?
✔ Answer
Step 1 — The real cause
The Earth’s axis of rotation is not upright with respect to its orbit — it is tilted, and the Earth maintains this same tilt all through its journey around the Sun. This tilt of the axis, together with the spherical shape of the Earth, gives rise to the seasons.
Fig. 12.9: Different positions of the Earth while revolving around the Sun (sizes and distances not to scale)
Step 2 — Intensity of sunlight (Fig. 12.10)
In June, the Northern Hemisphere is tilted towards the Sun. Because of the Earth’s curved surface, the same amount of sunrays is spread over a smaller area in the Northern Hemisphere ⇒ the sunlight is more intense and that area gets heated more. In the Southern Hemisphere the same rays are spread over a larger area ⇒ less intense.
Fig. 12.10: (a) More intense sunlight in the Northern Hemisphere and less intense in the Southern Hemisphere in June (b) The opposite happens in December
Step 3 — Length of daytime (Fig. 12.11)
In June, places in the Northern Hemisphere stay in the lit half for more than 12 hours during the 24-hour rotation ⇒ longer days, shorter nights. (The North Pole gets sunlight for all 24 hours!) In December, the opposite happens ⇒ shorter days, longer nights in the Northern Hemisphere.
Fig. 12.11: In the Northern Hemisphere (a) Longer daytime in June (b) Shorter daytime in December
More intense sunlight + for a longer time ⇒ SUMMER (June in the Northern Hemisphere). Less intense sunlight + for a shorter time ⇒ WINTER (December in the Northern Hemisphere). The seasons and day lengths are reversed in the Southern Hemisphere.
⚠️ Dive Deeper — Two INCORRECT reasons often given for the seasons:
“When the Northern Hemisphere tilts towards the Sun, it is closer to the Sun.”
“The Earth’s orbit is an oval with the Sun slightly off-centre, so the Earth is at different distances from the Sun during the year.”
In both cases the change in distance is extremely small — these are NOT the reasons for seasons. In fact, the Earth is closest to the Sun in January, which is winter in the Northern Hemisphere!
📅 Solstices & Equinoxes (Northern Hemisphere): Longest day ≈ 21 June (summer solstice). Shortest day ≈ 22 December (winter solstice). Around 21 March (spring equinox) and 23 September (autumn equinox), day and night are each about 12 hours. On the Equator, there are always ~12 hours of day and ~12 hours of night, so seasons are not very prominent in the southern states of India.
9
Activity 12.4
Could the light from the Sun get blocked by the two planets (Mercury and Venus) revolving between the Earth and the Sun? In Activity 12.4, are you able to cover the entire head of your friend with your thumb? How is this possible, and how does it explain a solar eclipse?
✔ AnswerFig. 12.12: Trying to cover a friend’s head with the thumb
Activity resultYes — you can cover your friend’s entire head with your thumb, even though your thumb is far smaller than his head. This works because your thumb is very close to your eye, while your friend is about 5 metres away.
The concept — Apparent size
The size of an object as seen by your eye is called its apparent size. It depends on two things:
the object’s actual physical size, and
its distance from you.
A small object placed close by can have the same apparent size as a large object placed far away.
Applying it to the Moon and the Sun
The Moon is much smaller than the Sun in physical size, but it is also much closer to us. As a result, the apparent sizes of the Moon and the Sun in the sky are nearly the same. That is why the Moon can appear to cover the entire Sun and cause a solar eclipse.
Why can’t Mercury and Venus block the Sun? Although they are much larger than the Moon, they are also very much farther from the Earth. So their apparent sizes are far smaller than the Sun’s, and they can never block it. When Venus passes between the Sun and the Earth, it appears only as a tiny black dot moving across the bright face of the Sun — a rare event called the Transit of Venus.
10
Section 12.3.1
What is a solar eclipse? Explain total and partial solar eclipse using Fig. 12.13, and state how a solar eclipse should be viewed safely.
✔ AnswerFig. 12.13: Geometry of a solar eclipse (sizes and distances not to scale)
What is a solar eclipse?
At certain times, the Moon comes in between the Sun and the Earth in such a way that it obstructs the sunlight from reaching us. This is called a solar eclipse.
Order: Sun → Moon → Earth.
Type
What happens
What the observer sees
Total solar eclipse
The Moon’s shadow falls on a small area of the Earth’s surface. That area is in total darkness.
No part of the Sun is visible; for a few minutes it turns dark during the day.
Partial solar eclipse
In areas where the Moon blocks out only some regions of the Sun.
A part of the Sun’s disc remains visible.
Fig. 12.14: Solar eclipse — (a) Total (b) Partial (c) The ‘diamond ring’ seen just as the Moon starts to move away
Because of the Earth’s rotation and the Moon’s motion in its orbit, the Moon’s shadow keeps moving across the Earth’s surface. That is why a total solar eclipse lasts only for a few minutes at any place. As the Moon moves away from in front of the Sun, we see a partial eclipse and daylight begins to return — sometimes producing the beautiful ‘diamond ring’ effect.
⚠️ Caution — Safe viewing of a solar eclipse: Even during an eclipse, the Sun is intense enough to damage the eyes and cause blindness. NEVER look at the Sun directly, and never view it through sunglasses, binoculars or telescopes.
Fig. 12.15: A public solar eclipse viewing organised in Ooty, Tamil Nadu (a mirror fixed on a ball projects the Sun’s image on a screen)
Safe ways to observe:
Use special solar eclipse goggles / filters made for solar viewing.
Project the Sun’s image onto a wall or screen using a small mirror (as in Fig. 12.15) or a pinhole — and look at the screen, never at the Sun.
Best of all — attend an eclipse viewing event organised by a planetarium or an astronomy club, where safe eye protection and scientific explanations are provided.
Note: eclipses were feared in ancient times and many superstitions grew around them. Now that we know why they occur, there is nothing to fear — as long as we never look at the Sun directly.
11
Section 12.3.2
What is a lunar eclipse? Explain total and partial lunar eclipse using Fig. 12.16. Can we watch it with the naked eye?
✔ AnswerFig. 12.16: Geometry of a lunar eclipse (sizes and distances not to scale)
What is a lunar eclipse?
As the Moon revolves around the Earth, sometimes the Earth comes in between the Sun and the Moon and blocks the sunlight from reaching the Moon. This is a lunar eclipse. On such days we see the Earth’s shadow falling on the full disc of the Moon.
Order: Sun → Earth → Moon.
Type
Condition
What we see
Total lunar eclipse
The Moon is completely inside the Earth’s shadow.
The bright disc of the Moon turns dark red and stays that way until it moves out of the shadow.
Partial lunar eclipse
Only part of the Moon is in the Earth’s shadow.
A part of the Moon looks dark while the rest stays bright.
Unlike the Sun, we can safely watch an eclipsed full Moon with the naked eye — no special filters are needed.
📝
Exercise — “Let Us Enhance Our Learning”
Questions 1 to 12 (pages 184–186) with complete, detailed answers + Exploratory Projects
1
In Fig. 12.17, how many hours of sunlight do the North Pole and the South Pole receive during one rotation of the Earth?
✔ AnswerFig. 12.17
Step 1 — Read the figure
In Fig. 12.17 the Earth’s axis is tilted. The North Pole (NP) lies in the lit (day) half, and the South Pole (SP) lies in the dark (night) half. (This is the situation in June.)
Step 2 — Rotate the Earth mentally
As the Earth rotates once (24 hours) about this axis, the North Pole keeps turning but never leaves the lit half — so it stays in sunlight the whole time. The South Pole keeps turning but never enters the lit half — so it stays in darkness the whole time.
North Pole → 24 hours of sunlight (continuous daylight, no sunset). South Pole → 0 hours of sunlight (i.e., 24 hours of darkness).
In December the situation is exactly the reverse — the South Pole gets 24 hours of sunlight and the North Pole gets 24 hours of darkness. This is why the polar regions have six months of continuous day followed by six months of night.
2
Fill in the blanks — (i) Stars rise in the ______ and set in the ______. (ii) Day and night are caused by the Earth’s ______. (iii) When the Moon fully covers the Sun from our view, it is called a ______ solar eclipse.
✔ Answer
#
Statement
Answer
Reason
(i)
Stars rise in the ____ and set in the ____.
East … West
The Earth rotates from West to East, so all sky objects (Sun, Moon, stars) appear to move from East to West.
(ii)
Day and night are caused by the Earth’s ____.
rotation (on its axis, from West to East)
Only the half facing the Sun gets light; rotation brings each place alternately into light and darkness.
(iii)
When the Moon fully covers the Sun from our view, it is called a ____ solar eclipse.
total
Observers inside the Moon’s full shadow see no part of the Sun at all.
3
State whether True or False — (i) Lunar eclipse occurs when the Sun comes between the Earth and the Moon. (ii) Sunrise happens earlier in Gujarat than in Jharkhand. (iii) In Chennai, the longest day occurs on the summer solstice. (iv) We should watch the solar eclipse directly with our naked eye. (v) Seasons occur due to the tilt of Earth’s axis of rotation and its spherical shape. (vi) The Earth’s revolution around the Sun causes day and night.
✔ Answer
#
Statement
True / False
Explanation
(i)
Lunar eclipse occurs when the Sun comes between the Earth and the Moon.
FALSE
A lunar eclipse occurs when the Earth comes between the Sun and the Moon (Sun → Earth → Moon).
(ii)
Sunrise happens earlier in Gujarat than in Jharkhand.
FALSE
The Earth rotates West → East, so eastern places see sunrise first. Jharkhand lies to the east of Gujarat, so sunrise happens earlier in Jharkhand.
(iii)
In Chennai, the longest day occurs on the summer solstice.
TRUE
Chennai is in the Northern Hemisphere, where the longest day is the summer solstice (around 21 June). (Being close to the Equator, the difference is small — but it is still the longest day.)
(iv)
We should watch the solar eclipse directly with our naked eye.
FALSE
The Sun is intense enough even during an eclipse to damage the eyes and cause blindness. Use special solar filters/goggles or projection methods.
(v)
Seasons occur due to the tilt of Earth’s axis of rotation and its spherical shape.
TRUE
These two together decide the intensity of sunlight and the length of daytime at a place, which produce the seasons.
(vi)
The Earth’s revolution around the Sun causes day and night.
FALSE
Day and night are caused by the Earth’s rotation (≈24 hours). Revolution (≈1 year) causes the seasons and the changing night sky.
4
Padmashree saw the Orion constellation nearly overhead at 8 pm yesterday. When will she see Orion overhead today?
✔ Answer
Step 1 — The idea
As the Earth revolves around the Sun, our view of the night sky shifts a little every day. As a result, a given star or constellation reaches the same position in the sky about 4 minutes earlier each day.
Step 2 — Where does “4 minutes” come from?
The Earth takes about 365 days to complete one revolution ($360^\circ$). So in one day it moves through
$$\frac{360^\circ}{365\ \text{days}} \approx 1^\circ \text{ per day}$$
The Earth rotates $360^\circ$ in 24 hours, so it turns $1^\circ$ in
$$\frac{24 \times 60\ \text{minutes}}{360} = \frac{1440}{360} = 4\ \text{minutes}$$
Step 3 — Apply it
Orion was overhead at 8:00 pm yesterday. Today it will be overhead 4 minutes earlier:
$$8{:}00\ \text{pm} – 4\ \text{minutes} = 7{:}56\ \text{pm}$$
Padmashree will see Orion nearly overhead at about 7:56 pm today — roughly 4 minutes earlier than yesterday.
This is also why the same constellation appears about 2 hours earlier after a month, and why we see different constellations in different seasons.
5
Nandhini saw a group of stars rising at midnight on 21 June. When will she see the same group of stars rising at midnight next year?
✔ Answer
Step 1 — Reason
The stars visible at a particular time of night depend on where the Earth is in its orbit. The Earth returns to the same position in its orbit only after it completes one full revolution around the Sun.
Step 2 — Time for one revolution
$$\text{One revolution} \approx 365\ \text{days and }6\ \text{hours} \approx 1\ \text{year}$$
Step 3 — Conclusion
So the Earth will again be at the same point in its orbit — and the same group of stars will again rise at midnight — after one year, i.e., on about 21 June next year.
She will see the same group of stars rising at midnight on 21 June of next year (about one year later).
6
Abhay noticed that when it was daytime in India, his uncle who was in the USA was generally sleeping as it was night-time there. What is the reason behind this difference?
✔ Answer
Step 1 — The Earth is spherical and opaque
At any moment, sunlight can fall on only one half of the Earth. That half has day; the other half is dark and has night.
Step 2 — India and the USA are on nearly opposite sides
India and the USA are located on almost opposite sides of the globe. So when India is on the half facing the Sun (daytime), the USA is on the half turned away from the Sun (night-time).
Step 3 — Rotation keeps swapping them
As the Earth rotates from West to East (one rotation in about 24 hours), India moves out of the sunlight and the USA moves into it. That is why their day and night are roughly opposite — and why their clocks show very different times.
Reason: the Earth is spherical, only half of it is lit by the Sun at a time, and it rotates on its axis. Because India and the USA lie on nearly opposite sides of the Earth, it is day in one when it is night in the other.
7
Four friends used the following ways to see the solar eclipse. Who among them was being careless? (i) Ravikiran used a solar eclipse goggle. (ii) Jyothi used a mirror to project the Sun’s image. (iii) Adithya saw the Sun directly with his eyes. (iv) Aruna attended a programme arranged by a planetarium.
✔ Answer
Friend
Method used
Safe?
Why
(i) Ravikiran
Solar eclipse goggle
✔ Safe
Special solar filters cut down the Sun’s intense light to a safe level.
(ii) Jyothi
Mirror to project the Sun’s image
✔ Safe
She looks at the projected image on a screen/wall, not at the Sun itself.
(iii) Adithya
Saw the Sun directly with his eyes
✘ CARELESS & DANGEROUS
Even during an eclipse the Sun is intense enough to damage the eyes and cause blindness.
(iv) Aruna
Planetarium programme
✔ Safe
Organisers provide specialised eye protection and scientific guidance.
Adithya (iii) was being careless. Never look at the Sun directly — and never through sunglasses, binoculars or telescopes either.
8
Fill in the circles in Fig. 12.18 appropriately with one of the following: Sun, Moon, Earth.
✔ AnswerFig. 12.18 (the question as given in the textbook)
Step 1 — Recall the two arrangementsSolar eclipse: the Moon comes between the Sun and the Earth ⇒ order is Sun – Moon – Earth. Lunar eclipse: the Earth comes between the Sun and the Moon ⇒ order is Sun – Earth – Moon.
Step 2 — Fill the circlesIn the solar-eclipse row the first circle is already “Sun”, so the next two are Moon and Earth. In the lunar-eclipse row the last circle is already “Moon”, so the first two are Sun and Earth.
The completed Fig. 12.18 — correct order for each type of eclipse
Solar eclipse: Sun → Moon → Earth Lunar eclipse:Sun → Earth → Moon
9
The Moon is much smaller than the Sun, yet it can block the Sun completely from our view during a total solar eclipse. Why is it possible?
✔ Answer
Step 1 — The key idea: apparent size
What matters is not the actual size of an object but its apparent size — the size it appears to be to our eye. Apparent size depends on both the object’s actual size and its distance from us. A nearby small object can look as big as a distant large object.
Step 2 — Compare the Sun and the Moon
The Sun is huge but extremely far away; the Moon is small but very close.
Step 3 — Conclusion
The two ratios are almost equal. So even though the Sun is about 400 times bigger than the Moon, it is also about 400 times farther away. Therefore the apparent sizes of the Sun and the Moon in the sky are nearly the same, and the Moon can just cover the Sun’s disc completely — giving a total solar eclipse.
This is exactly what Activity 12.4 showed: a small thumb held close to the eye can completely cover a much bigger head that is 5 metres away.
10
The Indian cricket team matches in Australia are often held in December. Should they pack winter or summer clothes for their trip?
✔ Answer
Step 1 — Where is Australia?Australia lies in the Southern Hemisphere; India lies in the Northern Hemisphere.
Step 2 — What happens in December?In December, the Southern Hemisphere is tilted towards the Sun. So it receives more intense sunlight for a longer time (days longer than 12 hours) ⇒ it is SUMMER there. The Northern Hemisphere (India) is tilted away ⇒ winter in India.
Step 3 — ConclusionSeasons in the two hemispheres are opposite. So while it is winter in India in December, it is summer in Australia.
The team should pack SUMMER clothes (light clothing) for their December trip to Australia.
11
Why do you think lunar eclipses can be seen from a large part of the Earth when they happen, but a total solar eclipse can be seen by only a small part of the Earth?
✔ Answer
Step 1 — Lunar eclipse
The Earth is much bigger than the Moon, so it casts a very large shadow. During a lunar eclipse, this big shadow falls on the Moon and the Moon itself becomes dark. Since the darkened Moon is up in the sky, anyone on the entire night side of the Earth (from where the Moon is visible) can see it. That is a very large part of the Earth — almost half of it.
Step 2 — Solar eclipse
The Moon is much smaller than the Earth, so the dark cone of its shadow (umbra) that reaches the Earth is very small — it covers only a narrow region a few hundred kilometres wide on the Earth’s surface. Only observers standing inside this small shadow patch can see a total solar eclipse. People outside it see only a partial eclipse, or nothing at all.
Total solar eclipse
Lunar eclipse
Whose shadow?
The Moon’s shadow falls on the Earth
The Earth’s shadow falls on the Moon
Size of shadow
Very small patch on the Earth’s surface
Large — big enough to cover the whole Moon
Who can see it?
Only people inside that small patch
Everyone on the night side of the Earth
Duration
Only a few minutes (the shadow moves quickly)
Can last for an hour or more
In a solar eclipse we must be inside a small shadow; in a lunar eclipse we simply have to be able to see the darkened Moon — which is possible from anywhere on the night side of the Earth.
12
If the Earth’s axis were not tilted with respect to the axis of revolution, explain what would be the effect on seasons?
✔ Answer
Step 1 — Why seasons happen now
Seasons occur because the Earth’s axis is tilted. Due to this tilt, a hemisphere is sometimes tilted towards the Sun (more intense sunlight + longer days ⇒ summer) and six months later tilted away from it (less intense sunlight + shorter days ⇒ winter).
Step 2 — Remove the tilt
If the axis were upright (not tilted), then no hemisphere would ever lean towards or away from the Sun. Throughout the year:
The intensity of sunlight at any place would stay nearly the same all year round.
Every place on the Earth would have about 12 hours of day and 12 hours of night, every single day.
The North Pole and the South Pole would not have six months of day and six months of night.
Step 3 — Effect on seasons
There would be NO cycle of seasons — no summer and no winter. Instead, each place would have one fixed kind of weather all year, depending only on its latitude: regions near the Equator would be permanently hot, the mid-latitudes permanently mild, and the polar regions permanently cold.
Without the tilt, the seasons would disappear. Every day would be like an equinox day — this proves that the tilt of the axis (and not the Earth’s distance from the Sun) is the real cause of the seasons.
★
Exploratory Projects — (a) Repeat Activity 12.2 with a lamp and a tilted globe placed at different positions on a circle. (b) Draw two circles of radius 14.7 cm and 15.2 cm (1 cm = 10 million km) to represent the closest and farthest distances from the Sun. (c) If the tilt of the Earth’s axis increased, would seasons be more extreme? What about Uranus?
✔ Answer / Guidance
(a) Globe + lamp around a circle
Keep the tilt of the globe fixed (always pointing in the same direction) as you move it to different positions around the lamp — just as the real Earth does.
(i) At one position, the Northern Hemisphere is more illuminated (and the North Pole is fully lit) — this is June. Half a circle away, the Southern Hemisphere is more illuminated — this is December. At the two in-between positions (March, September), both hemispheres are equally illuminated.
(ii) & (iii) On rotating the globe, you will find that in June the places in the Northern Hemisphere stay in the light for more than half of each rotation (longer days), while in December they stay in the light for less than half (shorter days). In March and September, day and night are almost equal (12 h each) everywhere.
This demonstrates that the tilt + revolution together produce the seasons and the changing length of the day.
(b) How different are the closest and farthest distances?
Step-wise calculation
Scale given: $1\ \text{cm} = 10\ \text{million km}$
$$\text{Closest distance} = 14.7\ \text{cm} \times 10 = 147\ \text{million km}$$
$$\text{Farthest distance} = 15.2\ \text{cm} \times 10 = 152\ \text{million km}$$
$$\text{Difference} = 152 – 147 = 5\ \text{million km}$$
As a percentage of the average distance (about 150 million km):
$$\frac{5}{150}\times 100 \approx 3.3\%$$
The two circles look almost identical — the difference is only about 3%. This tiny change in distance cannot cause the seasons. (In fact, the Earth is closest to the Sun in January, which is winter in the Northern Hemisphere!)
(c) What if the tilt were larger? What about Uranus?
Yes — a larger tilt would make the seasons more extreme. A hemisphere would lean much more sharply towards the Sun (fiercely hot, very long summer days) and then much more sharply away (very cold, very long winter nights).
The Earth’s axis is tilted at about 23.5°. Uranus is tilted by about 98° — it is practically lying on its side as it orbits the Sun. So its tilt is far more than the Earth’s.
Because of this extreme tilt, each pole of Uranus faces the Sun almost directly for about 42 Earth-years of continuous daylight, followed by about 42 years of continuous darkness (Uranus takes about 84 Earth-years to go once around the Sun) — giving it the most extreme seasons in the Solar System.
Use these points to write your newspaper / school-magazine article, adding a diagram comparing the tilt of the Earth (23.5°) with that of Uranus (98°).
Solutions prepared from NCERT Curiosity — Textbook of Science, Grade 7, Chapter 12 “Earth, Moon, and the Sun” (pages 169–186). Diagrams reproduced from the chapter for study purposes.
