Full Chapter Summary & Detailed Notes - Earth, Moon, and the Sun
Introduction – Rashmika’s Observation
The chapter opens with a beautiful real-life story of Rashmika, a 12-year-old girl in Kanniyakumari, Tamil Nadu. While cycling to school in the morning, she notices that the shadows of coconut trees are very long. On her way back in the afternoon, the same shadows are much shorter. She wonders:
“Why do shadows change their length? Is it because the Sun is moving across the sky?”
But she remembers from Grade 6 (chapter “Beyond Earth”) that the Earth moves around the Sun. So she gets puzzled: Does the Sun move? Or does the Earth move? This question becomes the central theme of the entire chapter.
12.1 Rotation of the Earth – The Real Reason
We all see the Sun “rising” in the East and “setting” in the West every day. But actually, the Sun is almost stationary. It only appears to move because the Earth is rotating (spinning) on its own axis.
Activity 12.1: Let us explore – The Merry-Go-Round Experiment
This is the most important activity of the chapter and is asked in almost every exam.
- Sit on a merry-go-round facing outward.
- Ask someone to rotate it slowly in the anti-clockwise direction.
- Observation 1: All stationary objects (trees, buildings, people) around you appear to move in the opposite direction — clockwise direction.
- Observation 2: Fix your gaze on one tree. It appears to come from your left side, move across in front of you, and disappear on your right side.
Conclusion from Activity: When the observer (you) rotates anti-clockwise, stationary objects appear to move clockwise. Exactly the same thing happens on Earth!
Applying the Merry-Go-Round Logic to Earth
- Earth rotates from West to East (anti-clockwise if seen from above North Pole).
- Because of this rotation, the stationary Sun appears to move from East to West.
- That is why the Sun “rises” in the East and “sets” in the West.
Major Effects of Earth’s Rotation
| Effect | Explanation |
|---|
| Day and Night | Earth takes 24 hours for one rotation. The side facing Sun → Day; opposite side → Night. |
| Changing Length of Shadows | Morning/Evening: Sun low → long shadows
Noon: Sun overhead → shortest shadows (Rashmika’s observation) |
| Apparent Motion of Sun & Stars | All celestial objects appear to move East to West daily. |
Important Facts about Rotation
- Time period: 24 hours (1 day)
- Direction: West to East
- Axis: Imaginary line from North Pole to South Pole (tilted 23.5° – causes seasons, but not in this chapter)
- Speed: ~1670 km/h at equator, but we don’t feel it (constant speed + gravity)
Scientific Proofs of Earth’s Rotation
- Foucault’s Pendulum (1851): Pendulum swing direction changes slowly → Earth rotates beneath it.
- Coriolis Effect: Moving objects (winds, missiles) deflect right in Northern Hemisphere, left in Southern.
- Satellite Images: Direct visual proof of spinning Earth.
Common Mistakes & Clarifications
- Day-night is caused by rotation (24 hrs), not revolution.
- Seasons are caused by revolution + axial tilt (365 days).
- We don’t feel rotation because speed is constant and atmosphere rotates with us.
Key Takeaways for Exams
| Concept | One-Liner Answer |
|---|
| Why does the Sun appear to rise in East & set in West? | Due to Earth’s rotation from West to East |
| Activity 12.1 conclusion? | Merry-go-round anti-clockwise → objects appear clockwise → same as Earth’s rotation makes Sun appear to move East to West |
| Shadow length change? | Morning & evening long, noon shortest |
| Time for one rotation? | 24 hours |
| Direction of rotation? | West to East |
Golden Exam Answer (Write this in every 3/5 mark question)
“The Sun appears to rise in the east and set in the west because the Earth rotates on its axis from west to east. This rotation takes 24 hours and also causes day and night as well as the change in length of shadows during the day.”
Activities + 25+ Questions and Answers - Earth, Moon, and the Sun
Activities with Detailed Observations and Inferences
Activity 12.1: Let Us Explore (Merry-Go-Round)
- Setup: Sit on a merry-go-round facing outward. Turn it slowly anti-clockwise.
- Observation 1: Stationary objects (trees, buildings) appear to move in the opposite direction (clockwise).
- Observation 2: A fixed tree appears from the left, moves across view, disappears on the right.
- Reason: When the observer rotates, stationary objects seem to move oppositely.
- Inference: Earth's West to East rotation makes stationary Sun appear to move East to West.
- Exam Tip: This activity proves apparent motion; always mention direction.
NCERT In-Text Questions - Solved
1. Do the objects around you appear to be moving? In which direction do they appear to be moving?
Yes, objects appear to move clockwise when merry-go-round turns anti-clockwise.
2. In which direction do you find the tree turning around you? Is it in your view all the time?
Tree turns clockwise; appears from left, disappears on right – not in view all time.
3. Does it indicate that the Sun is moving in the sky? Or might it be that the Earth itself is turning around and the Sun just appears to move?
Earth is turning; Sun appears to move.
Let Us Enhance Our Learning (End-of-Chapter Exercises) - Fully Solved
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?
During one rotation of the Earth, the North and South Poles experience 24 hours of either sunlight or darkness, depending on the season. This is because the Earth's axial tilt causes these poles to either face the sun constantly (24 hours of sunlight) or be completely shielded from the sun (24 hours of darkness) for periods of several months. According to the figure, the South Pole is receiving 24 hours of sunlight, while the North Pole is in complete darkness.
2. Fill in the blanks.
(i) Stars rise in the East and set in the West.
(ii) Day and night are caused by the Earth’s rotation.
(iii) When the Moon fully covers the Sun from our view, it is called a total solar eclipse.
3. State whether True or False.
(i) Lunar eclipse occurs when the Sun comes between the Earth and the Moon. → False (Lunar eclipse happens when Earth is between Sun and Moon.)
(ii) Sunrise happens earlier in Gujarat than in Jharkhand. → True (Gujarat is west, so Sun rises earlier there.)
(iii) In Chennai, the longest day occurs on the summer solstice. → True (Summer solstice around June 21 has longest day in Northern Hemisphere.)
(iv) We should watch the solar eclipse directly with our naked eye. → False (Can damage eyes; use safe methods like pinhole projector.)
(v) Seasons occur due to the tilt of Earth’s axis of rotation and its spherical shape. → True (Tilt causes varying sunlight; spherical shape affects angle.)
(vi) The Earth’s revolution around the Sun causes day and night. → False (Day-night caused by rotation; revolution causes seasons.)
4. Padmashree saw the Orion constellation nearly overhead at 8 pm yesterday. When will she see Orion overhead today?
She will see Orion overhead approximately 4 minutes earlier each day due to the Earth's rotation and revolution.
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?
Nandhini will see the same group of stars rising at midnight on 21 June next year as well, since the Earth completes one full revolution around the Sun in about 365 days, so the position of the stars is similar each year.
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?
This difference is due to the Earth’s rotation. As the Earth rotates, different parts of the world experience day and night at different times. India and the USA are in different time zones.
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. → Safe
(ii) Jyothi used a mirror to project the Sun’s image. → Safe
(iii) Adithya saw the Sun directly with his eyes. → Careless (Can damage eyes)
(iv) Aruna attended a programme arranged by a planetarium. → Safe
8. Fill in the circles in Fig. 12.18 appropriately with one of the following: Sun, Moon, Earth.
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?
The Moon can block the Sun completely during a total solar eclipse because it is much closer to the Earth than the Sun. The apparent sizes of the Moon and the Sun are similar when viewed from Earth.
10. The Indian cricket team matches in Australia are often held in December. Should they pack winter or summer clothes for their trip?
The Indian cricket team should pack summer clothes for their trip to Australia in December because, in the Southern Hemisphere, December is summer. While India experiences winter during this time, Australia has warm weather, so they will need light and comfortable clothing suitable for summer.
11. Why do you think lunar eclipses can be seen from a large part of the Earth when they happen, but total solar eclipse can be seen by only a small part of the Earth?
Lunar eclipses are visible from a larger part of the Earth because the Earth’s shadow is large, and it covers a wide area. A solar eclipse, however, is visible only from a small part of the Earth because the Moon’s shadow is much smaller and concentrated in specific regions.
12. If the Earth’s axis were not tilted with respect to the axis of revolution, explain what would be the effect on seasons?
If the Earth's axis were not tilted, there would be no seasonal changes. The amount of sunlight received at different latitudes would remain the same throughout the year, leading to a uniform climate year-round.
Extra Practice Questions (Exam Booster – 25+ Questions)
1. Why does the Sun appear to rise in the East and set in the West?
Due to Earth's rotation from West to East, the Sun appears to move from East to West.
2. Explain the merry-go-round activity and its relation to Earth's rotation.
When merry-go-round turns anti-clockwise, stationary objects appear clockwise. Similarly, Earth's West to East rotation makes Sun appear East to West.
3. What is the direction of Earth's rotation?
West to East (anti-clockwise from North Pole).
4. How long does Earth take to complete one rotation?
24 hours.
5. What causes day and night?
Earth's rotation on its axis.
6. Why are shadows longer in the morning and evening?
Sun is low in the sky, so light rays are slanted, making shadows long.
7. Why are shadows shortest at noon?
Sun is directly overhead, so light rays are vertical, making shadows short.
8. What is the axis of rotation?
Imaginary line from North Pole to South Pole.
9. Why don't we feel Earth's rotation?
It is smooth and constant, with no sudden changes, and the atmosphere rotates with us.
10. What is Foucault's pendulum?
A long pendulum that shows Earth's rotation by changing swing direction over time.
11. What is the Coriolis effect?
Deflection of moving objects (winds, currents) due to Earth's rotation – right in Northern Hemisphere, left in Southern.
12. How does Earth's rotation cause the apparent motion of stars?
Stars appear to move from East to West, just like the Sun.
13. What would happen if Earth didn't rotate?
No day-night cycle; one side always day, other always night.
14. What is the speed of Earth's rotation at the equator?
Approximately 1670 km/h.
15. How is the Earth's axis tilted?
23.5 degrees (causes seasons).
16. Differentiate between rotation and revolution.
Rotation: Spin on axis (day-night, 24 hrs). Revolution: Orbit around Sun (seasons, 365 days).
17. Why does the Sun's position change throughout the day?
Due to Earth's rotation causing apparent motion.
18. Explain Rashmika's observation of shadows.
Morning: Sun low → long shadows; Afternoon: Sun high → short shadows, due to Earth's rotation.
19. What is the geocentric model?
Ancient belief: Earth stationary, Sun moves around it.
20. What is the heliocentric model?
Sun stationary, Earth revolves around it (Copernicus).
21. How does rotation affect time zones?
Earth divided into 24 time zones, each 15° longitude, due to 24-hour rotation.
22. What is a sidereal day?
Time for one rotation relative to stars (23 hours 56 minutes).
23. What is a solar day?
24 hours (relative to Sun).
24. Why is Kanniyakumari mentioned?
Tropical location with clear shadow changes; southern tip of India.
25. If Earth rotated faster, what would happen?
Shorter days, stronger winds, more extreme weather.
Scientific Skills Step-by-Step (Chapter 12)
Skill 1: Understanding Apparent Motion through Analogy
• Sit on merry-go-round facing outward
• Turn anti-clockwise slowly
• Observe stationary objects appear to move clockwise
• Fix gaze on one tree — it moves from left to right
Conclusion: When observer rotates, stationary objects appear to move in opposite direction — same as Earth’s rotation makes Sun appear to move.
Skill 2: Observing Daily Shadow Observation (DIY Activity)
• Fix a stick vertically in ground
• Mark shadow length at 8 AM, 12 noon, 4 PM
• Measure with scale
• Observe: longest in morning/evening, shortest at noon
Conclusion: Changing shadow length proves Earth’s rotation and apparent Sun motion.
Skill 3: Foucault Pendulum Demonstration (Advanced)
• Long pendulum allowed to swing freely
• Over hours, swing direction appears to change
• Actually Earth rotates beneath it
Conclusion: Direct proof of Earth’s rotation.
Skill 4: Sunrise-Sunset Direction Observation
• Note exact direction of sunrise and sunset for a week
• Sunrise always in East, sunset in West
Conclusion: Consistent direction due to Earth’s fixed rotation.
Best Exam Answer Tip:
“The Sun appears to rise in the east and set in the west due to the rotation of the Earth from west to east.”
→ Write this line in every 3-mark/5-mark question on this topic!
