Notes Chapter 12 Class 7 Science Curiosity NCERT

Notes For All Chapters – Science Curiosity Class 7

Earth, Moon, and the Sun

1. Introduction

• Rashmika observed that shadows of coconut trees are long in the morning and shorter in the afternoon.
• She thought this happened because the Sun moves across the sky.
• But she also remembered that Earth moves around the Sun.
• This raises an important question:

  Does the Sun move or does the Earth move?

• The truth is that the Earth rotates on its axis, which makes the Sun appear to move.

2. Rotation of the Earth

Understanding Rotation

• Rotation is the motion of an object in which all its parts move in circles around an imaginary line passing through it.
• This imaginary line is called the axis of rotation.

Examples

• Spinning top
• Rotating fan
• Spinning ball

Earth’s Axis

• The Earth spins on an imaginary axis.
• This axis passes through:
  • North Pole
  • South Pole

Direction of Earth’s Rotation

• When viewed from above the North Pole, the Earth rotates:
  from West to East (anticlockwise direction).

Time for One Rotation

• The Earth completes
  one rotation in about 24 hours
  .

3. Activity to Understand Rotation

Using a merry-go-round activity:

• Sit on a merry-go-round facing outward.
• When it rotates anticlockwise, surrounding objects appear to move clockwise.
• A fixed tree appears:
  • From the left side
  • Moves across
  • Disappears on theright side.

Conclusion

• When the observer moves, objects appear to move in the opposite direction.
• Similarly, because the Earth rotates, the Sun appears to move across the sky.

4. Day and Night

Cause of Day and Night

• Day and night occur because the Earth rotates on its axis.

Explanation

• Only half of the Earth receives sunlight at a time.
• The other half remains in darkness.

Therefore

• Sun-facing side → Day
• Opposite side → Night

Sunrise and Sunset

• Because the Earth rotates from West to East:
  • The Sun appears to rise in the East.
  • The Sun appears to move across the sky.
  • The Sun sets in the West.

5. Understanding Day–Night Using a Globe

Activity with globe and torchlight:

Steps:

1. Use a globe to represent Earth.
2. Use a torch as the Sun.
3. Shine light on the globe in a dark room.

Observations:

• Half of the globe becomes bright (day).
• The other half remains dark (night) .

Conclusion:

• The rotation of Earth causes the day-night cycle.

6. Sunrise in India

• Sunrise occurs earlier in eastern India.
• Then sunlight gradually reaches other parts of the country.

Reason:

• Earth rotates West to East.

7. Apparent Motion of the Sun

Because of Earth’s rotation:

• The Sun appears to:
  1. Rise in the East
  2. Move across the sky
  3. Set in the West

At night:

• The stars become visible.

8. Demonstration of Earth’s Rotation

Foucault Pendulum

• A long pendulum used to demonstrate Earth’s rotation.

Scientist:

• Leon Foucault

Example in India:

• A 22-metre Foucault pendulum is installed in the new Parliament building in New Delhi.

9. Apparent Motion of Stars

Observation:

• Stars appear to move across the sky during the night.

Example Activity:

• Observe the Big Dipper (Saptarishi) and Pole Star (Dhruva Tara).
• Draw their positions every two hours.

Observation:

• The Big Dipper appears to move around the Pole Star.

Why Pole Star Appears Fixed

• Earth’s axis points close to the Pole Star.
• Therefore the Pole Star appears almost stationary.

Movement of Moon

• Just like the Sun and stars, the Moon also appears to rise in the East and set in the West due to Earth’s rotation.

10. Star Trails

Astrophotographers use long-exposure photography.

Result:

• Stars appear as circular arcs in photographs.

These arcs are called star trails.

11. Ancient Indian Astronomy

Aryabhata

• Famous Indian mathematician and astronomer.
• Lived in the 5th century CE.

Contributions:

• Explained the apparent motion of stars.
• Stated that stars appear to move because Earth rotates.

His calculation of Earth’s rotation:

• 23 hours 56 minutes 4.1 seconds

Very close to modern measurement.

12. Revolution of the Earth

Definition

Revolution is the motion of an object around another object.

Example:

• Earth revolves around the Sun.

Orbit

• The path followed during revolution is called an orbit.

Shape of Earth’s Orbit

• The orbit is nearly circular.

Time for One Revolution

• Earth takes 365 days and 6 hours to complete one revolution.

This period forms one year.

13. Changing View of Night Sky

Because Earth revolves around the Sun:

• The stars visible after sunset change during the year.

Example:

• Different stars rise at sunset in March, June, September, and December.

Indigenous Knowledge

• Communities like Bhil and Pawara in western India used patterns of stars to predict monsoon.

14. Seasons on the Earth

Earth experiences a cycle of seasons every year.

Examples:

• Summer
• Winter
• Spring
• Autumn

15. Cause of Seasons

Seasons occur due to:

1. Tilt of Earth’s axis
2. Spherical shape of Earth

16. Tilt of Earth’s Axis

• The Earth’s axis is tilted with respect to its orbit.
• The tilt remains constant while Earth revolves around the Sun.

17. Summer in Northern Hemisphere

Occurs around June.

Features:

• Northern Hemisphere tilted towards the Sun.
• Sunlight falls on smaller area → more intense heat.
• Daytime lasts more than 12 hours.

Result:

• Summer season

18. Winter in Northern Hemisphere

Occurs around December.

Features:

• Northern Hemisphere tilted away from the Sun.
• Sunlight spreads over larger area → less heat.
• Daytime is shorter than night.

Result:

• Winter season

19. Southern Hemisphere Seasons

Seasons are opposite in the Southern Hemisphere.

20. Solstices

Summer Solstice

• Occurs around 21 June.
• Longest day of the year in Northern Hemisphere.

Winter Solstice

• Occurs around 22 December.
• Shortest day and longest night in Northern Hemisphere.

21. Equinox

Two days when day and night are equal (12 hours each):

1. Spring Equinox – around 21 March
2. Autumn Equinox – around 23 September

22. Special Situation at the Poles

North Pole

• Sun rises on 21 March.
• Sun remains visible for 6 months.
• Darkness lasts next 6 months.

South Pole

• Experiences opposite situation.

23. Equator Region

At the Equator:

• Day and night are almost equal throughout the year.
• Seasonal changes are less noticeable.

24. Eclipses

Many natural phenomena depend on the Sun.

But sometimes sunlight gets blocked, causing eclipses.

Types:

1. Solar eclipse
2. Lunar eclipse

25. Solar Eclipse

Definition

A solar eclipse occurs when the Moon comes between the Sun and Earth, blocking sunlight.

Arrangement:

Sun → Moon → Earth

Apparent Size Concept

Even though the Moon is smaller than the Sun, it can block the Sun because:

• The Moon is much closer to Earth.
• Therefore its apparent size looks similar to the Sun.

Types of Solar Eclipse

Total Solar Eclipse

• Moon completely blocks the Sun.
• Small area on Earth becomes dark for a few minutes.

Partial Solar Eclipse

• Moon blocks only part of the Sun.

26. Diamond Ring Effect

• Just when the Moon begins to move away from the Sun,
• A bright ring appears called the Diamond Ring.

27. Safety During Solar Eclipse

Never look at the Sun directly.

Do NOT use:

• Naked eyes
• Sunglasses
• Binoculars
• Telescopes

Safe ways:

• Solar eclipse glasses
• Projection methods
• Observing through astronomy events.

28. Superstitions About Eclipses

Earlier people:

• Did not know the scientific reason.
• Believed eclipses were bad events.

Examples of beliefs:

• Not eating food
• Not going outside

Now we know eclipses are natural astronomical events.

29. Lunar Eclipse

Definition

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, blocking sunlight from reaching the Moon.

Arrangement:

Sun → Earth → Moon

Types of Lunar Eclipse

Total Lunar Eclipse

• Moon fully enters Earth’s shadow.
• Moon appears dark red.

Partial Lunar Eclipse

• Only part of the Moon enters the shadow.

Observation

• Lunar eclipses are safe to observe with naked eyes.

30. Astronomy in India

Kodaikanal Solar Observatory

• Located in Tamil Nadu.
• Established in 1899.
• Provides scientific data about the Sun.

31. Indian Scientist

M. K. Vainu Bappu

• Known as Father of Modern Indian Astronomy.

Contributions:

• Established telescopes in India:
  • Manora Peak (Nainital)
  • Kavalur Observatory (Tamil Nadu)
• Studied stars and discovered a comet.
• Travelled worldwide to study solar eclipses.

32. Summary of the Chapter

• The Earth rotates on its axis in about 24 hours.
• Rotation causes day and night.
• Earth revolves around the Sun in about 365 days.
• The tilt of Earth’s axis causes seasons.
• The Moon can block sunlight and cause a solar eclipse.
• The Earth can block sunlight reaching the Moon and cause a lunar eclipse.