NCERT Class 7 Science Curiosity Chapter 12 Earth Moon and the Sun PDF Download

Chapter 12: Earth, Moon, and the Sun

One morning in Kanniyakumari, Tamil Nadu, 12-year-old Rashmika was eagerly cycling to school. She was excited. That was the day her science teacher had dedicated a class for students to share and try to explain interesting observations.

Rashmika had been noticing that the coconut tree shadows were long in the morning but shorter in the afternoon on her way back. She thought about it and decided that the size of the shadows changed because the Sun moved across the sky during the day. But she also remembered learning that the Earth moved around the Sun (in chapter 'Beyond Earth' in the Grade 6 Science textbook Curiosity) so she was puzzled. She wondered - does the Sun move in the sky? Or does the Earth move?

[Figure: Children cycling past coconut trees with long and short shadows visible, See in your textbook]

12.1 Rotation of the Earth

You might have also noticed that the Sun rises in the East and sets in the West. Have you ever wondered why? Let us try to understand why. Have you ever enjoyed riding a merry-go-round at a park or at your school? Let us go back to riding a merry-go-round!

Activity 12.1: Let us explore

• Sit on a merry-go-round facing towards the outer side as shown in Fig. 12.1.
• Ask someone to turn the merry-go-round slowly in the anti-clockwise direction as shown in Fig. 12.1. While you are sitting on the moving merry-go-round, look around you. Do the objects around you appear to be moving? In which direction do they appear to be moving?

While you turn in the anti-clockwise direction, the objects appear to turn around you in the opposite direction, that is, in the clockwise direction.

• Now fix your gaze at a particular tree (or a building) ahead of you while sitting on the merry-go-round turning around in anti-clockwise direction.

In which direction do you find the tree turning around you? Is it in your view all the time?

The tree appears to turn around you in the opposite direction, that is, the clockwise direction. The tree appears in your view from your left-hand side and then moves out of your view on the right-hand side when you view it from a merry-go-round turning around in anti-clockwise direction.

[Figure 12.1: A girl observing objects around her while riding a merry-go-round, See in your textbook]

Using the observations made by us while riding a merry-go-round, let us now think. When we view from the Earth, the Sun appears in the East, moves across the sky from the East to the West and disappears in the West. 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?

The fact is that the Sun appears to be moving because we view it from the Earth, which is turning around itself.

Teacher's Note

Think of yourself on the merry-go-round: when you spin, everything around you appears to move in the opposite direction. The Sun and our sky work the same way. The Sun looks like it travels across the sky from East to West, but really the Earth is spinning beneath us from West to East. This makes the Sun appear to move. Remember this backwards motion whenever you see the sunrise or sunset.

In which way is the Earth turning around itself? To visualise this, let us recall some of the objects which turn around themselves. Have you watched a top spinning around its spindle (Fig. 12.2a)? Or a spinning fan (Fig. 12.2b)? Or tried spinning a ball (Fig. 12.2c)?

[Figure 12.2: (a) A spinning top (b) A spinning fan (c) A spinning ball, See in your textbook]

In a similar manner, the Earth also spins (or rotates) on its own axis in space as shown in Fig. 12.3. The Earth's axis of rotation passes through its geographic North Pole and the South Pole. The Earth completes one rotation in about 24 hours.

When viewed from the top of the North Pole (Fig. 12.3), the Earth is rotating in the anti-clockwise direction, that is, from West to East.

[Figure 12.3: Rotation of the Earth, showing the North Pole, South Pole, Equator, Northern Hemisphere, Southern Hemisphere, Earth's axis of rotation, and the direction of rotation from West to East, See in your textbook]

Let us try to understand this with the help of a globe. You have used a globe in Grade 6 to represent the Earth and identified North Pole, South Pole, and Equator on it. You also learnt that its axis passed through its North and South poles (in the Grade 6 Social Science textbook Exploring Society India and Beyond).

Activity 12.2: Let us explore

• Use a globe to represent the Earth and place a small sticker to mark your location on it (Fig. 12.4a).
• While viewing from above the North Pole, slowly rotate the globe on its axis in anti-clockwise direction.
• Observe how your location turns around and finally comes back to its original position completing one rotation.

[Figure 12.4(a): Using a globe to understand rotation of the Earth, See in your textbook]

Dive Deeper

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.

Teacher's Note

One rotation takes 24 hours, which is why we have day and night. When your location on the globe faces the Sun during rotation, you experience day. When it rotates away from the Sun, you experience night. This happens every 24 hours because that is how long one complete rotation takes.

Key Points

• The Sun appears to rise in the East and set in the West, but this is because the Earth is rotating, not because the Sun is moving.
• The Earth rotates (or spins) on its own axis, which passes through the North Pole and South Pole.
• When viewed from above the North Pole, the Earth rotates in an anti-clockwise direction, from West to East.
• The Earth completes one full rotation in about 24 hours, which is why we have day and night.
• Rotation is the spinning motion of an object around an imaginary line called the axis of rotation.

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