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๐ฌ Think about it
On a merry-go-round, the trees seem to spin past you, but you know it is you who is turning. Could the Sun's daily journey across the sky be the same kind of trick? Let us explore the Earth's spin.
The Earth spinning on its axis through the poles.
What is the rotation of the Earth?
- The Earth spins on its own axis in space.
- Its axis passes through the North Pole and South Pole.
- The Earth completes one rotation in about 24 hours.
- Seen from above the North Pole, it spins West to East.
Example:
A spinning top turns around its spindle, just as the Earth turns around its axis.
A spinning top, fan, and ball turning on themselves.
Why does the Sun appear to move across the sky?
- We view the Sun from the spinning Earth.
- So the Sun appears to rise in the East.
- It seems to move across the sky and set in the West.
- Really, the Earth is turning, not the Sun.
Example:
On a merry-go-round turning anti-clockwise, a tree seems to move clockwise past you, though the tree is still.
A girl seeing the Sun rise, peak, and set as the Earth turns.
In this Activity, we will use a merry-go-round to see how moving makes still things appear to move.
A girl on a turning merry-go-round watching trees move past.
Materials needed
A merry-go-round and a helper to turn it.
Procedure
1. Sit on a merry-go-round facing the outer side.
2. Ask someone to turn it slowly in the anti-clockwise direction.
3. Look around and note which way the objects appear to move.
4. Fix your gaze on one tree or building ahead of you.
5. Watch how the tree comes into and goes out of your view.
2. Ask someone to turn it slowly in the anti-clockwise direction.
3. Look around and note which way the objects appear to move.
4. Fix your gaze on one tree or building ahead of you.
5. Watch how the tree comes into and goes out of your view.
Observation
As you turn anti-clockwise, the trees appear to move clockwise. A tree comes into view from the left and leaves on the right.
Explanation
The trees only appear to move because you are turning. In the same way, the Sun appears to move because the Earth spins.
โ Summary
- Sat on turning merry-go-round
- Trees seemed to move
- Really you were turning
- Same as Earth's spin
๐งฌ Dive Deeper
An object turning around its axis of rotation.
- Rotation is the motion of an object turning around itself.
- All its parts move in circles around an imaginary line.
- This line is called the axis of rotation.
- The Earth's axis passes through its North and South Poles.
In this Activity, we will use a globe and a torch to understand rotation and the day-night cycle.
A globe lit on one half by a torch acting as the Sun.
Materials needed
A globe, a small sticker, a torch, and a dark room.
Procedure
1. Mark your location on the globe with a small sticker.
2. Viewing from above the North Pole, rotate the globe anti-clockwise.
3. Watch your location turn around and return after one rotation.
4. In a dark room, shine a torch (the Sun) on the globe from about 1.5 m.
5. Notice one half is lit and the other half is dark.
6. Rotate from West to East and watch your location move through day and night.
2. Viewing from above the North Pole, rotate the globe anti-clockwise.
3. Watch your location turn around and return after one rotation.
4. In a dark room, shine a torch (the Sun) on the globe from about 1.5 m.
5. Notice one half is lit and the other half is dark.
6. Rotate from West to East and watch your location move through day and night.
Observation
One half of the globe is lit (day) and the other is dark (night). As it rotates West to East, your location moves into light at sunrise and into darkness at sunset.
Explanation
The Earth's rotation from West to East causes the day-night cycle. The side facing the Sun has day; the other side has night.
โ Summary
- Globe rotated on axis
- Torch lit one half
- Lit half is day
- Rotation causes day-night
In this Activity, we will watch the Big Dipper to see the effect of the Earth's rotation on the stars.
The Big Dipper sketched moving around the Pole Star.
Materials needed
A clear night sky between March and May, paper, and a pencil.
Procedure
1. On an early evening, identify the Big Dipper and the Pole Star.
2. Note your location and date; do it all on the same night.
3. Draw the Big Dipper's position with respect to the Pole Star and mark the time.
4. After two hours, observe and draw it again, noting the time.
5. Repeat after another two hours.
6. See whether the Big Dipper appears to move around the Pole Star.
2. Note your location and date; do it all on the same night.
3. Draw the Big Dipper's position with respect to the Pole Star and mark the time.
4. After two hours, observe and draw it again, noting the time.
5. Repeat after another two hours.
6. See whether the Big Dipper appears to move around the Pole Star.
Observation
The Big Dipper appears to move around the Pole Star, which stays nearly fixed.
Explanation
The Earth's axis points close to the Pole Star, so it appears still while other stars seem to circle it. This is due to the Earth's rotation.
โ Summary
- Big Dipper sketched thrice
- It moved each time
- Pole Star stayed fixed
- Caused by Earth's rotation
โฒ Fascinating Facts
A long Foucault pendulum hanging from a high ceiling.
- Leon Foucault used a long pendulum to show the Earth's rotation.
- It is now called a Foucault pendulum in his honour.
- A 22-metre Foucault pendulum hangs in the new Parliament building, New Delhi.
- It links the idea of India with the vastness of the cosmos.
๐ท Fascinating Facts
A night photo showing star trails as curved arcs.
- Astrophotographers keep a camera's shutter open for a long time.
- The stars' apparent motion is then recorded as arcs.
- These arcs of a circle are called star trails.
- They show the effect of the Earth's rotation.
๐ Fascinating Facts
An old manuscript of Aryabhata's Aryabhatiya.
- Ancient Indian astronomers noticed the daily motion of the sky.
- Aryabhata wrote the Aryabhatiya around the fifth century CE.
- He explained the apparent star motion using a boat example.
- His value for one rotation is very close to today's value.
Important Points
- The Earth rotates on its axis in about 24 hours.
- It rotates from West to East, causing day and night.
- The Sun, Moon, and stars only appear to move.
โ Test Yourself
-
What is rotation?
View Answer
The motion of an object turning around its own axis. -
How long does one rotation of the Earth take?
View Answer
About 24 hours. -
In which direction does the Earth rotate?
View Answer
From West to East. -
What does the Earth's rotation cause?
View Answer
The day-night cycle. -
Why does the Pole Star appear fixed?
View Answer
The Earth's axis points very close to it.
Important Definitions
- Rotation — the motion of an object in which all its parts move in circles around an imaginary line through it.
- Axis of rotation — the imaginary line through the North and South Poles about which the Earth spins.
๐ NCERT Question 2 — Fill in the blanks
Fill in the blanks about where stars rise and set, what causes day and night, and the total solar eclipse.
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๐ NCERT Question 6 — Abhay noticed that when
Abhay noticed it was daytime in India while his uncle in the USA was sleeping at night. What is the reason?
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