Hinglish

Rotation of the Earth

Class 7 Science · Chapter 12: Earth, Moon, and the Sun · NCERT Curiosity section 12.1

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Cycling to school by the sea, Meher notices that coconut-tree shadows are long in the morning and short in the afternoon. "The Sun must move across the sky," she thinks. "But we learnt that the Earth moves. So which is it?"

On a merry-go-round turning anticlockwise, Kabir sees trees and buildings seem to sweep past clockwise, the opposite way. In the same way, the Earth spins (rotates) on its axis from West to East (anticlockwise seen from above the North Pole), so the Sun only appears to rise in the East, cross the sky and set in the West. One rotation takes about 24 hours.

With a globe and a torch in a dark room, the friends see that half the globe is lit (day) and half is dark (night). Turning the globe West to East, a sticker on India moves into light (sunrise) and later into darkness (sunset). Eastern India sees sunrise first. A huge Foucault pendulum, 22 m long, hangs in India's new Parliament building as a demonstration of the Earth's rotation.

At night, the Big Dipper (Saptarishi) seems to wheel around the Pole Star (Dhruv taara), which hardly moves because the Earth's axis points almost at it. Long-exposure photos show star trails as arcs. Over 1500 years ago, Aryabhata compared this to a person in a moving boat seeing the shore move backwards, and his value for one rotation, about 23 h 56 min, is remarkably close to today's.

Remember

  • The Earth rotates on its axis from West to East in about 24 hours, causing day and night.
  • So the Sun, Moon and stars appear to rise in the East and set in the West; the Pole Star seems fixed.

Quick check

Why does the Sun appear to rise in the East?

  1. the Earth rotates from West to East
  2. the Sun moves around the Earth each day
  3. the East is warmer
  4. the Moon pushes the Sun up
Answer

the Earth rotates from West to East. We see the Sun from a spinning Earth.

Think about it

If the Earth rotated from East to West instead, where would the Sun rise?

One answer

It would appear to rise in the West and set in the East, since the Sun's apparent motion is always opposite to the Earth's spin.

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.