NCERT Solutions for Class 7th Science Chapter 12 and 17212.1 Rotation of the Earth — Activity 12.2: Let us explore

Book page 171 Updated on2026-09-19

Q1.

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.

Answer

Stick a small paper sticker on the globe at your own town. Now look down on the globe from straight above the North Pole and turn it slowly anti-clockwise.

Observation: the sticker does not go up or down — it travels along a circle, parallel to the equator, and after one complete turn it arrives back exactly where it started. Every other point of the globe does the same thing on its own circle, and all these circles have their centres on the axis.

Why it happens: that is precisely what rotation means — all parts of the object move in circles about an imaginary line passing through it. The two points that do not travel at all are the North Pole and the South Pole, because they lie on the axis.

Try this: stick a second sticker near the equator and a third close to the North Pole. In one turn both complete a full circle in the same time, but the equator sticker covers a much bigger circle — so a place on the equator is actually racing through space much faster than a place near the pole.

Q2.

Now, shine light from the torch placed at some distance, say 1.5 metres, on the globe as shown in Fig. 12.4b. Do you notice how half of the globe receives the light from the torch, while the other half stays dark?

Fig. 12.4(b), page 172 — a torch about 1.5 m from a globe standing on a board, in a darkened room. Redrawn sketch.

Answer

Yes. With the torch acting as the Sun and the room dark, exactly one half of the globe is lit and the other half stays dark, and the boundary between them is a circle running through both poles' side of the globe.

Half of the globe	What it represents
The half facing the torch	Daytime
The half turned away from the torch	Night-time

Why it happens: light travels in straight lines and the Earth is an opaque sphere. A sphere can present only half of its surface to a distant source at any instant; the rest is in its own shadow. Since the real Sun is very far away, its rays reach us almost parallel — which is why the lit part is very nearly an exact half.

Tip: keep the torch at least about 1.5 m away. If you bring it too close, the beam spreads over less than half the globe and the day–night boundary is no longer sharp.

Q3.

In India, sunrise first occurs in the eastern part and then in other parts. While looking at the eastern part of India on the globe, rotate the globe in one direction and then in the opposite direction. What is the direction of rotation when light falls on the eastern part of India first?

Answer

The light falls on the eastern part of India first when the globe is rotated from West to East about its North–South axis.

Turn the globe the other way (East to West) and you will see the torchlight reach Gujarat before it reaches Arunachal Pradesh — the opposite of what really happens in India. So that direction must be wrong.

Why it happens: the place that meets the sunlight first is the one that is carried into the lit half first. Turning the globe eastward pushes the eastern edge of the dark half into the light, so eastern India crosses the sunrise line before central and western India do.

Real-life check: on the same date the Sun rises in Itanagar (Arunachal Pradesh) roughly two hours before it rises in Dwarka (Gujarat) — yet both places keep the same clock time, because India uses a single standard time.

Q4.

Now while rotating the globe from West to East, observe your location on the Earth. Does it go through a cycle of day and night?

Answer

Yes. The sticker marking your location passes through a complete cycle in every single turn of the globe.

The sticker crosses from the dark half into the lit half — this is sunrise.
It travels through the lit half — this is daytime, and it is brightest when the sticker faces the torch squarely (noon).
It crosses back into the dark half — this is sunset.
It travels through the dark half — this is night, and then sunrise comes again.

Why it happens: the Sun's light is always there and always lighting one half of the Earth. Nothing about the light changes during a day; what changes is which half of the Earth your town is standing in. One rotation of the Earth, about 24 hours, gives exactly one day and one night.

Key point: day and night are caused by the Earth's rotation, not by its revolution around the Sun. This is a very common mistake in tests.

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