12.1 Rotation of the Earth
1 · Does the Sun really move across the sky or is it the Earth that moves?
The Sun only appears to cross the sky. In reality the Earth turns (rotates) from west to east; that rotation makes the Sun seem to rise in the east, move westward, and set.
Activity 12.1 (“merry-go-round”)
2 a · While you rotate anticlockwise on the merry-go-round, in which direction do trees and buildings seem to move?
They appear to move the opposite way—clockwise.
2 b · If you fix your gaze on a single tree, how does it enter and leave your view?
It first appears on your left, passes in front of you, then disappears on your right (clockwise path).
Lesson: What looks like the tree’s motion is actually due to your rotation; the same illusion makes the Sun appear to move when the Earth spins.
Activity 12.2 (globe + torch)
3 a · What fraction of the globe is lit when you shine a torch on it?
Exactly half is illuminated; the other half remains dark.
3 b · When you rotate the globe west-to-east, does sunrise reach India’s eastern side first?
Yes. The eastern edge (e.g. Arunachal Pradesh) enters the light before the western edge (e.g. Gujarat).
3 c · What does an observer at one spot experience during the rotation?
A cycle of day (while in light) and night (while in darkness).
4 · When viewed from above the North Pole, which way is the Earth rotating?
Anti-clockwise (west → east).
Activity 12.3 (Big Dipper watch)
5 · After two-hour intervals the Big Dipper seems to have moved. Why?
Because as the Earth rotates, the entire sky appears to turn westward around the (almost fixed) Pole Star. Successive drawings show that apparent motion.
6 · Why do stars appear to move but the Pole Star stays almost fixed?
Earth’s rotation axis points almost directly at the Pole Star, so that star is nearly aligned and appears stationary while others trace circles around it.
12.2 Revolution of the Earth
7 · Why do the constellations that rise at sunset change from month to month?
Because the Earth revolves around the Sun once each year. At sunset we always face the night-side of Earth; that direction gradually shifts, revealing new groups of stars through the seasons.
8 · How long does one revolution of the Earth take and what is its path called?
About 365 days 6 hours along a nearly circular orbit around the Sun.
12.2.2 Seasons
9 · Why is it summer in June in the Northern Hemisphere and winter in June in the Southern Hemisphere?
In June the Northern Hemisphere is tilted toward the Sun. Sun-rays strike it more directly and for more than 12 hours a day—so it warms up (summer). The Southern Hemisphere is tilted away, gets lower-angle light for fewer hours, so it cools (winter). Six months later the situation reverses.
10 · Which dates mark solstices and equinoxes in the Northern Hemisphere?
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~21 June – Summer solstice (longest day)
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~22 December – Winter solstice (shortest day)
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~21 March – Spring (vernal) equinox (12 h day / 12 h night)
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~23 September – Autumn equinox (12 h day / 12 h night)
12.3 Eclipses
12.3.1 Solar eclipse
11 · How can the small Moon hide the huge Sun?
Although the Moon’s actual diameter is 400 times smaller than the Sun’s, it is also about 400 times closer to Earth. Their apparent angular sizes in the sky are therefore nearly equal, so the Moon can completely cover the Sun.
12 · What is the difference between a total and a partial solar eclipse?
Where the Moon’s full umbral shadow touches Earth observers see a total eclipse—the Sun’s bright disc is completely hidden. Nearby, under the penumbral shadow, observers see only part of the Sun covered: a partial eclipse.
13 · Why must we never view a solar eclipse with the naked eye?
Even a thin crescent Sun is intensely bright and can permanently damage the retina. Safe viewing requires special solar filters or indirect projection.
12.3.2 Lunar eclipse
14 · What causes a lunar eclipse and why can everyone on Earth’s night-side see it?
When the Earth comes between the Sun and the Full Moon, Earth’s shadow falls on the Moon. Because the shadow is large, anyone who has the Moon above the horizon during the event can witness the eclipse.
Blue-Box Exercise Section
(“Let Us Enhance Our Learning”)
1 · How many hours of sunlight do the North and South Poles receive in Fig 12.17?
The diagram corresponds to an equinox: each pole’s horizon lies exactly on the terminator, so both receive 12 hours of daylight and 12 hours of darkness during one rotation.
2 · Fill in the blanks
(i) Stars rise in the east and set in the west.
(ii) Day and night are caused by the Earth’s rotation on its axis.
(iii) When the Moon fully covers the Sun it is a total solar eclipse.
3 · True or False
| Statement | True / False | Why |
|---|---|---|
| (i) Lunar eclipse occurs when the Sun comes between Earth & Moon | False – Earth must be between Sun and Moon. | |
| (ii) Sunrise happens earlier in Gujarat than Jharkhand | False – Jharkhand is farther east; it sees sunrise first. | |
| (iii) In Chennai the longest day is on the summer solstice | True – Chennai is in Northern Hemisphere. | |
| (iv) We should watch a solar eclipse directly with naked eye | False – Dangerous for eyesight. | |
| (v) Seasons occur due to tilt of Earth’s axis and its spherical shape | True | |
| (vi) Earth’s revolution around Sun causes day and night | False – Rotation causes day/night; revolution causes yearly changes. |
4 · Orion overhead at 8 pm yesterday—when tonight?
Star patterns rise about 4 minutes earlier each successive night. After one day Orion will be overhead roughly 7 h 56 min after 8 pm, i.e. right about 7 h 56 pm ≈ 7:56 pm (roughly 8 pm again minus four minutes).
5 · Stars rising at midnight on 21 June — when next year?
One sidereal day is 365.25/366.25 of a solar day; effectively, after one year minus 4 minutes the same stars rise at the same clock time. So on 21 June next year they will again rise around midnight.
6 · Why is it night in USA when it’s day in India?
Because Earth’s east-to-west rotation places India’s longitude in sunlight while longitudes ~180 ° away (USA) face away from the Sun simultaneously—hence opposite day/night.
7 · Four eclipse-viewing methods—who was careless?
Adithya (iii) was careless by looking directly at the Sun without protection.
8 · Fill the solar-/lunar-eclipse diagrams (Fig 12.18).
For solar eclipse the middle circle must be Moon; for lunar eclipse the middle circle is Earth. The other two are the Sun and the remaining body accordingly.
9 · How can the small Moon block the Sun entirely?
Its apparent diameter equals the Sun’s because it is proportionally nearer. Thus it can fully cover the Sun’s disc during alignment.
10 · Indian cricket team touring Australia in December: pack winter or summer clothes?
Summer clothes. December is summer in the Southern Hemisphere.
11 · Why does a lunar eclipse cover a large part of Earth but a total solar eclipse only a small patch?
Earth’s shadow at Moon’s distance is wider than the Moon, so half the night side can see it. The Moon’s umbral shadow on Earth is only ~100–200 km wide, so a total solar eclipse is visible along a narrow track.
12 · If Earth’s axis were not tilted, what about seasons?
There would be no significant seasons; every place would receive nearly the same daily length and intensity of sunlight throughout the year.
Exploratory Projects — Hints
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Globe + lamp experiment: You will notice varying daylight areas and unequal day lengths, reproducing seasonal effects.
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Two concentric circles: The difference between perihelion (closest) and aphelion (farthest) distances is only ~3 %.
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Greater axial tilt (e.g., Uranus ~98°): Seasons would be extremely extreme; each pole would point at the Sun for ~42 years then spend 42 years in darkness.
(Students prepare reports; no single “right” answer is required.)