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NCERT Solutions Curiosity Chapter 12 Dive Deeper boxes of the chapter — Dive Deeper

Book page 171 to 186 Updated on2026-09-05

Q1.
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.
Answer

This is the exact definition you should learn. Three things are packed into it:

  1. All parts move — not just one point. Every particle of the object takes part.
  2. Each part moves in a circle — not along a straight line and not randomly.
  3. The circles are centred on one imaginary line that passes through the object — the axis of rotation.
Spinning objectIts axis of rotationWhich parts stay still
Spinning topIts spindleThe points on the spindle
Ceiling fanThe rod through its centreThe centre of the hub
EarthThe line through the North Pole and the South PoleThe two poles
Rotation is not revolution: in rotation the axis passes through the object itself (the Earth spinning, once in 24 hours). In revolution the object goes around another object (the Earth going around the Sun, once in about 365 days and 6 hours).
Q2.
Two incorrect reasons often given to explain why seasons occur on the Earth are: When the Northern Hemisphere tilts towards the Sun, it is closer to the Sun. The orbit of the Earth is an oval with the Sun slightly displaced from its centre so the Earth is at different distances from the Sun over the year.
Answer

Both of these sound convincing, and both are wrong. The difference in distance is far too small to matter in either case.

  • "The tilted hemisphere is nearer." The Earth's radius is only about 6,400 km, while the Earth–Sun distance is about 150 million km. Tilting brings the Northern Hemisphere nearer by a few thousand kilometres out of 150 million — a change of well under one-hundredth of one per cent. It cannot make a season.
  • "The Earth is nearer to the Sun in some months." The orbit is only very slightly oval. The Earth's closest and farthest distances are about 147 million km and 152 million km — a difference of roughly 3%.
Farthest − closest = 152 − 147 = 5 million km
As a fraction of the distance: 5 ÷ 150 ≈ 0.033 = about 3% only.
The decisive fact: the Earth is closest to the Sun in January — the depth of the Northern Hemisphere's winter, and the height of the Southern Hemisphere's summer. If distance decided the seasons, both hemispheres would have summer together in January. They do not. So distance is not the cause; the tilt of the axis and the spherical shape of the Earth are.
Q3.
In the Northern Hemisphere, the longest day occurs around 21 June—this is known as summer solstice. After the summer solstice, the duration of a day becomes shorter while that of a night becomes longer. The shortest day and longest night in this Hemisphere occur around 22 December known as the winter solstice. Around 21 March and 23 September, the daytime lasts for 12 hours. In the Northern Hemisphere, these days are called the spring and the autumn equinox, respectively.
Answer

These four dates are the milestones of the year. Learn them as a table.

Date (approx.)Name in the Northern HemisphereDaytime hereWhat is happening
21 MarchSpring equinox12 hoursNeither pole leans towards the Sun
21 JuneSummer solsticeLongest of the yearNorthern Hemisphere leans most towards the Sun
23 SeptemberAutumn equinox12 hoursNeither pole leans towards the Sun
22 DecemberWinter solsticeShortest of the yearNorthern Hemisphere leans most away from the Sun
Why it happens: "solstice" marks the turning points of the tilt, "equinox" the two in-between days on which the day–night boundary passes through both poles, giving equal day and night everywhere. Between 21 June and 22 December the day shortens; between 22 December and 21 June it lengthens again.
Did you know? At the North Pole the Sun rises on the equinox day, 21 March, and stays continuously in the sky for six months, setting on 22 September. The South Pole behaves in exactly the opposite way.
Q4.
Though the planets Mercury and Venus are much larger than the Moon in size, they are also much farther from the Earth as compared to the Moon. Thus, their apparent sizes are very much smaller than the Sun and they cannot block the Sun. For example, when Venus passes between the Sun and the Earth, it appears as a tiny black dot passing against the bright face of the Sun. This event, known as a Transit of Venus, is a rare event.
Answer

The box is really a second application of Activity 12.4 — apparent size decides everything, and apparent size depends on both actual size and distance.

BodyActual diameterDistance from the EarthApparent size in our sky
MoonAbout 3,500 km — smallest of the threeAbout 3.8 lakh km — nearestAbout the same as the Sun's — it can cover the Sun
VenusAbout 12,000 km — bigger than the MoonTens of millions of km — far awayA tiny dot
MercuryAbout 4,900 km — bigger than the MoonEven farther than Venus at transitA tinier dot
Why it happens: being big is not enough. The Moon wins the contest only because it is so close. A transit of Venus therefore never darkens the day; it just puts a black speck on the Sun's disc, which can be seen only with proper, safe solar projection — never by looking at the Sun.
Did you know? Transits of Venus come in pairs eight years apart, and then not again for more than a century. The last pair was in 2004 and 2012.
Q5.
This activity, similar to Activity 11.5, is to be set up by your teacher. A mirror can be used to project an image of the Sun onto a wall. However, holding it at the correct angle throughout a solar eclipse can be difficult. To solve this, make a movable stand for the mirror. Use a hollow ball with a small hole, half-fill it with sand (to keep it stable), and attach a small mirror (such as an embroidery mirror) to it. Place the ball on a circular ring, like an adhesive tape ring, so it can be turned around easily. Adjust it until the Sun's image appears on a wall or screen. Fig. 12.15 shows this set-up where the mirror is fixed to a green ball.
Answer

This is the safe way to watch a solar eclipse — you look at a picture of the Sun on a wall, never at the Sun itself. Here is how to build and use it.

  1. Take a hollow ball with a small hole and half-fill it with dry sand. The sand lowers the centre of gravity so the ball sits steadily instead of rolling away.
  2. Stick a small mirror — an embroidery mirror is ideal — on the outside of the ball.
  3. Rest the ball on a ring, for example a roll of adhesive tape. The ball can now be nudged in any direction and it stays where you leave it, so it works as a movable stand.
  4. Turn the ball until the patch of reflected sunlight lands on a wall or a white screen in the shade. Move the screen farther away to get a bigger but dimmer image, nearer for a smaller but brighter one.
  5. During the eclipse, the round patch on the wall is a real image of the Sun. Watch the Moon's dark bite grow and shrink on the wall.
Why a small mirror works: a small flat mirror far from the screen behaves like a pinhole — the bright patch stops being a square copy of the mirror and becomes a round image of the Sun itself. That is why the shape of the patch changes during the eclipse.
Caution: this activity must be done strictly under the supervision of a teacher. Never direct the reflected beam into anyone's eyes, and never look at the Sun or at the mirror itself.
Q6.
In reality, the planets move around a special point in the solar system, which is very close to the Sun but not exactly at its centre! The Sun also moves around the same point a little instead of staying perfectly still. Scientists use such tiny wobbles in the movement of other stars to discover exoplanets around them!
Answer

The picture of a fixed Sun with planets going round it is a very good approximation, but not the whole truth.

The Sun and its planets pull on each other. So the Sun and a planet both go around a common balance point that lies between them — extremely close to the Sun's centre, because the Sun is so much more massive, but not exactly at it. The Sun therefore traces a tiny loop of its own instead of standing perfectly still.

Why this is so useful: other stars must wobble in the same way if they have planets. Those planets are far too faint to photograph, but the star is bright — and a careful measurement of its small, regular wobble reveals a hidden planet. Planets found around other stars in this way are called exoplanets.
Everyday model: hold hands with a friend of about your own weight and spin. Neither of you stands still — you both circle a point midway between you. Now let a much heavier adult spin a small child in the same way: the adult barely shifts, but does shift a little. That small shift is the Sun's wobble.
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