NCERT Solutions Curiosity Chapter 11 Chapter opener — Probe and ponder

Book page 170 Updated on2026-09-05

Q1.

Have you ever seen the Moon during the day? Why do you think it is sometimes visible when the Sun is up?

Answer

Yes — the Moon is in the daytime sky far more often than most people notice. Meera saw it over the kites at the Patang Mahotsav on Makar Sankranti. It is visible in daylight because it shines by reflecting sunlight, and that reflected light is bright enough to show against the blue sky whenever the Moon is above the horizon and far enough away from the Sun.

Why it happens: The Moon is above the horizon for roughly 12 hours out of every 24, but those hours are not fixed to the night. The Moon rises about 50 minutes later each day, so its "up time" slides all the way round the clock in one month. For about half the month, a good part of that time falls in daylight. The book's own example: on 7 April 2025 moonrise was at 14:23 — the middle of the afternoon.

You will not see it near new Moon, for two reasons at once: the Moon is then in almost the same direction as the Sun, so it is lost in the glare, and its non-illuminated half is the one turned towards us.

Check it yourself: Look towards the east in the late afternoon during the week after full Moon, or towards the west in the morning during the week after new Moon. A pale, washed-out Moon against a blue sky is a common sight once you start looking.

Q2.

Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month or a year?

Answer

Every unit of time comes from a natural cycle seen from where you stand, so all three would change.

A day — one sunrise to the next. The chapter tells us that only one half of the Moon always faces the Earth. For that to be true the Moon must turn once on its own axis in exactly the time it takes to go once round the Earth. So a day on the Moon lasts about 29.5 Earth days — roughly 14¾ Earth days of continuous sunlight followed by 14¾ of darkness.
A month — on Earth a month is one cycle of the phases of our companion in the sky. From the Moon that companion is the Earth, and the Earth would show phases in the very same 29.5-day rhythm. But that is the same as the lunar day, so on the Moon the month and the day would be one and the same unit. A Moon-dweller would probably not need a separate month at all.
A year — the Moon travels round the Sun along with the Earth, so its cycle of seasons is the Earth's cycle. A year on the Moon would still be about 365¼ days.

Why it happens: A "day" measures rotation, a "month" measures the companion body's revolution, and a "year" measures the journey round the Sun. On the Moon the first two cycles have become the same length, which is why its calendar would look so strange to us.

Q3.

What would happen if Earth had two moons instead of one? How would that change the night sky?

Answer

Two moons would almost certainly have different orbits and different periods, and that single fact changes everything that follows.

Two sets of phases at once. Each moon would go through its own cycle at its own rate. On most nights one might be a waxing crescent while the other is a waning gibbous, and only rarely would both be full together.
Brighter, more varied nights. Truly dark nights — when both moons are new or both are below the horizon — would become rare.
The "month" would stop being one clean unit. Our month exists because one cycle of phases repeats reliably. With two moons a calendar-maker would have to pick one of them, or invent a longer unit in which both cycles fit — much harder than fitting 12 lunar months into a year.
More eclipses. Two moons mean two chances every cycle for a moon to pass into Earth's shadow or across the Sun's face, and the moons could even eclipse each other.
More complicated tides. Tides follow the Moon's position and phase. With two pulls, high tides would sometimes add together and sometimes cancel, so their timing and height would be far less regular.

Did you know? Mars really does have two small moons, Phobos and Deimos. Phobos goes round Mars in only about 7½ hours — faster than Mars turns — so from the surface it rises in the west and sets in the east.

Q4.

If we didn’t have clocks or calendars, how else could we measure time?

Answer

By counting natural periodic events — events that repeat with a fixed period. That is exactly how every calendar in history began, and the chapter uses three of them.

Natural cycle	What is really moving	How you would use it	Length
Sunrise, noon, sunset	Earth turning on its axis	Fix a stick upright and mark the tip of its shadow. The shadow is shortest at noon; the gap between two noons is one day (Activity 11.3)	24 hours
Phases of the Moon	Moon revolving round the Earth	Count nights from one full Moon to the next; each night's shape tells you where you are in the count	About 29.5 days
Cycle of seasons; sunrise point moving along the horizon	Earth revolving round the Sun	Mark where the Sun rises against a tree or a hill. It creeps north till about 21 June and south till about 21 December — Uttarayan and Dakshinayan	About 365 days

Within a day, people also used devices built on steady processes rather than on the sky — a sundial, a water clock, a sand glass, or the burning of a marked lamp.

Why it works: Measuring time is nothing more than counting repetitions of something that repeats reliably. The sky supplies the most reliable repeating events there are, which is why ancient astronomers could fix the length of the year at about 365 days with no instruments at all.

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