Class 8 Science Curiosity Chapter 11 Solutions (NCERT 2026–27) – Keeping Time with the Skies

These Class 8 Science Curiosity Chapter 11 solutions cover Keeping Time with the Skies from the new NCF-2023 textbook (2026–27), with every “Keep the curiosity alive” question answered step by step.

Class: 8 Subject: Science Book: Curiosity Chapter: 11 Exercise: Keep the curiosity alive (12 Qs) Session: 2026–27
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Class 8 Science Curiosity Chapter 11 Solutions – Overview

Chapter 11 of Curiosity, Keeping Time with the Skies, explains how the sky helps us measure time. It shows why the Moon changes its shape (phases) — from new Moon to crescent, half, gibbous and full Moon — because we see different fractions of its illuminated half as it revolves around the Earth in about a month. The Moon’s phases are not caused by Earth’s shadow. The chapter then builds the units of time — the day (Earth’s rotation), the month (Moon’s phase cycle) and the year (Earth’s revolution and seasons) — and how these gave rise to lunar, solar and luni-solar calendars, the Indian National Calendar, festival dates, and modern artificial satellites. These Class 8 Science Curiosity Chapter 11 solutions answer every textbook question.

Key Concepts & Definitions

Phases of the Moon: the changing shapes of the bright (illuminated) portion of the Moon as seen from Earth from one day to the next.

Why phases happen: the Moon shines by reflecting sunlight; only the half facing the Sun is lit. As the Moon revolves around the Earth, we see different fractions of that lit half — so its shape appears to change. (Phases are NOT due to Earth’s shadow.)

Waxing & waning: waxing = bright part growing (Shukla Paksha); waning = bright part shrinking (Krishna Paksha). Full Moon = Purnima; New Moon = Amavasya. One full cycle ≈ one month (about 29.5 days).

Gibbous & crescent: gibbous = more than half lit portion visible; crescent = less than half visible.

Mean solar day: the average time (24 hours) for the Sun to return to its highest point in the sky — due to Earth’s rotation.

Calendars: lunar (follows Moon’s phases, ≈354-day year), solar (follows seasons, 365¼ days, e.g. Gregorian), luni-solar (uses Moon’s phases but adds an extra month, Adhika Maasa, to stay in step with seasons).

Leap year: an extra day (29 February) usually added every four years in the Gregorian calendar; century years must be divisible by 400. This keeps dates close to the seasonal year. Artificial satellite: a human-made object launched to orbit the Earth.

“Probe and Ponder” & Activity Answers

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

ANSWERYes — the Moon can be seen by day. The Moon rises about 50 minutes later each day, so on many days moonrise happens in the afternoon and the Moon is already above the horizon while the Sun is still up. Since the Moon shines by reflecting sunlight, it is bright enough to be spotted in the daytime sky, especially in its gibbous or crescent phases.

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

ANSWERTime units come from natural cycles of the place you live on. On the Moon a “day” (one sunrise to the next) lasts about a month of Earth-time, because the Moon rotates much more slowly and always keeps one face towards the Earth. A “month” and “year” would have to be defined from other cycles — for example the Moon’s orbit of the Earth, or the Earth’s orbit of the Sun — so they would not match our Earth definitions.

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

ANSWERTwo moons would each have their own orbit and so their own set of phases and rise/set times. The night sky would often show two lit shapes at once, brighter nights would be more common, and tides would be more complicated (each moon would pull on the oceans). Timekeeping by a single Moon-cycle would become harder.

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

ANSWERWe could use natural sky cycles: the Sun’s daily rise and set and the length of shadows (a day); the changing phases of the Moon (a month); and the cycle of seasons and the Sun’s shifting rising point on the horizon (a year). Ancient people used exactly these to make calendars and sundials.

Activity 11.1 & 11.2 (key conclusions)

Activity 11.1: After observing the Moon at sunrise/sunset for a month, did the Moon appear different each day, and at the same position?

ANSWERThe Moon looked different each day — its bright portion grew or shrank — and it was not visible on all days (around new Moon it cannot be seen). Its position in the sky also shifted from the previous day, because the Moon rises about 50 minutes later each day. This shows the Moon goes through a regular cycle of phases in about a month.

Activity 11.2: Why does the illuminated portion of the Moon seen from the Earth decrease when it appears closer to the Sun?

ANSWEROnly the Moon’s half facing the Sun is lit. When the Moon is near the Sun in the sky (positions D, E, F in the ball model), it is almost between us and the Sun, so its lit half faces mostly away from Earth — we see only a thin crescent or, at new Moon, nothing. When it is opposite the Sun (full Moon), its whole lit half faces us. Thus the visible lit fraction shrinks as the Moon appears to move closer to the Sun.

Class 8 Science Curiosity Chapter 11 Solutions — Keep the Curiosity Alive

1. State whether the following statements are True or False. (i) We can only see that part of the Moon which reflects sunlight towards us. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases. (iii) Calendars are based on various astronomical cycles which repeat in a predictable manner. (iv) The Moon can only be seen at night.

ANSWER (i) True — the Moon has no light of its own; we see only the sunlit portion that reflects light towards us. (ii) False — phases are caused by the changing position of the Moon as it revolves around the Earth, not by Earth’s shadow. (Earth’s shadow causes a lunar eclipse, not the phases.) (iii) True — the day, month and year all come from repeating astronomical cycles (Earth’s rotation, Moon’s phases, Earth’s revolution). (iv) False — the Moon is often visible during the daytime too, because it rises about 50 minutes later each day.

2. Amol was born on 6th of May on a full Moon day. Does his birthday fall on the full Moon day every year? Explain your answer.

ANSWERNo. The Gregorian date repeats after about 365 days, whereas a full-Moon cycle is about 29.5 days. Twelve phase cycles take about 354 days, roughly 11 days less than a solar year. Therefore the Moon does not return to the same phase on every 6 May. The “11 days” compares twelve cycles with a year; it is not a rule that every successive May full Moon falls exactly 11 days earlier.

3. Name two things that are incorrect in Fig. 11.10.

ANSWERFirst error: stars are drawn on the dark part of the Moon’s disc. That part is still solid Moon and blocks stars behind it; it is not a transparent gap.Second error: the left cloud is drawn behind the Moon, as though the Moon covers it. Clouds are in Earth’s atmosphere and are much nearer than the Moon, so a cloud crossing the same line of sight must appear in front of it. The right cloud being in front is not the error.

4. Look at the pictures of the Moon in Fig. 11.11, and answer the following questions. (i) Write the correct panel number corresponding to the phases of the Moon shown in the pictures above. (ii) List the picture labels of the phases of the Moon that are never seen from Earth. Hint: You can use your observations from Activity 11.1 or Fig. 11.2 as reference.

ANSWER
Phase requestedNCERT Fig. 11.11 label
Three days after new Moon: crescentD
Full MoonE
Three days after full Moon: gibbousA
A week after full Moon: half MoonC
New MoonB
(ii) F is not an ordinary lunar phase. Its curved boundary cuts off a corner, instead of joining opposite points on the lunar limb as the projected day–night boundary does. Such a bitten shape should not be confused with the ordinary phase cycle.B is the valid new-Moon position, represented by a dark disc in a diagram, even though it is normally not visible against daylight. A and C identify the lit fraction; image orientation alone cannot distinguish waxing from waning without observing context.
Eight lunar phases from new Moon through waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter and waning crescent.
Original phase guide. Conventional north-up view: apparent tilt changes with location and viewing time. Dark areas are unilluminated Moon, not missing material. Tap to view at full size.

5. Malini saw the Moon overhead in the sky at sunset. (i) Draw the phase of the Moon that Malini saw. (ii) Is the Moon in the waxing or the waning phase?

ANSWER(i) Draw a half-lit Moon: the first-quarter phase, about a week after new Moon. In the conventional north-up drawing below, the right half is bright. In the actual sky, the bright side points towards the Sun; the apparent tilt varies.(ii) Waxing (Shukla Paksha). A first-quarter Moon is highest in the sky around sunset, rising near noon and setting around midnight. “Overhead” here means high in the sky, not necessarily exactly at the zenith.

For phase names and approximate timing, see NASA’s Moon phases guide.

6. Ravi said, “I saw a crescent Moon, and it was rising in the East, when the Sun was setting.” Kaushalya said, “Once I saw the gibbous Moon during the afternoon in the East.” Who out of the two is telling the truth?

ANSWER Kaushalya is telling the truth. A crescent Moon is close to the Sun in the sky, so it rises and sets near the Sun — it is seen low in the west just after sunset (waxing) or low in the east just before sunrise (waning). It cannot rise in the east exactly as the Sun sets. So Ravi’s statement is wrong. A gibbous Moon is far from the Sun, so it rises in the afternoon and is well up in the eastern sky by late afternoon — it can indeed be seen in the east during the afternoon. So Kaushalya is correct.

7. Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will luni-solar calendars need an intercalary month more often or less often?

ANSWER Less often. As the Moon moves farther away it revolves more slowly, so each lunar month becomes longer. Then 12 lunar months add up to more days, closer to the 365-day solar year, so the shortfall (currently about 11 days a year) shrinks. It would then take longer for the difference to build up to a full month, so the extra (intercalary) month, Adhika Maasa, would be needed less frequently.

8. A total of 37 full Moons happen during 3 years in a solar calendar. Show that at least two of the 37 full moons must happen during the same month of the solar calendar.

ANSWER Use the pigeonhole principle. In 3 years there are 3 × 12 = 36 months in the solar calendar (the “boxes”). The 37 full Moons (the “objects”) must each fall in one of these 36 months. Since 37 full Moons are being placed into only 36 months, and 37 > 36, at least one month must contain more than one full Moon. Therefore at least two of the 37 full Moons must occur in the same month of the solar calendar. (This is why an extra full Moon in a single month is called a “Blue Moon” — it happens because the lunar month, ≈29.5 days, is slightly shorter than most calendar months.)

9. On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?

ANSWER The Moon was visible the whole night, rising at sunset and setting at sunrise. This happens only when the Moon is opposite the Sun in the sky — that is, on the full Moon day. So Vaishali saw the full Moon.

10. If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?

ANSWERUsing the textbook approximation, a 365-day calendar falls short of the seasonal year by about ¼ day per year. Thus a fixed date such as 15 August would occur progressively earlier in the seasonal cycle.If we take a shift of roughly six months (about 180 days, towards February) as reaching winter, then 180 ÷ 0.25 = 720 years. This is an estimate, not an exact date. Choosing January or December as the winter target gives a larger estimate, roughly 800–960 years. State the assumed seasonal shift in your calculation.

11. What is the purpose of launching artificial satellites?

ANSWER Artificial satellites are launched to serve many useful purposes: communication (TV, phone, internet), navigation (GPS/NavIC), weather monitoring and forecasting, disaster management, Earth observation and mapping (e.g. ISRO’s Cartosat and the Bhuvan platform), and scientific research (e.g. AstroSat studying stars). They give us vital information for daily life, safety and space science.

12. On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?

ANSWER
Measure of timePeriodic phenomenon it is based on
(i) DayThe rotation of the Earth on its own axis — the Sun returning to its highest point in the sky (about 24 hours).
(ii) MonthThe cycle of the phases of the Moon as it revolves around the Earth (about 29.5 days).
(iii) YearThe revolution of the Earth around the Sun and the resulting cycle of seasons (about 365¼ days).

The Indian National Calendar begins Chaitra on 22 March, or 21 March in Gregorian leap years: IMD Positional Astronomy Centre.

Gregorian leap-year test: divisible by 4, with centuries excluded unless divisible by 400; 2000 and 2024 are leap years, 1900 and 2100 are not.
The precise Gregorian rule refines the textbook approximation of one extra day every four years. Tap to view at full size.

Common Misconceptions to Avoid

Watch out for these

  • Thinking the Moon’s phases are caused by Earth’s shadow — they are caused by seeing different parts of the lit half as the Moon revolves; Earth’s shadow causes a lunar eclipse.
  • Believing the Moon’s actual shape changes — only the visible lit portion changes; the Moon stays a sphere.
  • Assuming the Moon is seen only at night — it rises about 50 minutes later each day and is often visible by day.
  • Thinking the Moon rises exactly when the Sun sets — only the full Moon does; other phases rise and set at different times.
  • Confusing waxing and waning — waxing (Shukla Paksha) grows, best seen at sunset; waning (Krishna Paksha) shrinks, best seen at sunrise.
  • Mixing up calendars — lunar follows the Moon (≈354 days), solar follows seasons (365 days), luni-solar adds an extra month to match both.

Extra Practice Questions

These questions and MCQs are ClearStudy practice, separate from the NCERT exercise.

Very Short Answer Type Questions

Q1. What is the day when the Moon appears as a full bright circle called in India?

ANSWERPurnima (full Moon day).

Q2. About how many days does the Moon take to cycle through all its phases?

ANSWERAbout 29.5 days (nearly a month).

Q3. Name the natural satellite of the Earth.

ANSWERThe Moon.

Short Answer Type Questions

Q1. Why does the Moon rise about 50 minutes later each day?

ANSWERWhile the Earth completes one rotation in 24 hours, the Moon has moved a little further ahead in its orbit around the Earth. The Earth must rotate a bit more (about 50 minutes’ worth) for the Moon to appear in nearly the same spot in the sky. So moonrise is delayed by roughly 50 minutes daily.

Q2. Why does a lunar calendar slowly fall out of step with the seasons?

ANSWERA lunar year of 12 lunar months is about 354 days, but the seasons repeat in about 365 days. The lunar year is therefore about 11 days short, so the same lunar months drift earlier against the seasons year after year. Luni-solar calendars fix this by adding an extra month (Adhika Maasa) every 2–3 years.

Long Answer Type Question

Q1. Explain how the day, the month and the year are each defined from natural sky cycles, and how leap years keep the solar calendar accurate.

ANSWER The day comes from the Earth’s rotation: the Sun rises, reaches its highest point, and returns there in about 24 hours (the mean solar day). The month comes from the Moon’s phase cycle — about 29.5 days from one full Moon to the next as the Moon revolves around the Earth. The year comes from the Earth’s revolution around the Sun, taking about 365¼ days, during which one full cycle of seasons occurs. Because the year is 365¼ days, an extra quarter-day builds up each year and adds to nearly one full day every four years. To absorb this, solar calendars add a leap day (29 February) every fourth year. Finer corrections (skipping leap years in century years not divisible by 400) keep the Gregorian calendar closely matched to the seasons over long periods.

MCQs & Assertion–Reason

1. The phases of the Moon are caused by:

(a) Earth’s shadow on the Moon    (b) the Moon changing its real shape    (c) seeing different parts of the lit half as the Moon revolves around Earth    (d) clouds covering the Moon

2. The day when the Moon is not visible is called:

(a) Purnima    (b) Amavasya (new Moon)    (c) full Moon    (d) gibbous day

3. The Moon takes about how long to complete one cycle of phases?

(a) one day    (b) one week    (c) about 29.5 days    (d) one year

4. Each day, the Moon rises approximately:

(a) at the same time    (b) 50 minutes earlier    (c) 50 minutes later    (d) exactly when the Sun sets

5. The mean solar day is based on the:

(a) revolution of the Earth around the Sun    (b) rotation of the Earth on its axis    (c) phases of the Moon    (d) movement of the stars

6. The Gregorian calendar is an example of a:

(a) lunar calendar    (b) solar calendar    (c) luni-solar calendar    (d) sidereal-only calendar

7. In a leap year an extra day is added to:

(a) January    (b) February    (c) June    (d) December

8. The extra month added to a luni-solar calendar to match the seasons is called:

(a) Shukla Paksha    (b) Krishna Paksha    (c) Adhika Maasa (intercalary month)    (d) Uttarayan

9. When the Moon shows more than half of its lit portion, the phase is called:

(a) crescent    (b) gibbous    (c) new Moon    (d) half Moon

10. The Indian National Calendar begins its year on:

(a) 1 January    (b) 22 March normally; 21 March in a Gregorian leap year    (c) 21 June    (d) 14 November

Answer key: 1-(c), 2-(b), 3-(c), 4-(c), 5-(b), 6-(b), 7-(b), 8-(c), 9-(b), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: The Moon shines because it reflects sunlight.

Reason: Sunlight strikes the Moon and some of it is reflected towards Earth.

A-R 2. Assertion: A full Moon is visible in the sky the whole night, from sunset to sunrise.

Reason: On full Moon day the Moon is nearly opposite the Sun in the sky.

A-R 3. Assertion: The phases of the Moon are caused by Earth’s shadow falling on the Moon.

Reason: We see different fractions of the Moon’s illuminated half as it revolves around the Earth.

A-R 4. Assertion: In the Gregorian calendar, a leap day is usually added every four years, with century-year exceptions.

Reason: The Earth takes about 365 and a quarter days to revolve once around the Sun.

A-R 5. Assertion: A purely lunar calendar always keeps the seasons in the same months.

Reason: A lunar year of 354 days is shorter than the solar year of about 365 days.

Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(D).

Quick Revision Summary

  • The lit part of the Moon changes shape from day to day through phases — new Moon, crescent, half, gibbous and full Moon.
  • Phases happen because we see different parts of the Moon’s illuminated half as it moves around the Earth (not due to Earth’s shadow).
  • One full cycle of the Moon’s phases takes about a month (≈29.5 days); the Moon rises about 50 minutes later each day.
  • Natural cycles give the units of time: day (Earth’s rotation), month (Moon’s phases), year (Earth’s revolution and seasons).
  • Lunar calendars follow the Moon; solar calendars (e.g. Gregorian, Indian National Calendar) follow the seasons; luni-solar calendars add an extra month (Adhika Maasa) to match both. Gregorian leap years add 29 February: years divisible by 4 qualify, except century years unless divisible by 400.
  • Artificial satellites are human-made objects orbiting Earth, used for communication, navigation, weather, mapping and research.

Real-life Applications

Sky timekeeping shapes daily Indian life. Lunar and luni-solar calendars set the dates of festivals — Diwali on the new Moon of Kartika, Holi and Buddha Purnima on full Moons, Eid-ul-Fitr after the crescent is sighted — which is why their Gregorian dates shift each year. Solar festivals like Makar Sankranti and Pongal stay near the same date. Farmers use seasonal (solar) calendars to plan sowing and harvest, fishers and coastal communities track the Moon to predict tides, and the Indian National Calendar and the Rashtriya Panchang fix official festival holidays. Earth-orbiting satellites support services such as weather observation, navigation and mapping. Cartosat is associated with Earth observation and AstroSat with astronomy; Chandrayaan and Mangalyaan are planetary exploration missions and should not be grouped as routine weather or navigation satellites.

How to write a clear answer

Always state clearly that phases are due to the Moon’s revolution (not Earth’s shadow). Learn which phase rises/sets when: full Moon all night, waxing best at sunset, waning best at sunrise. Remember the three cycles — day (rotation), month (Moon’s phases ≈29.5 days), year (revolution ≈365¼ days) — and the difference between lunar (354 days), solar (365 days) and luni-solar calendars. For numerical questions (full-Moon counting, leap-year drift) use the pigeonhole idea and the quarter-day per year, and show your working.

Frequently Asked Questions

What is Class 8 Science Curiosity Chapter 11 about?

Chapter 11, Keeping Time with the Skies, explains the phases of the Moon, why the Moon’s appearance changes, how the day, month and year are defined from sky cycles, lunar, solar and luni-solar calendars, the Indian National Calendar, festival dates and artificial satellites.

Why does the Moon change its shape?

The Moon shines by reflecting sunlight, so only the half facing the Sun is lit. As the Moon revolves around the Earth, we see different fractions of that lit half, so its shape appears to change. This is what we call the phases of the Moon — it is not caused by Earth’s shadow.

How many questions are in the “Keep the curiosity alive” exercise of Chapter 11?

There are 12 questions in the “Keep the curiosity alive” exercise, all solved on this page, along with the “Probe and ponder” prompts and activity conclusions.

Are these Class 8 Science Curiosity Chapter 11 solutions free?

Yes. All solutions are free and follow the official NCERT Curiosity textbook for Grade 8, session 2026–27.

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