Keep the Curiosity Alive — Chapter Exercises — "Keeping Time with the Skies"

1. 14 marks

   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.

   Hint. Statement (ii) is exactly the misconception the chapter's 'A step further' box corrects directly.

   ✦ (i) True — we only see the part of the Moon's illuminated half that also happens to face Earth. (ii) False — the chapter states directly that Earth's shadow does NOT cause the Moon's phases (it causes lunar eclipses, a separate, occasional event); phases come from the changing Sun–Earth–Moon geometry. (iii) True — day, month and year are all built from repeating astronomical cycles (Earth's rotation, the Moon's phase cycle, Earth's revolution). (iv) False — since the Moon is visible whenever it happens to be above the horizon and bright enough to see, exactly as Meera's own kite-festival sighting shows.

2. 23 marks

   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.

   Hint. A full-Moon cycle takes about 29.5 days, but a solar year is about 365.25 days — is 365.25 a clean multiple of 29.5?

   ✦ No — the Moon's phase cycle (about 29.5 days) and the solar year (about 365.25 days) are two independent natural cycles that do not divide evenly into each other. A solar year contains roughly 12.37 lunar cycles, not a whole number, so the full Moon does not land back on 6th May the following year; the date of the full Moon nearest to 6th May will drift from year to year, only occasionally lining back up with his birthday by coincidence.

3. 33 marks

   Name two things that are incorrect in Fig. 11.10, which shows a crescent Moon at night among clouds and stars.

   Hint. Look at exactly where the stars are drawn relative to the Moon's disc and the clouds — which of these objects should be blocking which?

   ✦ Looking closely at the figure: two stars are drawn appearing directly on top of the Moon's dark disc, and one star is drawn overlapping a cloud. Both are physically impossible for the same reason the chapter uses to explain eclipses — a nearer, opaque object blocks light from anything farther behind it. Stars are vastly farther away than the Moon, so the Moon's own solid disc (even its unlit side) would block a star positioned behind it, not let it appear to shine on top of the disc; likewise, clouds sit within Earth's atmosphere, far closer than any star, and would block a star from view rather than let it appear layered in front of or through the cloud.

4. 45 marks

   Look at the pictures of the Moon in Fig. 11.11 (panels A–F). (i) Write the correct panel letter for each of these phases: three days after New Moon; Full Moon; three days after Full Moon; a week after Full Moon; day of New Moon. (ii) List the picture labels of the phases that are never seen from Earth.

   Hint. Match how much of each panel is lit to how far along the waxing/waning cycle each description describes, then check which single panel's dark patch doesn't reach both the top and bottom of the disc.

   ✦ (i) Three days after New Moon (thin waxing crescent) → D; Full Moon → E; three days after Full Moon (still mostly lit, just starting to wane) → A; a week after Full Moon (roughly half the fortnight to new Moon has passed) → C; day of New Moon → B.

   (ii) F is never seen from Earth. Examined closely, every real Moon phase's boundary between light and dark is a curve that runs from the very top of the disc to the very bottom (as Activity 11.2 itself observes — the dividing line is always curved, spanning the full ball). Panel F's dark patch is confined to a small region near the top-left only, never reaching the bottom of the disc — a shape that cannot arise from the actual geometry of sunlight falling on a sphere, which is why A, C and D — whose curves genuinely do span the full height of the disc — are the ones that represent real phases.

5. 53 marks

   Malini saw the Moon overhead in the sky at sunset. (i) What phase of the Moon did she see? (ii) Is the Moon in the waxing or the waning phase?

   Hint. The chapter states exactly which phase puts the Moon overhead at a specific time of day — but that fact was given for sunrise. Think about the corresponding waxing-side event at sunset instead.

   ✦ The chapter states that the Moon is overhead at sunrise when its bright part has decreased to a half circle — during the waning fortnight. By the same logic applied to the waxing fortnight, the Moon is overhead at sunset when its bright part has grown to a half circle during the waxing period. This means (i) Malini saw a half Moon, and (ii) it was in its waxing phase (Shukla Paksha), which is why this is consistent with the chapter's own note that 'a waxing Moon is easiest to spot at sunset.'

6. 63 marks

   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?

   Hint. A crescent Moon is close to the Sun in the sky (near new Moon); a gibbous Moon is far from the Sun (near full Moon) — check each claim against that separation.

   ✦ Kaushalya is telling the truth; Ravi is not. A crescent Moon is close to new Moon, meaning it stays close to the Sun in the sky at all times — it could never be rising in the East right as the Sun sets in the West, since that would place it nearly opposite the Sun, which only happens near full Moon. A gibbous Moon, however, is close to full Moon and does sit far from the Sun in the sky, so it CAN rise in the East during the afternoon while the Sun is still up, exactly as the 'A step further' box describes for a bright Moon appearing before sunset.

7. 73 marks

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

   Hint. A slower Moon means a longer lunar month — how does a longer lunar month change the size of the yearly lunar-vs-solar shortfall that Adhika Maasa corrects?

   ✦ Less often. A slower-revolving Moon means each lunar month grows slightly longer, so 12 lunar months add up to a number of days closer to the 365-day solar year than the current 354 days — shrinking the yearly shortfall that Adhika Maasa exists to correct. A smaller shortfall takes longer to accumulate into a full extra month, so the intercalary month would need to be inserted less frequently than the current roughly-every-2–3-years pattern.

8. 84 marks

   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.

   Hint. 3 years contain 36 solar-calendar months in total — compare that to the number of full moons being distributed among them.

   ✦ Three years contain 3 × 12 = 36 months in the solar calendar. There are 37 full Moons to be distributed among these 36 months. If every month contained at most one full Moon, the maximum possible number of full Moons across 3 years would be only 36 — but there are 37, one more than that maximum. By the pigeonhole principle, at least one month must therefore contain two of the 37 full Moons, which is why at least two full Moons must fall within the same month of the solar calendar.

9. 92 marks

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

   Hint. Only one phase stays visible for a full night, rising right at sunset and setting right at sunrise.

   ✦ She would have seen a full Moon. Only the full Moon rises at nearly the moment the Sun sets and sets at nearly the moment the Sun rises the next morning, since the full Moon sits nearly opposite the Sun in the sky — making it the one phase visible continuously across an entire night.

10. 103 marks

    If we stopped having leap years, in approximately how many years would the Indian Independence Day (15 August) happen in winter?

    Hint. Without leap years, the calendar loses about a quarter-day every year against the real seasons — work out how many quarter-days add up to roughly half a year's worth of seasonal drift.

    ✦ Without leap years, the calendar falls behind the true solar year by about a quarter of a day (≈6 hours) every year, since that quarter-day is exactly what the leap day exists to correct. Independence Day (August, currently late summer/monsoon in India) would need to drift by roughly half a year's worth of seasons — about 182–183 days — to land in winter instead. At about 0.25 days of drift per year, that takes roughly 182 ÷ 0.25 ≈ 730 years.

11. 112 marks

    What is the purpose of launching artificial satellites?

    Hint. The chapter lists several practical applications directly.

    ✦ Artificial satellites help with communication, navigation, weather monitoring, disaster management, and scientific research — since ISRO's Cartosat, AstroSat, and interplanetary missions (Chandrayaan, Aditya L1, Mangalyaan) between them cover Earth-imaging/mapping, astronomy, and space exploration.

12. 123 marks

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

    Hint. Match each unit to the specific astronomical cycle the chapter names for it.

    ✦ (i) Day — the Sun returning to its highest position in the sky, caused by Earth's rotation about its own axis (the mean solar day, ≈24 hours). (ii) Month — the Moon cycling through all its phases (≈29.5 days). (iii) Year — Earth completing one revolution around the Sun, which gives its associated cycle of seasons (≈365¼ days).

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 11 (hecu111.pdf), Reprint 2026-27, pages 170-189. Questions are referenced from the NCERT textbook for identification.