Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 11 — every Probe and Ponder prompt, all 4 Activities, all 12 "Keep the Curiosity Alive" exercise questions, and all 5 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter explains why the Moon looks different every night, how the day, month, and year all come from natural cycles in the sky, how lunar, solar, and luni-solar calendars came into being, why festival dates shift, and why countries launch artificial satellites.
Chapter 11 opens with Meera spotting the Moon during the daytime at a kite festival, which sets up the central puzzle: why does the Moon look different every night? The chapter builds the answer using a ball-and-lamp model, showing that only the illuminated half of the Moon faces the Sun, and phases occur because we see different fractions of that illuminated half as the Moon orbits the Earth. It then connects three natural cycles — the day (Earth's rotation), the month (the Moon's phases), and the year (Earth's revolution around the Sun) — to how lunar, solar, and luni-solar calendars came into being, including the Indian National Calendar and why festival dates shift. It closes with why artificial satellites are launched and how to spot them. Every Activity and exercise is solved here exactly as the textbook presents it.
The Moon reflects sunlight; as it orbits the Earth, we see changing fractions of its illuminated half — waxing, full, waning, and new Moon.
Calendars are built from natural cycles — the Moon's phases (month), Earth's rotation (day), and Earth's revolution (year).
Human-made satellites orbit Earth for communication, navigation, weather monitoring, and scientific research.
Phases of the Moon
The Moon does not emit its own light — it shines by reflecting sunlight. Only the half facing the Sun is illuminated at any moment. Since we can only see the illuminated portion of the half of the Moon facing the Earth, and this changes as the Moon orbits the Earth, the Moon's visible shape — its phase — changes from day to day.
| Term | Meaning |
|---|---|
| Full Moon (Purnima) | Entire illuminated half faces the Earth — full bright circle |
| New Moon (Amavasya) | Non-illuminated half faces the Earth — Moon not visible |
| Waxing period (Shukla Paksha) | Bright portion grows from new Moon to full Moon (~2 weeks) |
| Waning period (Krishna Paksha) | Bright portion shrinks from full Moon to new Moon (~2 weeks) |
| Gibbous phase | More than half of the illuminated portion is visible |
| Crescent phase | Less than half of the illuminated portion is visible |
The Moon's phases are not caused by the Earth's shadow falling on it — that is what causes a lunar eclipse, a separate, much rarer event. Phases happen purely because of the changing relative positions of the Sun, Moon, and Earth as the Moon orbits the Earth. A lunar eclipse can only happen on a full Moon day, and a solar eclipse only on a new Moon day, but neither happens every month because the Moon's orbit is tilted slightly with respect to the Earth's orbit around the Sun.
Units of time and their natural basis
| Unit | Based on | Approximate duration |
|---|---|---|
| Day (mean solar day) | Earth's rotation on its own axis | 24 hours |
| Month (lunar month) | The Moon's revolution around the Earth (one full cycle of phases) | ~29.5 days |
| Year (solar year) | Earth's revolution around the Sun (one cycle of seasons) | ~365.25 days |
Types of calendars
| Calendar | Based on | Length of year | Synced with seasons? |
|---|---|---|---|
| Lunar calendar | 12 lunar months | ~354 days | No — drifts against seasons |
| Solar calendar (e.g. Gregorian) | Earth's revolution around the Sun | 365 days (366 in a leap year) | Yes |
| Luni-solar calendar | Lunar months + periodic intercalary month (Adhika Maasa) | ~354 days, corrected every 2–3 years | Yes, approximately |
A leap year adds one extra day (29 February) every four years to correct for the Earth taking about a quarter-day more than 365 days to orbit the Sun. In the Gregorian calendar, a year divisible by 4 is a leap year, with further corrections for century years (skipped unless divisible by 400).
Indian National Calendar
A solar calendar based on the Saka Era, used alongside the Gregorian calendar for official purposes. The year begins on 22 March (21 March in a leap year), has 365 days (366 in a leap year), and uses traditional Indian month names such as Chaitra and Vaisakha.
Artificial satellites
Human-made objects that orbit the Earth, typically around 800 km above the surface, completing one orbit in roughly 100 minutes. They support communication, navigation, weather monitoring, disaster management, and scientific research.
Sample answer: yes, the Moon is often visible during the day, especially near the first- or last-quarter phase — this happens because the Moon's rising and setting times shift compared to the Sun's, so their paths across the sky overlap for many hours in any 24-hour period, and moonlight is faint enough that it doesn't disappear in daylight the way stars do.
If we lived on the Moon, a "day" would likely be defined by the lunar day-night cycle (which lasts about 29.5 Earth days, since the Moon rotates once on its axis in the same time it takes to orbit the Earth); a "month" would need a different natural cycle altogether, perhaps based on watching Earth's own phases as seen from the Moon; and a "year" could still be tied to one full revolution around the Sun, which stays the same regardless of which body we stand on.
If Earth had two moons, the night sky would show two separate sets of phases cycling at different rates (unless both moons had identical orbits), tides would likely become more complex due to two competing gravitational pulls, and ancient calendars built around a single Moon's cycle would have needed an entirely different (probably more complicated) natural basis.
Without clocks or calendars, time could still be measured using natural periodic events — the Sun's daily rising and setting (for a day), the Moon's repeating phases (for a month), the cycle of seasons or the position of sunrise/sunset on the horizon (for a year), and even shadows cast by a stick in the ground, exactly as ancient civilisations did before mechanical timekeeping existed.
This is an open reflection prompt meant to set up the chapter's central ideas — Moon phases, natural cycles, and calendars — all explained in full through the Activities below.
Sample recorded data (Table 11.1):
| Day | Moon seen at | Bright portion vs. previous day | Moon–Sun separation vs. previous day |
|---|---|---|---|
| 1 (day after full Moon) | Sunrise | — | — |
| 2–14 | Sunrise | Decreased each day | Closer each day |
| ~15 (new Moon) | Not visible | Not visible | Closest (near the Sun) |
| 16–29 | Sunset | Increased each day | Farther each day |
Analysis: yes, the Moon appears different almost every day of the month, since its bright (illuminated) portion keeps shrinking for about two weeks and then growing for the next two weeks. The Moon is not visible on every single day at a fixed time (around the new Moon, it's too close to the Sun in the sky to be easily seen). The Moon's position in the sky, relative to the Sun, also shifts night to night — moving closer to the Sun during the waning half of the cycle and farther from it during the waxing half.
At position E (ball held towards the lamp): the side of the ball facing you is the non-illuminated side, so it appears completely dark — this matches the new Moon.
At position A (ball held opposite the lamp): the side of the ball facing you is the entire illuminated side, so it appears fully bright — this matches the full Moon.
At in-between positions (B, C, D and F, G, H): only part of the illuminated side faces you, and the boundary between the bright and dark portions appears as a curved line — matching the gibbous, half, and crescent phases seen from Earth. Turning anti-clockwise traces the Moon's phases through one complete cycle, just as the Moon does over about a month while orbiting the Earth.
Conclusion: the Moon's phases occur because the illuminated half of the Moon (the half facing the Sun) is not always the same half that faces the Earth — as the Moon moves along its orbit, we see a changing fraction of its illuminated half, from none (new Moon) to all of it (full Moon) and back again.
Method: the shadow is shortest exactly when the Sun is at its highest point in the sky for that day (local solar noon). Counting the dots marked up to that shortest shadow gives the precise time it occurred.
Sample recorded data (Table 11.2):
| Date | Time of shortest shadow | Duration of day |
|---|---|---|
| 22 March | 12:20 | — |
| 23 March | 12:20 | 24:00 (24 h 0 min) |
| 24 March | 12:19 | 23:59 (23 h 59 min) |
Conclusion: averaging the durations found across several days gives a value very close to 24 hours — this is exactly the definition of the mean solar day: the average time the Sun takes to return to its highest position in the sky from one day to the next.
Method: with an adult present, go to a spot with a clear, unobstructed view of the sky just before sunrise or shortly after sunset — this is when satellites, lit up by sunlight from below the horizon, are most visible against a dark sky.
What to look for: a small point of light, with steady or slowly flickering brightness, moving noticeably and quickly across the sky over a few minutes — unlike stars, which appear fixed relative to each other, or aircraft, which usually show flashing red/green lights and an audible engine sound.
Helpful tools: mobile apps or websites that track satellite positions can tell you exactly which satellites are visible from your location and at what time, making them much easier to spot on purpose.
For thousands of years, Indian astronomers tracked the sky without modern instruments or knowledge that the Earth revolves around the Sun, yet careful observation let them work out the year's length as approximately 365 days. They noticed the Sun doesn't always rise due East — it rises slightly north of East in summer and south of East in winter, reaching these extremes at the solstices (around 21 June and 21 December). The Sun's apparent northward drift from December to June is called Uttarayan, and its southward drift from June to December is Dakshinayan — a cycle recorded in the ancient Taittirīya Saṁhitā. Ancient texts like the Surya Siddhanta also tracked the solstices and equinoxes by noting which constellation — such as Capricorn (Makar) — appeared behind the Sun at the time.
Meghnad Saha was a pioneering Indian astrophysicist who studied stars and their temperatures, and is famously known for developing the Saha equation, a mathematical relationship used to understand the physical conditions inside stars. The Saha Institute of Nuclear Physics in Kolkata is named in his honour. He also chaired the Calendar Reform Committee, whose work led to the creation of the Indian National Calendar.
The Indian Space Research Organisation (ISRO) has launched satellite series like Cartosat, which captures high-quality images of the Earth used for mapping, city planning, and disaster management (feeding into platforms like Bhuvan), and AstroSat, which makes scientific observations of stars and other celestial objects. India's other missions include Chandrayaan 1, 2, and 3 to the Moon, Aditya L1 to study the Sun, and Mangalyaan to Mars — alongside student-built satellites such as AzaadiSat, InspireSat-1, and Jugnu.
Vikram Sarabhai, a researcher in space science and nuclear physics, is regarded as the Father of the Indian Space Programme, having pioneered India's effort to launch its first artificial satellites. The Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram — the ISRO centre responsible for developing rockets and launch vehicle technology — is named in his honour.
Many people assume the Moon rises exactly when the Sun sets, but this isn't true. The Moon actually rises roughly 50 minutes later each day, since it moves ahead in its orbit while the Earth completes one rotation every 24 hours. Moonrise sometimes happens in the afternoon, so the Moon can be spotted in the eastern sky even in daylight.
Adding a leap day every four years slightly over-corrects the calendar over long periods, since the true solar year is a little less than 365.25 days. To fix this, the Gregorian calendar skips the leap year in century years (like 1700, 1800, 1900) — but skipping every one of them would under-correct instead, so a leap year is added back every 400 years (like 1600 and 2000), keeping the calendar closely matched to the seasons.
The tropical year is the time between successive spring equinoxes, and the Gregorian calendar is based on it. The sidereal year is the time for the same background stars to rise again at sunset, and it's about 20 minutes longer than the tropical year — a gap so small that it takes a very long time to become noticeable, though astronomers use the sidereal year to track Earth's exact position in its orbit.
Traditional Indian luni-solar calendars name their twelve months Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha, and Phalguna. In Amant calendars, a month starts the day after the new Moon and ends on the new Moon; in Purnimant calendars, a month starts the day after the full Moon and ends on the full Moon.
In 1952, the Government of India formed a Calendar Reform Committee (chaired by Meghnad Saha) to study every calendar in use across the country and recommend one accurate, uniform calendar. Its recommended "Unified National Calendar" — the Indian National Calendar — came into effect from 21 March 1956 CE (1 Chaitra, 1878 Saka), and it follows the general principles of the ancient Surya Siddhanta.
Since sunrise happens earlier in Eastern India than in Western India, festival dates based on the exact lunar phase at sunrise can occasionally shift by a day between regions in the same year. To keep this consistent nationwide, the Positional Astronomy Centre publishes the Rashtriya Panchang every year — detailed calculations of celestial positions for a central Indian location — which the Government of India uses to officially declare festival holidays.
The Moon has inspired Indian classical ragas like Chandrakauns, Chandranandan, and Shubhapantuvarali, and hand gestures (mudras) such as Chandrakala and Ardhachandran appear in Bharatanatyam and other classical dance forms like Kathak, Odissi, and Kuchipudi. Traditional painting styles such as Madhubani and Warli, along with sculpture and pottery from tribal art traditions like Saura and Gond, also prominently depict the Moon and Sun.
Many artificial satellites and their spent rocket parts remain in orbit after their useful life ends, becoming "space junk" that crowds space and risks colliding with active satellites. Small debris usually burns up harmlessly in the atmosphere on re-entry, but larger pieces can survive and crash to the ground — a growing problem that countries are now working together to manage.
(i) True. We can only see the illuminated portion of the half of the Moon that faces the Earth — never the non-illuminated part, and never the far side that always faces away from Earth.
(ii) False. Earth's shadow falling on the Moon causes a lunar eclipse, not the regular phases of the Moon. Phases happen because of the changing relative positions of the Sun, Moon, and Earth as the Moon orbits — with no shadow involved.
(iii) True. The day, month, and year are all based on predictable, repeating astronomical cycles — Earth's rotation, the Moon's phases, and Earth's revolution around the Sun respectively.
(iv) False. The Moon can often be seen during the day too, especially near the first- or last-quarter phases, since its position in the sky relative to the Sun shifts throughout its monthly cycle.
Answer: No.
Reason: the Moon's phase repeats on a cycle of about 29.5 days, while the solar (calendar) year, which fixes the date "6th May" every year, is about 365.25 days long. Since 365.25 days is not an exact whole-number multiple of 29.5 days (365.25 ÷ 29.5 ≈ 12.4 lunar cycles), the Moon's phase on any fixed calendar date drifts from year to year rather than repeating exactly — so 6th May will only occasionally, by coincidence, land on a full Moon day again.
Likely error 1 — Stars shown too close to and as bright as the Moon: real stars are enormously fainter than reflected moonlight. In an actual night sky, faint stars close to a bright Moon get washed out by its glare and would not be clearly visible right beside it, as the illustration shows.
Likely error 2 — The crescent's shape/orientation: a real crescent Moon's two pointed tips ("horns") always lie exactly opposite each other along a straight line through the Moon's centre, and both horns always point directly away from the Sun's position in the sky. A cartoon-style crescent that doesn't follow this — for example, with horns that aren't aligned through the centre, or that don't point consistently away from a plausible Sun direction — would be scientifically inaccurate.
Since this question depends on comparing fine visual details in your specific printed copy of Fig. 11.10, please check the illustration in your textbook against these two commonly tested errors (unrealistic star brightness/placement near the Moon, and an incorrectly shaped or oriented crescent) to confirm which two apply to your figure.
(i) How to match each phase to its appearance:
| Phase | Expected appearance |
|---|---|
| Day of new Moon | Entirely dark disc — no illuminated portion visible at all |
| Three days after new Moon | Thin waxing crescent — a small sliver of light on one edge |
| Full Moon | Entirely bright disc — the whole illuminated half is visible |
| Three days after full Moon | Waning gibbous — slightly less than a full disc, just starting to shrink |
| A week after full Moon | Waning half Moon (last quarter) — exactly half the disc illuminated |
(ii) A phase that could never be seen from Earth: in reality, the Moon's illuminated portion always forms a single, continuous region bounded by one smooth, curved terminator line — it is never split into two separate illuminated slivers on opposite edges of the disc at the same time, and it never shows a perfectly straight (non-curved) division unless it's exactly a half-Moon. Any picture in Fig. 11.11 that shows two disconnected bright regions, or an oddly shaped division that isn't a smooth curve, depicts a phase that could never actually occur.
Matching the exact letters A–F requires comparing the actual shading in each picture in your copy of Fig. 11.11 against the appearances described above, since the specific image arrangement can vary between printings — use this table and the "impossible phase" rule to identify the correct letters in your own textbook.
(i) Phase: the Moon overhead at sunset is a half Moon — a circle with exactly one half shaded dark and the other half left bright, divided by a straight vertical line (this is the "first quarter" Moon).
(ii) Waxing or waning: it is in the waxing phase. The chapter established that the Moon is overhead at sunrise when it's a waning half Moon (last quarter); by the same reasoning, when the bright half-Moon is overhead at sunset, it must be the waxing half Moon (first quarter), since a waxing Moon is easiest to spot at sunset.
Kaushalya is telling the truth; Ravi is not.
Why Ravi's claim doesn't work: a crescent Moon phase only occurs when the Moon is positioned close to the Sun in the sky (a small illuminated sliver means most of the Moon's Earth-facing side is unlit, which happens only near the new Moon). If the Sun is setting in the West, a Moon close to the Sun in the sky should also be near the western horizon around that time — not rising in the East. A Moon rising in the East exactly as the Sun sets in the West would have to be roughly opposite the Sun, which matches a full Moon, not a crescent.
Why Kaushalya's claim works: a gibbous Moon (more than half illuminated) sits farther from the Sun in the sky than a crescent Moon does, closer to (but not quite at) the full Moon position. Such a Moon can rise well before sunset and be visible high in the eastern sky during the afternoon, which matches Kaushalya's observation.
Answer: Less often.
Reasoning: a slower-revolving Moon takes longer to complete each cycle of phases, so each lunar month becomes slightly longer. This means 12 lunar months would add up to a total closer to the length of the solar year (currently, 12 lunar months of ~29.5 days each total ~354 days, about 11 days short of the ~365-day solar year). As the lunar month lengthens, this 11-day gap would shrink, meaning intercalary (extra) months would be needed less frequently to keep the luni-solar calendar synced with the seasons.
Setup: 3 years in a solar (Gregorian-style) calendar contain 3 × 12 = 36 calendar months in total.
Applying the Pigeonhole Principle: we are distributing 37 full Moons among only 36 available calendar months. If each of the 36 months contained at most one full Moon, that could account for at most 36 full Moons in total — but there are 37 full Moons to place. Since 37 is greater than 36, it is impossible for every month to contain just one (or zero) full Moons; at least one month must therefore contain two full Moons.
Conclusion: this proves that at least two of the 37 full Moons must fall within the same calendar month — this is exactly the situation popularly called a "Blue Moon."
Answer: Full Moon.
Reasoning: the chapter establishes that on a full Moon day, the Moon is nearly opposite the Sun in the sky — so the Moon rises at almost exactly the same moment the Sun sets, stays visible all through the night, and sets at almost exactly the moment the Sun rises the next morning. Only the full Moon phase matches being visible continuously across the entire night, from sunset to sunrise.
Setting up the estimate: without leap years, every calendar year is treated as exactly 365 days, but the true solar year is about 365.25 days. This means the calendar "loses" about a quarter of a day relative to the true seasonal cycle every year, so fixed calendar dates gradually drift earlier relative to the seasons by roughly 1 day every 4 years.
Distance to shift into winter: 15 August (India's late-monsoon season) is roughly half a year away from the middle of winter (around mid-December to mid-January) — approximately 180–185 days.
Calculation: at a drift rate of about 0.25 day per year, shifting by roughly 183 days would take approximately 183 ÷ 0.25 ≈ 730 years.
Answer: it would take roughly 700–730 years for Independence Day to drift into winter if leap years were stopped.
Answer: artificial satellites are launched to support communication (relaying signals like TV, phone, and internet), navigation (like GPS-type positioning), weather monitoring (tracking clouds, storms, and cyclones), disaster management (mapping and responding to floods, earthquakes, and other emergencies), and scientific research (observing the Earth, the Sun, stars, and other celestial objects from space).
(i) Day: based on the Earth's rotation about its own axis — specifically, the average time the Sun takes to return to its highest position in the sky (the mean solar day, about 24 hours).
(ii) Month: based on the Moon's revolution around the Earth — one complete cycle of the Moon's phases, from one full (or new) Moon to the next, takes about 29.5 days.
(iii) Year: based on the Earth's revolution around the Sun — one complete cycle of the seasons, taking about 365.25 days (the solar year).
These five prompts are hands-on sky observations, research tasks, and long-term tracking projects rather than fixed-answer questions. Here's guidance on how to approach each one.
Guidance: tracing the shortest arc from the Sun's position to the crescent Moon, your finger should cross the illuminated (bright) part of the crescent first, before reaching its dark part — this directly demonstrates that the bright part of the Moon is the side facing the Sun, confirming that the Moon shines only by reflecting sunlight rather than glowing on its own.
Extra observation: the straight line joining the two pointed tips (horns) of the crescent corresponds to the actual diameter of the Moon's disc — useful for visually estimating the Moon's true size and orientation in the sky, even when most of it is in shadow.
Safety note: never point towards or look directly at the Sun itself — only trace the general direction/path across the sky towards the Moon.
Guidance: the Indian National Calendar's new year (1 Chaitra) begins on 22 March in a regular year but on 21 March in a leap year, since the leap day is added to the first month, Chaitra, rather than at the end of the year as in the Gregorian calendar. This means that in leap years, every date in the Indian National Calendar falls one day earlier against the Gregorian calendar compared to a regular year, until the Gregorian calendar's own leap day (29 February) evens things back out. Researching a full year-by-year comparison chart (available from Indian government almanac sources) will show exactly which months are affected and for how long in a given leap year.
Guidance: research regional New Year festivals such as Gudi Padwa (Maharashtra), Ugadi (Andhra Pradesh/Telangana/Karnataka), Baisakhi/Vaisakhi (Punjab), Poila Baisakh (West Bengal), Puthandu (Tamil Nadu), Vishu (Kerala), Bihu (Assam), Losar (regions of Himachal Pradesh/Sikkim), Cheti Chand (Sindhi community), and Navreh (Kashmir). For each, note whether the date is fixed to a solar/sidereal event (like a solstice or the Sun entering a particular zodiac sign — these tend to fall on nearly the same Gregorian date every year, similar to Makar Sankranti) or tied to a lunar/luni-solar month (in which case the Gregorian date shifts from year to year).
Guidance: tabulate the Gregorian date of Eid-ul-Fitr for each of the five years — since it follows a purely lunar calendar (12 lunar months, ~354 days), its Gregorian date should indeed shift earlier by roughly 11 days each year, with the underlying lunar month and day (per the Islamic calendar) staying the same throughout.
For Diwali: since it follows a luni-solar calendar, its Gregorian date will drift by a smaller, less regular amount most years, but you may spot one year where the date jumps forward instead of drifting backward as expected — this larger-than-usual jump is the signature of an intercalary month (Adhika Maasa) having been inserted that year to re-sync the lunar months with the solar seasons. Cross-checking against an actual luni-solar (Panchang) calendar for that year should confirm whether an intercalary month indeed falls between that year's Diwali and the previous year's.
Guidance: standing at the same spot each time and using fixed landmarks (trees, poles, or buildings) as reference points on the eastern horizon, mark where the Sun rises at the start of each month. Over a full year, the marked positions should visibly drift — shifting northward along the horizon from around December to June (matching Uttarayan) and southward from around June to December (matching Dakshinayan), exactly the cyclical pattern ancient Indian astronomers documented using solstices and equinoxes.
Related note on tides: if you live near a coast, you may notice the sea's water level rising and falling in a regular pattern called tides. Just as moonrise shifts about 50 minutes later each day, high and low tides at a given location also shift by roughly the same 50 minutes each day — a sign that tide timing is closely linked to the Moon's position and phase, not just the Sun's.
Now that Moon phases, calendars, and timekeeping are covered, move on to how ecosystems stay balanced, revisit Chapter 10 on mirrors and lenses, or book a free demo class for personalised coaching.
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