What is Time and Skies Exercises in Class 8 Science NCERT?

The Class 8 Science NCERT Chapter 11 — Keeping Time with the Skies exercise section tests students on all the key concepts covered in the chapter, including NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions. It includes multiple-choice que…

What does the NCERT Class 8 Science Keeping Time with the Skies exercise section cover?

The Class 8 Science NCERT Chapter 11 u2014 Keeping Time with the Skies exercise section tests students on all the key concepts covered in the chapter, including NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions. It includes multiple-choice questions (MCQs), assertion-reason questions, short-answer questions, long-answer questions, and competency-based questions that align with the NCERT Curiosity textbook. Working through these exercises helps Class 8 students reinforce their conceptual understanding, prepare for classroom assessments, and build exam readiness for CBSE school tests. Each question links back to a specific section of the chapter so students can revise the theory alongside the practice.

How should Class 8 students approach the Keeping Time with the Skies exercises?

Class 8 students should approach the Keeping Time with the Skies exercises by first revising the main theory and definitions from each part of the chapter, especially around NCERT exercises and day and year questions. Start with the MCQs and assertion-reason questions to quickly test recall, then attempt the short-answer questions that need a one or two-line explanation, and finally tackle the long-answer and competency-based questions that require applying concepts to new situations. Write neat diagrams wherever the question involves a process, cycle, or setup, and always use NCERT-style vocabulary in your answers to match CBSE marking schemes.

Are these Keeping Time with the Skies questions competency-based as per NEP 2020?

Yes, the Keeping Time with the Skies exercise set follows the NEP 2020 competency-based education (CBE) framework used by CBSE and NCERT for Class 8 Science. Instead of only asking students to recall definitions, many questions are scenario-based and require applying the concept to real-life situations, analysing data, or evaluating claims. This matches Bloom's Taxonomy levels L3 (Apply), L4 (Analyse), and L5 (Evaluate), going beyond rote learning. Students practising these questions build the higher-order thinking skills that modern CBSE school exams increasingly assess, and also prepare well for competitive foundation tests in later years.

Which NCERT Curiosity sections are tested in Chapter 11 exercises?

The Chapter 11 u2014 Keeping Time with the Skies exercise set systematically tests every major section of the NCERT Curiosity Class 8 Science textbook for this chapter. Topics covered include NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions. Every in-text question, activity-linked question, and end-of-chapter question from the Curiosity book is represented, so students can be confident that completing this exercise set gives them full coverage of the NCERT syllabus. Teachers also frequently pick questions from these sections for school unit tests and half-yearly exams, making thorough practice essential for strong performance.

How do assertion-reason questions work in this Class 8 Science chapter?

Assertion-reason questions in the Class 8 Science Keeping Time with the Skies exercise set present two statements u2014 an Assertion (A) and a Reason (R) u2014 and ask students to judge whether each is true and whether R correctly explains A. Students pick from four standard options: both true with R explaining A, both true but R not explaining A, A true and R false, or A false. This format tests whether Class 8 learners genuinely understand the cause-and-effect relationships behind NCERT exercises, not just surface facts. Mastering this format is valuable because CBSE Class 10 board exams use the same pattern.

What is the best revision strategy before attempting these exercises?

The best revision strategy before attempting Keeping Time with the Skies exercises is to read through all theory parts of NCERT Curiosity Chapter 11 once, write down the key terms such as NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions in your own words, and then re-do all activities mentally to understand the underlying concept. Next, make a one-page mind map linking the main ideas. Only then attempt the exercise questions u2014 first without looking back, then self-correcting. This active-recall approach builds much stronger long-term memory than repeatedly re-reading the textbook, and is widely recommended by CBSE teachers for Class 8 Science preparation.

What does the NCERT Class 8 Science Keeping Time with the Skies exercise section cover?

The Class 8 Science NCERT Chapter 11 — Keeping Time with the Skies exercise section tests students on all the key concepts covered in the chapter, including NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions. It includes multiple-choice questions (MCQs), assertion-reason questions, short-answer questions, long-answer questions, and competency-based questions that align with the NCERT Curiosity textbook. Working through these exercises helps Class 8 students reinforce their conceptual understanding, prepare for classroom assessments, and build exam readiness for CBSE school tests. Each question links back to a specific section of the chapter so students can revise the theory alongside the practice.

How should Class 8 students approach the Keeping Time with the Skies exercises?

Class 8 students should approach the Keeping Time with the Skies exercises by first revising the main theory and definitions from each part of the chapter, especially around NCERT exercises and day and year questions. Start with the MCQs and assertion-reason questions to quickly test recall, then attempt the short-answer questions that need a one or two-line explanation, and finally tackle the long-answer and competency-based questions that require applying concepts to new situations. Write neat diagrams wherever the question involves a process, cycle, or setup, and always use NCERT-style vocabulary in your answers to match CBSE marking schemes.

Are these Keeping Time with the Skies questions competency-based as per NEP 2020?

Yes, the Keeping Time with the Skies exercise set follows the NEP 2020 competency-based education (CBE) framework used by CBSE and NCERT for Class 8 Science. Instead of only asking students to recall definitions, many questions are scenario-based and require applying the concept to real-life situations, analysing data, or evaluating claims. This matches Bloom's Taxonomy levels L3 (Apply), L4 (Analyse), and L5 (Evaluate), going beyond rote learning. Students practising these questions build the higher-order thinking skills that modern CBSE school exams increasingly assess, and also prepare well for competitive foundation tests in later years.

Which NCERT Curiosity sections are tested in Chapter 11 exercises?

The Chapter 11 — Keeping Time with the Skies exercise set systematically tests every major section of the NCERT Curiosity Class 8 Science textbook for this chapter. Topics covered include NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions. Every in-text question, activity-linked question, and end-of-chapter question from the Curiosity book is represented, so students can be confident that completing this exercise set gives them full coverage of the NCERT syllabus. Teachers also frequently pick questions from these sections for school unit tests and half-yearly exams, making thorough practice essential for strong performance.

How do assertion-reason questions work in this Class 8 Science chapter?

Assertion-reason questions in the Class 8 Science Keeping Time with the Skies exercise set present two statements — an Assertion (A) and a Reason (R) — and ask students to judge whether each is true and whether R correctly explains A. Students pick from four standard options: both true with R explaining A, both true but R not explaining A, A true and R false, or A false. This format tests whether Class 8 learners genuinely understand the cause-and-effect relationships behind NCERT exercises, not just surface facts. Mastering this format is valuable because CBSE Class 10 board exams use the same pattern.

What is the best revision strategy before attempting these exercises?

The best revision strategy before attempting Keeping Time with the Skies exercises is to read through all theory parts of NCERT Curiosity Chapter 11 once, write down the key terms such as NCERT exercises, day and year questions, moon phase questions, calendar questions, competency-based questions in your own words, and then re-do all activities mentally to understand the underlying concept. Next, make a one-page mind map linking the main ideas. Only then attempt the exercise questions — first without looking back, then self-correcting. This active-recall approach builds much stronger long-term memory than repeatedly re-reading the textbook, and is widely recommended by CBSE teachers for Class 8 Science preparation.

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Keeping Time with the Skies — Exercises

🎓 Class 8 Science CBSE Theory Ch 11 — Friction ⏱ ~25 min

🌐 Language: [gtranslate]

🧠 AI-Powered MCQ Assessment ▲

This MCQ module is based on: Keeping Time with the Skies — Exercises

No. of Questions:

Difficulty Level:

Myaischool Lt Science Assessment ▲

[myaischool_lt_science_assessment grade_level="class_8" science_domain="physics" difficulty="basic"]

📚 Chapter Summary

Earth's Rotation

Earth spins once on its axis in 24 hours. This rotation causes day and night and makes the Sun appear to move east to west.

Earth's Revolution

Earth takes ~365.25 days to orbit the Sun once. This revolution gives us the year and the seasons.

Leap Year

The 0.25 day left over each year is saved for 4 years and added as 29 February, making a 366-day leap year.

Sundial & Jantar Mantar

A vertical rod (gnomon) casts a moving shadow that tells time. The Jantar Mantar observatories have huge stone versions.

Saur Maas

The Indian solar month — time the Sun takes to cross one zodiac sign (~30 days). Sankranti marks the transition.

Moon Phases

The Moon does not change shape — as it orbits Earth, different portions of its sunlit half face us, creating phases.

Amavasya & Purnima

Amavasya = new moon night (no Moon visible). Purnima = full moon night (whole lit face visible).

Pakshas

Shukla Paksha = 15-day bright half (Amavasya → Purnima). Krishna Paksha = 15-day dark half (Purnima → Amavasya).

Lunar Month

One full cycle of phases ≈ 29.5 days. Two pakshas make one month. 12 lunar months ≈ 354 days.

Three Calendar Types

Solar (Sun-based, e.g. Gregorian), Lunar (Moon-based, e.g. Hijri), Lunisolar (both, e.g. Hindu Panchang).

Adhik Maas

Extra lunar month added every 2–3 years to keep the Hindu lunisolar calendar aligned with the solar seasons.

Pole Star (Dhruv Tara)

Lies almost over Earth's North Pole. Appears fixed while all other stars circle around it. Used for navigation.

Constellations

Saptarishi (Ursa Major / Big Dipper) and Mriga (Orion) are prominent constellations visible from India.

Gregorian Calendar

Modern civil calendar introduced in 1582 CE. Accurate to about one day in 3,000 years. Year starts 1 January.

Saka Calendar

India's National Calendar (adopted 1957). Solar. Year starts on 1 Chaitra = 22 March (or 21 March in a leap year).

Indian Festivals

Makar Sankranti (solar, fixed ~14 Jan); Holi (Phalguna Purnima); Diwali (Kartika Amavasya); Eid (Hijri lunar).

🔑 Key Terms

RotationSpinning of Earth on its own axis; one rotation = 1 day = 24 hours.

RevolutionMotion of Earth around the Sun; one revolution = 1 year ≈ 365.25 days.

Leap YearA 366-day year with an extra 29 February; every year divisible by 4 (with century rule).

SundialInstrument that tells time from the shadow of a gnomon cast by the Sun.

Gnomon (Shanku)The vertical shadow-casting pointer of a sundial.

Jantar MantarGroup of 18th-century Indian stone observatories at Delhi, Jaipur, Ujjain, Varanasi and Mathura.

Saur MaasIndian solar month — time the Sun takes to cross one zodiac sign (~30 days).

SankrantiDay on which the Sun moves from one zodiac sign to the next.

Phases of the MoonChanging shapes of the Moon as seen from Earth over a month.

AmavasyaNew moon night — the Moon is invisible from Earth.

PurnimaFull moon night — the entire lit face of the Moon is visible.

Shukla PakshaBright fortnight; Moon waxes from new moon to full moon (15 tithis).

Krishna PakshaDark fortnight; Moon wanes from full moon to new moon (15 tithis).

TithiA lunar day; each paksha has 15 tithis.

Chaitra MaasFirst month of the Hindu calendar, corresponding to March-April.

PanchangTraditional Hindu lunisolar almanac listing tithi, weekday, nakshatra, yoga, karana.

Adhik MaasExtra lunar month added to the Hindu calendar every 2–3 years.

Hijri CalendarIslamic pure lunar calendar of ~354 days; year starts 1 Muharram.

Gregorian CalendarSolar civil calendar introduced in 1582 CE; year ≈ 365.2425 days.

Saka CalendarIndian National Calendar; solar; year begins 22 March.

Pole Star (Dhruv Tara)Star above Earth's North Pole; appears fixed; used for navigation.

SaptarishiGroup of seven bright stars — Indian name for Ursa Major / Big Dipper.

MrigaIndian name for the constellation Orion.

NakshatraOne of 27 star-groups along the Moon's monthly path through the sky.

📝 Exercises

Q1. Fill in the blanks:
(a) The time Earth takes to rotate once on its axis is ______ hours.
(b) The new moon night is called ______ in the Hindu calendar.
(c) The Pole Star is known as ______ in India.
(d) A leap year has ______ days.
(e) The Indian National Calendar is based on the ______ era.

(a) 24 (b) Amavasya (c) Dhruv Tara (d) 366 (e) Saka

Q2. State True (T) or False (F):
(a) The Moon produces its own light.
(b) A purely lunar year has 365 days.
(c) Saptarishi has seven bright stars.
(d) Makar Sankranti is a lunar festival.
(e) The Samrat Yantra at Jaipur is the world's largest sundial.

(a) F — the Moon only reflects sunlight. (b) F — a pure lunar year has about 354 days. (c) T. (d) F — Makar Sankranti is a solar festival, marking the Sun's entry into Makara (Capricorn). (e) T.

Q3. What gives us the idea of a "day", a "month" and a "year"? Link each unit of time to the correct astronomical motion.

Day (24 h): one complete rotation of the Earth on its own axis. Month (~29.5 days): one complete cycle of Moon phases, caused by the Moon's revolution around the Earth. Year (~365.25 days): one complete revolution of the Earth around the Sun.

Q4. Why was the concept of leap year introduced? How do we decide whether a given year is a leap year or not?

Earth takes about 365 days and 6 hours to revolve around the Sun, not a whole number of days. If we kept using only 365 days per year, every 4 years the calendar would slip one full day behind the Sun. To correct this, every fourth year is made 366 days long by adding 29 February — that is a leap year. The rule: a year divisible by 4 is a leap year. Century years (1900, 2000, 2100, …) are leap years only if they are also divisible by 400. So 2000 was a leap year, but 1900 was not.

Q5. With the help of a diagram, explain why the Moon appears in different shapes on different nights.

The Moon has no light of its own. The Sun always lights exactly half of the Moon's surface. As the Moon travels around the Earth, the angle between the Sun, the Moon and us changes. So we see different fractions of the sunlit half. When the lit half faces Earth fully, we see Purnima (full moon); when it faces completely away, Amavasya (new moon); between these, crescents and gibbous shapes. Draw: the Sun on one side, Earth in the middle, and the Moon shown at four orbit positions — new, first quarter, full, last quarter — with the lit half of each Moon pointed towards the Sun.

Q6. Differentiate between Shukla Paksha and Krishna Paksha. Which festivals are celebrated in each?

Shukla Paksha (bright half): 15 lunar days from Amavasya to Purnima. The Moon grows from invisible to fully lit. Festivals: Ram Navami, Ganesh Chaturthi, Raksha Bandhan, Sharad Purnima and Guru Purnima. Krishna Paksha (dark half): 15 lunar days from Purnima to the next Amavasya. The Moon shrinks from fully lit to invisible. Festivals: Janmashtami (Krishna Paksha Ashtami of Bhadrapada) and Diwali (Kartika Amavasya).

Q7. Why do Eid and Ramzan shift 10–11 days earlier in the Gregorian calendar every year, but Diwali does not?

Eid and Ramzan follow the Islamic Hijri calendar, which is purely lunar. Twelve lunar months total about 354 days, which is 11 days shorter than the Gregorian (solar) year of 365 days. So Ramzan slides 10–11 days earlier each Gregorian year. Diwali follows the Hindu lunisolar Panchang, which inserts an extra month (Adhik Maas) every 2–3 years. This correction keeps lunar months tied to the solar seasons, so Diwali always falls in October–November, shifting only within a few weeks year by year.

Q8. Describe, step by step, how you would locate Dhruv Tara (Pole Star) in the night sky using Saptarishi.

Step 1: Face the northern direction. Step 2: Look for the seven bright stars of Saptarishi, arranged like a big saucepan or ladle. Step 3: Identify the two stars that form the outer edge of the "bowl" of the saucepan — the "pointer stars". Step 4: Draw an imaginary straight line from the inner pointer star through the outer pointer star, and continue it onwards. Step 5: About five times the distance between the two pointer stars, your line ends on a single bright star — that is Dhruv Tara, the Pole Star. Unlike other stars, it stays almost perfectly still even after hours of watching.

Q9. State whether each of the following festivals is based on the Sun, the Moon, or a combination of both. Give a one-line reason.
(a) Makar Sankranti (b) Holi (c) Eid-ul-Fitr (d) Ugadi (e) Guru Purnima

(a) Sun — marks the Sun's entry into the Makara (Capricorn) zodiac sign; date nearly fixed (~14 January). (b) Moon — Phalguna Purnima; falls on a full moon of the Hindu lunisolar month. (c) Moon — determined by the sighting of the first crescent after Ramzan in the pure lunar Hijri calendar. (d) Combination (lunisolar) — first day of the Hindu lunisolar new year, Chaitra Shukla Pratipada, which stays in spring because of Adhik Maas. (e) Moon — celebrated on the Ashadha Purnima, a full moon.

Q10. Explain how sailors in ancient times could find the direction of north during a moonless night, far from any land.

On a clear, moonless night, a sailor would first locate Saptarishi (Ursa Major / Big Dipper) in the northern sky. Using the two pointer stars of Saptarishi, he would then identify the Pole Star (Dhruv Tara). Because the Pole Star lies almost directly over Earth's axis of rotation, it always marks true north with extraordinary steadiness — it does not rise, set or wander. Facing Dhruv Tara meant facing north. The angle of Dhruv Tara above the horizon (measured with a simple instrument like an astrolabe) even told the sailor his latitude — at the equator Dhruv Tara sits on the horizon; at the North Pole it is directly overhead.

Q11. Describe Activity 11.1 (building a sundial). Why are the hour marks not evenly spaced on it?

Fix a vertical stick (gnomon) upright on a level sunlit patch of ground. At 8 a.m., 9 a.m., 10 a.m. … until sunset, place a small marker at the tip of the stick's shadow. Label each marker with the time. On any future sunny day, the position of the shadow tip among the markers gives the time. The hour marks are not evenly spaced because the Sun's angular motion across the sky translates differently into shadow distance depending on where the Sun is. Near sunrise and sunset, the low Sun throws the shadow tip a long way for a small angular change — marks are far apart. Near noon, the high Sun throws the shadow tip only a short distance for the same angular change — marks are crowded close together.

Q12. What is a lunisolar calendar? Explain why the Hindu Panchang has to insert an Adhik Maas every few years.

A lunisolar calendar uses the Moon to measure months and the Sun to measure years — it tries to combine both. The Hindu Panchang is a lunisolar calendar. A problem arises because 12 lunar months (~354 days) are 11 days shorter than one solar year (~365.25 days). After about 3 years, this shortfall grows to nearly 30 days — almost a whole extra lunar month. If ignored, festivals like Holi would drift out of spring into winter. To fix this, the Panchang inserts an extra lunar month called Adhik Maas (also called Purushottam Maas) roughly every 2–3 years. This re-synchronises the lunar months with the solar seasons so that Holi always lands in spring and Diwali always in autumn.

Q13. Observe the sky on any cloudless night for 15 minutes. (i) Does the Moon (if visible) remain exactly in the same place? (ii) Do the stars move? Explain your observations.

(i) The Moon appears to drift slowly westwards; in 15 minutes you will notice that it has shifted perceptibly compared with a distant tree or rooftop. (ii) The stars also appear to drift west at nearly the same rate. Both movements are caused by Earth's rotation on its axis — the Earth turns from west to east, so everything in the sky seems to slide from east to west. The Pole Star (Dhruv Tara) is the one exception because it lies on the line of Earth's rotation axis.

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